DISSERTATION

Adsorption of non-ionic surfactants onto ultrafiltration membranes in aqueous and

organic solutions

vorgelegt von Diplom-Ingenieurin Le Anh Thu Nguyen

geb. in Đà Nẵng

Von der Fakultät II – Mathematik und Naturwissenschaften der Technischen Universität Berlin

zur Erlangung des akademischen Grades

Doktorin der Ingenieurwissenschaften - Dr.-Ing. -

genehmigte Dissertation

Promotionsausschuss:

Vorsitzende: Prof. Dr. M. A. Mroginski Berichter: Prof. Dr. rer. nat. R. Schomäcker Berichter: Prof. Dr.-Ing. T. Melin

Tag der wissenschaftlichen Aussprache: 06.Juli 2015

Berlin 2015 D 83

Affirmation

I make a solemn declaration, that I prepared the Ph.D. thesis independently and that I

cite all the utilised resources and references. Neither the Ph.D. thesis, nor parts of it are

published elsewhere.

Regensburg, 06 Juli 2015

_____________________________

Le Anh Thu Nguyen

Abstract The global aim of this thesis is to deliver the responsible mechanism for the technical infeasibility of the reverse micellar-enhanced ultrafiltration (reverse MEUF). The flux-reducing mechanism may be a cake layer formation, blocking the membrane pores or molecular interactions between surfactants and membrane surface. This thesis focuses on interactions between membrane and the non-ionic surfactants (nonylphenol ethoxylate NP5) either in a non-polar organic (1-dodecene) or in aqueous (water) solvent during the ultrafiltration (UF) at surfactant concentrations below and above the critical micelle concentration (cmc).

In the surfactant-based hydroformylation of 1-dodecene, reverse MEUF seems to be an attractive concept to recycle catalysts from any product stream as reverse micellar solutions. Owing to a lack of literature on reverse MEUF, this preliminary investigation is an essential step and fundamental for the beginning of the process design. Consequently, reverse MEUF was proven as unsuitable concept for catalyst recovery from the oily product phase of the hydroformylation of 1-dodecene.

Change in flux and retention were investigated by applying two models: (i) Hermia’s blocking models; (ii) Zhu and Gu’s adsorption models. Fundamental research revolves around contrasting MEUF and reverse MEUF using various membranes in aqueous and organic solutions.

Above the cmc, surfactants aggregate into “normal” micelles in water or “reverse” micelles in oil. Catalysts embedded in micelles can be recovered by using UF-membranes with a molecular weight cut-off (MWCO) of 10-30 kDa. Finding the responsible effect for the differing results of both MEUF types is a great motivation for the research in the next part of this study. To explain the changes in the permeability regarding to surfactant concentrations, the second stage of the study is divided into two parts: (i) UF of binary mixtures, i.e. surfactant/1-dodecene or surfactant/water; (ii) rinsing membranes by filtration with the same pure solvent.

As result, the study points out that the flux-reducing mechanism observed in this work is not caused by a mechanical membrane blocking. In the research field of membrane filtration, this work applies Zhu and Gu’s adsorption models for the first time, to the best knowledge of the author, to explain the adsorption mechanism of surfactants from oil, as well as from water onto UF-membranes. Additionally, different impacts such as the surface hydrophilicity, pore size and pore size distribution of membranes are observed with the help of Zhu and Gu’s models, which become to be an interesting tool for membrane screening suggested in this study.

Adsorption in membrane capillaries or at membrane surface has a negative impact on the retention of reverse micelles. The formation of reverse micelles in a non-polar

solvent is inhibited, even though the surfactant concentration of feed is increased. When adsorption reaches its saturation, the surfactants remaining in the bulk solution do not participate in the adsorption onto membrane. These molecules pass through the membrane capillaries. For this reason, reverse MEUF, which was originally expected to work successfully as a recycling process of the hydroformylation of 1-dodecene, is proven to be technically infeasible. Key word: Surfactant, Reverse micelle; Micellar-enhanced ultrafiltration; Catalyst recovery, Ultrafiltration membranes; Adsorption; Hydroformylation

Zusammenfassung Das globale Ziel der vorliegenden Dissertation ist es, eine Aussage über den grundlegenden Mechanismus zu treffen, der für die technische Undurchführbarkeit der inversen mizellengestützten Ultrafiltration (inverse MEUF) verantwortlich ist. Die Studie fokussiert auf den Wechselwirkungen zwischen Membran und den nichtionischen Tensiden (Nonylphenol ethoxylate NP5) für zwei Konzentrationsbereiche entweder in Wasser oder in 1-Dodecen: Unterhalb und oberhalb der kritischen Mizellbildungskonzentration (cmc). Aus Mangel an Literatur über inverse MEUF wurde eine Voruntersuchung zur Eignung des Membranverfahrens im ersten Teil der Studie durchgeführt. Dies gilt als erster Schritt einer Prozessauslegung. Das Ergebnis zeigt, dass kaum Rhodium durch inverse MEUF zurückgehalten wird, wobei hoher Rückhalt durch traditionelle MEUF technisch machbar ist. Änderungen an Fluss und Rückhaltevermögen wurden mittels zwei Modellen analysiert: (i) Hermias Blockungsmodelle; (ii) Zhu und Gus Adsorptionsmodelle. Dabei wurden als Grundlage der Forschungsarbeit unterschiedliche Membranen mit einer Ausschlussgrenze oder Cut-off (MWCO) von 10-30 kDa in wässrigen sowie organischen Lösungen systematisch eingesetzt. Um den Grund für die Permeabilitätsabnahme in Abhängigkeit der Tensidkonzentration herauszufinden, wurden im zweiten Teil der Studie zwei Versuchsgruppen gebildet: (i) UF von NP5/1-Dodecen und NP5/H2O; (ii) Membranspülung mit dem jeweiligen Reinstlösungsmittel. Die Auswertung der Fluss-Daten mittels Hermias Modellen wies drauf hin, dass der verantwortliche Mechanismus nicht aus einer mechanischen Membranverblockung stammt. Als Ergebnis liefert die vorliegende Studie, welche als allererste Studie in der Membranforschung die Modelle von Zhu und Gu anwendet, den Nachweis über die Tensidadsorption als ein für die Fluss-Abnahme entscheidender Mechanismus. Darüber hinaus wurden hierbei unterschiedliche Faktoren wie die Hydrophilie, die Porengröße, sowie die Porengrößenverteilung jeweiliger Membranoberfläche betrachtet. Adsorption in Kapillaren oder an der Oberfläche von der Membran hat einen negativen Einfluss auf den Rückhalt von inversen Mizellen. Die Bildung von inversen Mizellen in einem unpolaren organischen Lösungsmittel wird durch die Adsorption verhindert, selbst wenn die Tensidkonzentration im Feed weiter zunimmt. Bei der Adsorptionssättigung nehmen die Tenside, die in der Überschussmenge in der Bulklösung verbleiben, nicht an dem Adsorptionsvorgang teil. Diese Moleküle diffundieren an die Membrankapillaren. Aus diesem Grund, erweist sich die inverse MEUF, welche herkömmlich als ein effizientes Trennverfahren zum

Katalysatorrecycling in der Hydroformulation von 1-Dodecen erwartet wurde, als technisch ungeeignet. Schlagwörter: Tensid; inverse Mizelle; MEUF; Katalysatorrückgewinnung; Ultrafiltrationsmembranen; Adsorption; Hydroformylation

Acknowledgement The first sections of this dissertation is a part of the collaborative research centre InPROMPT (SFB/TR 63) financed in years from 2010 until 2012 by the Deutsche Forschungsgemeinschaft (DFG). The whole experimental work I carried out in the laboratory of Institute for Technical Chemistry (TC8) at the Technical University Berlin (TU Berlin). I acknowledge gratefully the financial support of DFG and TU Berlin. Moreover, special thanks go to the companies Sasol Germany GmbH and PolyAn GmbH for donating the surfactant samples Marlophen and polyacrylonitrile membranes, respectively.

I wish to express my sincere thanks to Prof. Schomäcker, Prof. Kraume, Prof. Drews, colleagues and students for their great support during my research years at TC8.

For my parents and Barbara

I

Table of contents

1 Introduction ................................................................................................ 1

2 State of the art ............................................................................................ 5

2.1 Surfactants and their classification ....................................................... 5

2.2 Micelle formation .................................................................................. 7

2.3 Thermodynamic models of micelle formation ....................................... 9

2.4 Surfactant selection for chemical process .......................................... 10

2.5 Ultrafiltration ....................................................................................... 12

2.5.1 Definition und classification ...................................................... 12

2.5.2 Mode of operation .................................................................... 13

2.5.3 Pore scale model ...................................................................... 15

2.6 Micellar-enhanced ultrafiltration ......................................................... 17

2.7 Reverse micelle enhanced ultrafiltration ............................................ 19

2.8 Models for flux data evaluation .......................................................... 21

2.8.1 Hermia’s blocking filtration models ........................................... 22

2.8.2 Zhu and Gu’s adsorption models .............................................. 25

3 Material and method ................................................................................ 27

3.1 Chemicals .......................................................................................... 27

3.1.1 Surfactants ............................................................................... 27

3.1.2 Solvents ................................................................................... 28

3.1.3 Catalyst-ligand-complex ........................................................... 28

3.1.4 Rhodium-containing reverse micellar solutions ........................ 30

3.2 Membranes ........................................................................................ 30

3.3 Experimental set-up ........................................................................... 32

3.4 Procedure of filtration experiment ...................................................... 33

3.5 Measurement of micelle and reverse micelle size .............................. 36

3.6 Determination of critical micelle concentration ................................... 36

3.6.1 Marlophen in aqueous solution................................................. 37

3.6.2 Marlophen in organic solution .................................................. 38

3.7 Surfactant concentration measurement ............................................. 39

3.7.1 Marlophen in aqueous solution................................................. 39

II

3.7.2 Marlophen in organic solution .................................................. 40

3.8 Rhodium concentration measurement ............................................... 40

4 Results and discussion ........................................................................... 42

4.1 Catalyst recovery by micellar enhanced ultrafiltration ........................ 42

4.1.1 Catalyst recovery by reverse micellar enhance ultrafiltration ... 43

4.1.2 Catalyst recovery by micellar enhance ultrafiltration ................ 45

4.2 Ultrafiltration of surfactant organic solutions ...................................... 47

4.2.1 Influence of surfactant concentration ........................................ 47

4.2.2 Permeability recovery ............................................................... 52

4.3 Ultrafiltration of surfactant aqueous solutions .................................... 57

4.3.1 Influence of surfactant concentration ........................................ 57

4.3.2 Permeability decline ................................................................. 59

4.3.3 Permeability recovery ............................................................... 62

4.4 Application of models for flux data evaluation .................................... 66

4.4.1 Hermia’s blocking models ........................................................ 66

4.4.2 Zhu and Gu’s adsorption models .............................................. 67

5 Conclusion ............................................................................................... 74

6 References ................................................................................................ 78

III

Abbreviation

AOT Sodium diethylhexyl sulfosuccinate

CA Cellulose acetate

CPC Cetylpyridinium chloride

CTAB Hexadecyltrimethyl ammonium bromide

Igepal CA520 Pentaethyleneglycolmonoisononylphenylether

Marlipal O 13/60 Hexaethoxylene-iso-tridecanol

MOAT Sodium bis(2-ethylhexyl polyoxyethylene)sulfosuccinate

PA Polyamide

PAN Polyacrylonitrile

PES Polyethersulfone

PTFE Polytetrafluoroethylene

PS Polysulfone

PVDF Polyvinylidene difluoride

RC Regenerated cellulose

SDS Sodium dodecyl sulfate

TiO2 Titanium dioxide

TTAB Tetradecyltrimethyl ammonium bromide

ZrO2 Zirconium dioxide

IV

List of figures Fig. 1. A catalyst recycling concept for hydroformylation of a long chain olefin: Reverse MEUF. .................................................................................................. 2

Fig. 2. Schematic surfactant molecule. ............................................................... 5

Fig. 3. Schematic representation of micelles and reverse micelles as spherical aggregates in water and oil, respectively. .......................................................... 7

Fig. 4. Concentration dependent properties of surfactant solutions undergo an abrupt change over a narrow cmc-region (adapted from [1,60]). ........................ 8

Fig. 5. Effect of polyethylene-oxide chain length on solubility of surfactant in solvents [46]. .................................................................................................... 11

Fig. 6. Classification of pressure driven membrane processes [74]. ................ 13

Fig. 7. Schematic representation of dead-end and cross-flow as two basic operation modes of UF process [74]. ............................................................... 14

Fig. 8. Pore-scale model for flux prediction of porous UF-membranes [78]. ..... 15

Fig. 9. Separation principle of o/w- and w/o-MEUF. ......................................... 17

Fig. 10. Flux-reducing mechanisms: (a) Adsorption of surfactant molecules based on the two-step model proposed by Zhu and Gu; (b) Pore blocking of micelles at the membrane due to Hermia’s blocking filtration models [194]. ... 21

Fig. 11. Chemical structure of the used non-ionic Marlophen NPn (NP5 with n=5; NP9 with n=9). .......................................................................................... 28

Fig. 12. Chemical structure of the used Acetylacetonatodicarbonylrhodium Rhacac(CO)2. ................................................................................................... 28

Fig. 13. Chemical structure of the used ligand SulfoXantPhos (SX). ................ 29

Fig. 14. The set-up to prepare rhodium-ligand-complex. .................................. 29

Fig. 15. A contact angle determined by a sessile water drop on a membrane surface (a self-shot photograph). ...................................................................... 31

Fig. 16. Flow-sheet of the dead-end ultrafiltration test system (1: pressure gauge, 2: dead-end stirred cell, 3: permeate, 4: electronic balance) [182]. ...... 32

Fig. 17. Filtration steps of experimental series 1 [182]. .................................... 33

Fig. 18. Filtration steps of experimental series 2 [182]. .................................... 33

Fig. 19. Feed concentration of each step in the experimental series 1. ............ 34

Fig. 20. Procedures of the experimental series 2. ............................................ 35

V

Fig. 21. Cmc of NP9 in water is determined at 0.006% by surface tension measurement. .................................................................................................. 37

Fig. 22. Cmc determination of NP5 in 1-dodecene based on Gibbs’ rule [182]. 38

Fig. 23. Set-up of pressurized microwave to digest organic solvents as a preparation step for ICP OES analysis (from CEM GmbH [189]). .................... 41

Fig. 24. Negligible rhodium retentions by w/o-MEUF of a reverse micellar solution consisting of 9.0% NP5, 86.5% 1-dodecene and 4.5% water [182]. ... 44

Fig. 25. High micelle retentions (85% - 100%) by o/w-MEUF of microemulsion (5.5% NP9, 0.2% 1-dodecene and 94.3% water) using various membranes. .. 46

Fig. 26. High rhodium retention (91% - 93%) by o/w-MEUF using PES5. ........ 46

Fig. 27. Permeability development during the stepwise increase of surfactant concentration for the three membranes PTFE10, PAN10 and TiO2 [182]. ........ 49

Fig. 28. Relative permeability as a function of surfactant concentration [182]. . 50

Fig. 29. Interacting phenomena like adsorption, desorption at membrane surface as well as the micellisation and surface diffusion at a concentration (a) below; (b) at the threshold of and above the cmc [182]. ................................... 51

Fig. 30. Rinsing of PTFE10, PAN10 and TiO2 membrane after filtration with a surfactant concentration below the cmc [182]. ................................................. 53

Fig. 31. Rinsing of PAN10, TiO2 and PTFE10 membrane after filtration with a surfactant concentration above the cmc [182]. ................................................. 56

Fig. 32. Permeability, calculated from the initial flux, as a function of the surfactant concentration during filtrations of experimental series 1 with various membranes (cmc at 0.00274%) [194]. ............................................................. 58

Fig. 33. Comparison of flux behaviour between polar and non-polar solution in the experimental series 1: (A) NP5/H2O [194] und (B) NP5/1-dodecene [182]. 60

Fig. 34. Layer formation models with the orientation of head groups of non-ionic surfactant to aqueous medium under influence of: (a) hydrophilic membrane surface; (b) hydrophobic membrane surface (adapted from [163]). .................. 62

Fig. 35. Permeability decline (PD) from UF of surfactant solutions: (A) NP5/H2O (< cmc), (B) NP5/H2O (> cmc), (C) NP5/1-dodecene (< cmc) and (D) NP5/1-dodecene ( >cmc) and permeability recovery (PR) after rinsing with pure solvents [194]. .......................................................................................... 64

Fig. 36. Direct and inverse proportional relationship between PD and MWCO (6-500 kDa) in accordance to the type of applied surfactants and membrane

VI

materials (PS: Polysulphone; PVDF: Polyvinylidene fluoride; CA: Cellulose acetate), adapted from [39]. ............................................................................. 65

Fig. 37. Hermia’s four blocking models applied to the data obtained for the filtration of NP5/H2O with PAN10 at 1 bar, 20°C and 200 min-1 [194]. .............. 67

Fig. 38. Zhu and Gu’s adsorption model in combination with Hagen-Poiseuille’s law [194]. ….. ................................................................................................... 68

Fig. 39. Determination of n and K by using Eq. (40) based on the combination of the two-step adsorption model with S-shape proposed by Zhu and Gu and the PD from UF of surfactant solutions [194]: (A) NP5/H2O (< cmc), (B) NP5/H2O (≥ cmc), (C) NP5/1-dodecene (< cmc) and (D) NP5/1-dodecene (≥ cmc). 69

Fig. 40. Interactive phenomena as responsible effects on the negative filtration performance of reverse MEUF [194]. ............................................................... 77

INTRODUCTION

1

1 Introduction Surfactants have been known in various practical fields for their ability to self-associate into micelles [1], to alter physicochemical solution properties [2] and for their tendency to adsorb at interfaces [3,4]. Representative examples of the first property of surfactant for membrane process are micellar-enhanced ultrafiltration (MEUF) and reverse micellar-enhanced ultrafiltration (reverse MEUF). To understand the responsible mechanism for the technical infeasibility of reverse MEUF is the global goal of this study. The flux-reducing mechanism may be a cake layer formation, blocking in membrane pores or molecular interactions between surfactants and membrane surface etc. For example, in the surfactant-based hydroformylation of 1-dodecene, reverse MEUF seems to be an attractive concept to recycle catalysts from product stream as reverse micellar solutions. Owing to lack of literature on the separation efficiency of reverse MEUF this preliminary investigation is an essential step at the beginning of the process design. During ultrafiltration (UF) of micellar solutions, influence of surfactant adsorption on the change in flux and in micelle retention has been extensively investigated in several studies [5–12]. To evaluate the membrane performance from flux data, this study applies Hermia’s models and Zhu and Gu’s models under consideration of mechanical blocking mechanisms and surfactant adsorption at various UF-membranes, respectively. Due to their amphipathic structure, adding surfactants to a biphasic liquid system enables the aqueous and organic phases to form a single stable micellar medium. The ternary mixtures of water, oil and surfactant, called microemulsions, are thermodynamically stable, optically transparent and macroscopically homogeneous [13]. A microemulsion consists of a dispersed phase and a continuous phase. In many cases, water is the continuous phase and the microemulsion contains micelles as dispersed phase. Otherwise, in a non-polar organic continuous phase, the microemulsion contains reverse micelles. Basically, traditional UF is ineffective at separating any dissolved low molecular weight species having molecular weight less than 500 Daltons [14]. Using MEUF, such species can be solubilised into the interior of micelles or reverse micelles at surfactant concentrations above the critical micelle concentration (cmc). With respect to surfactants’ natures, interactions between surfactants and membrane surface are considered to a certain degree as a phenomenal background for the

INTRODUCTION

2

discussion on the permeability and micelle retention during MEUF. Based on the formation of micelles in water or in oil, MEUF can work as oil-in-water (o/w) MEUF or water-in-oil (w/o) MEUF. The former type has been shown as an established separation method [15–17]. By contrast, knowledge on the so-called reverse MEUF (w/o-MEUF) is limited to just a few studies on enzymatic reactions [18–21]. Reverse micelles attracts nowadays a great deal of attention in many application fields, ranging from synthesis of inorganic nanoparticles [22–24] and enzyme catalysis [25,26] to bioseparation [27,28]. With a particular interest in non-ionic reverse micelles the original target of the present study was to investigate their use in reverse MEUF for the recovery of catalysts from the oxo synthesis of 1-dodecene. Against the background of increased awareness and demand on sustainability by the German Catalysis Society [29], finding a way to integrate catalyst recycling into chemical processes is responding to this call [15,30–32]. Using catalysts in a chemical reaction allows higher reaction performances, expressed in terms of activity and selectivity. Demand on recovery and reuse of catalysts has increased over time due to their rarity and high expense. Therefore, combining reaction and separation makes chemical processes more sustainable and commercially feasible. Pursuing the same idea, a concept of a chemical process composed of three stages was designed as schematically shown in Figure 1 in the collaborative research SFB TR63 (Sonderforschungsbereich/Transregio 63): (1) Hydroformylation of 1-dodecene, (2) phase separation and (3) reverse MEUF.

Fig. 1. A catalyst recycling concept for hydroformylation of a long chain olefin: Reverse MEUF.

The initial focus of the work in hand was the filtration unit in coordination with other projects involving the reaction and the surfactant-based phase separation (cloud-point extraction). Since its discovery at Ruhrchemie AG in 1938 by Dr. Otto

INTRODUCTION

3

Roelen, oxo synthesis is a commercially important reaction with permanent growth [33]. Oxo products and their derivates are used for production of a large variety of chemicals, e.g. solvents, detergents, plasticizers, fragrance and intermediates for fine and specialty chemical industry [34]. Worldwide about six million tonnes of aldehydes are hydroformylated per annum [35]. However, this chemical process is limited to short chain olefins. Due to their sufficient solubility, short chain olefins are hydroformylated successfully in aqueous media without addition of surfactant or other phase transfer agents [34,36]. In presence of homogenous catalyst complexes, alkenes higher than hexene are hydroformylated under severe conditions [36]. According to the concept designed by SFB TR63, in the first process stage, 1-dodecene reacts in presence of a water-soluble rhodium complex with carbon monoxide and hydrogen to form tridecanal. In the second stage, non-polar organic mixture and water are separated by a surfactant-mediated phase separation. In the third stage, reverse MEUF was proposed as a promising process to recycle rhodium from the hydroformylation organic mixture. Working in reverse micellar phases allows that the reverse micelles dissolve molecules which are less soluble in oil than in water. In the study on hydroformylation of short and long chain olefins the authors Vyve et al. pointed out that reverse micelles containing catalytic complexes (Rh/TPPTS) in their interior have a diameter ranging from 10 to 100 nm [35]. Thereby, such solubilised rhodium complexes are purposed to be retained by ultrafiltration membranes with the molecular weight cut-off (MWCO) of 10 kDa or the nominal pore diameter of 5 nm. If the rhodium is completely solubilised in the reverse micelles, the ultrafiltration satisfies the described concept as an appropriate recycling tool. In the first part of the experiment, reverse MEUF was investigated under consideration of the hydroformylation of 1-dodecene in microemulsion. Therefore, some of non-ionic surfactants tested in the reaction [37], e.g. nonylphenol ethoxylates, were selected here to prepare feed solutions. To serve as a fundamental reference, o/w-MEUF is conducted using the same type of surfactant and the same dead-end test cell. An overview of the impact of the organic and aqueous solvent on the micelle formation, and thereby comparison between reverse MEUF and MEUF for the catalyst separation are carried out as aim of the first study part. Both MEUF types showed different results of retention. High rhodium recovery is reached by the traditional MEUF, but not by the reverse MEUF. There is an open question about a mechanism, which may be responsible for the inhabitation of the formation of reverse micelles

INTRODUCTION

4

in non-polar organic media or the solubilisation of rhodium complexes. Thereby, the catalysts pass through membranes during the reverse MEUF. Finding reasons for the lack of reverse MEUF becomes a great motivation for the second part of this study. For this purpose, solutions consisting of either surfactant/water or surfactant/oil are used for the filtration with various membranes made from polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), titanium dioxide (TiO2) and regenerated cellulose (RC). Adding surfactants stepwise to feed is an effective way to gain a fundamental understanding of concentration-depending interactions between membrane surface and surfactants or surfactant aggregates. In order to understand flux-reducing mechanisms, flux data are evaluated using two different models: (i) Hermia’s filtration blocking models; (ii) Adsorption isotherm models by Zhu and Gu. The former ones have been proven as a useful method applied in the micro- and ultrafiltration field since three decades [38–40]. To the author’s best knowledge, no prior work has applied Zhu and Gu’s model to ultrafiltration. According to Hermia, porous membranes can be blocked physically by deposition of organic solutes during filtration. Consequently, Hermia’s model affirms that a decline in the number of active pores causes flux reduction. With respect to surfactant physio-chemical properties, adsorption and surface aggregation occur according to Zhu and Gu, when an interface solid/liquid exists. The flux evaluation results from both theoretical models aimed to offer an overview of the surfactant influence on flux behaviour for various membranes in two concentration regions below and above cmc, as well as an approach to explain the technical infeasibility of reverse MEUF.

STATE OF THE ART

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2 State of the art 2.1 Surfactants and their classification Surfactants are organic versatile applications owing to their remarkable ability to self-associate in solutions, to influence significantly surface and interface properties as well as solubilise themselves in micelles [41]. As surface-active agents, surfactants have been used in numerous industrial productions, particularly where high surface areas, modification of interfacial activity or stability of colloidal systems is required such as processing of foods, agrochemicals, pharmaceuticals, lubricants, laundry detergents and additives [42]. Surfactants consist of a lipophilic and a hydrophilic part, called “tail” and “head”, respectively. A schematic representation of a surfactant molecule is shown in Fig. 2.

Fig. 2. Schematic surfactant molecule.

Lipophilic tails are usually hydrocarbon chain, polymeric short chain or siloxane chain. Hydrophilic heads bear an electrical charge or possess a polar group, in many cased, with oxygen atoms. Depending to the nature of hydrophilic part, surfactants are classified in three categories: ionic, zwitterionic and non-ionic surfactants [43]. Ionic surfactants possess a hydrophilic part on the rest of its lipophilic part.

In polar solvents both of them dissociate into two oppositely charged species: Surfactant ion and its counterion. According to its electrical charge ionic surfactant are subdivided into cationic and anionic surfactant. Anionics are characterized by their solubilisation, which can be attributed

to its counterion M+. Carboxylate (e.g. the so-called soap R−COO−M+), sulfate (e.g. R−SO3−M+) and phosphate (e.g. R–OPO3−M+) are the

hydrophilichead group

hydrophobictail

non-polar partpolar part

STATE OF THE ART

6

encountered anionic surfactants. Most of laundry detergents contain either anionics or mixtures of anionics and non-ionics.

Cationics have positively charged heads and include usually ammonium salts compounds (e.g. RxHyN+X− and R4N+X−). Relating to their adsorption at surfaces or surface modification cationic surfactants are applied as anticorrosion, flotation collectors, hair conditioners, fabric softeners and bactericides. These surfaces – e.g. fibres, plactics, hairs and cell membranes – are originally negatively charged.

Zwitterionics or amphoterics is a special case of surfactants. They exhibit both cationic and anionic parts in their molecular structure. That are synthetic products such as betaines or sulfobetaines and natural matters like amino acids and phospholipids. For instance, a long-chain amino acid RN+(CH3)2CH2CH2SO3− contains the both positively and negatively charged ions. Due to their surface activity in a neutral pH-range zwitterionics are usually used to increase specialised properties such as foam and detergency in personal care products (e.g. shower gels, foam baths, etc.) [44].

Non-ionics are for example polyoxyethylene alcohols, ethers and esters. They have a weaker hydrophilic effect than ionics, since their hydrophilic head carries no electrical charge. On the one hand, the head is indissociable into ions. On the other hand, it contains commonly hydrogen bonding, e.g. ethoxylates –(OCH2CH2)nOH. Therefore, non-ionic surfactants have a strong affinity for water because of its dipole-dipole interactions. One advantage over ionics is that the length of both hydrophilic and lipophilic part are variable. For instance, alkylphenol polyethoxylates (APE) are interesting products for studying the variation of properties with structure. Their hydrophilic-lipophilic balance (HLB) can be varied easily, either by changing the alkyl chain on the phenol or the ethylene oxide chain. From this point of view, APE represents approximately 10% of overall surfactant consumption [4]. Among those, nonylphenol ethoxylates (NPn, n denoting the number of ethoxy units in the molecule) make up around 80% [45]. For some applications, the value of n can reach even 50 [46]. In this work nonylphenol ethoxylates with n = 5 and 9, commercially known as Marlophen NP5 and NP9, are used. With focus on these surfactants, following sections are devoted to the important characteristics of non-ionic surfactants.

STATE OF THE ART

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2.2 Micelle formation In dilute solutions, surfactants are dissolved as unimeric species. When the concentration of these surfactant molecules exceeds a particular concentration known as critical micelle concentration (cmc), they associate to form micelles. Adding surfactant more in solution leads to increase in formed micelles, whereas the concentration of free surfactant, i.e. unassociated monomers, remains almost constant [47]. Above cmc, aggregation of surfactant molecules in aqueous media results formation of “normal” micelles or “micelles”. Hydrophilic surfactant head groups are organised in the shell of micelles towards water, while hydrophobic tails packed together in core try to minimise the contact with water by being in oil [48].

Contrary to micelles in water, the structure of micelles formed in non-aqueous solvents is absolutely inverse, i.e. head groups in polar core and hydrophobic tails in the shell. Fig. 3 illustrates the structure of micelles and reverse micelles schematically.

Fig. 3. Schematic representation of micelles and reverse micelles as spherical aggregates in water and oil, respectively.

Micelles are dynamic species where a constant interchange of molecules between the aggregates and solution pseudo-phases takes place on a microsecond scale [41]. The number of surfactant molecules in micelles (N) varies from 50 to 200 [41]. Contrary to polar media, the reverse micellar aggregates are in non-polar media smaller because of their lower aggregation number (between 7 and 30) [49]. The dipole-dipole interactions between the polar head groups are perhaps the reason. Generally, non-ionic surfactants have higher N than anionic and cationic surfactants. Moreover, non-ionic surfactants associate to form micelle at much lower concentrations than ionic surfactants [1]. The shape and size of micelles are governed by geometric and energetic aspects [2], e.g. the nature, the concentration of surfactant, the solvent and other species

Oil H2O

Oil H2O

STATE OF THE ART

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in solution. The formed micelles can be cylindrical, lamellar or spherical, while their radii vary in a range from 10 nm to over 100 nm [48]. Frequently used terms in connection with micellar systems are critical micelle concentration, aggregation number, micelle radius and Krafft point [50]. Increase in temperature leads to a continuous increase in the water solubility of solutes. This temperature dependence is also applied for surfactant molecules in water below cmc. At a particular temperature, the so-called Krafft temperature or cloud point, the solubility is increased radically. Krafft point is experimentally found out as an intersection of solubility and cmc curves [47]. Generally Krafft point is defined as the temperature where a surfactant solution reaches its cmc and the formed micelles produce a higher solubility [47]. For measuring micelle size and shape nuclear magnetic resonance [51–53], self-diffusion, fluorescence [54] and small-angle scattering [55,56] and light scattering have been known as powerful experimental techniques [1]. Some important manifestations of micelle formations as abrupt changes in physicochemical properties of surfactant solutions around cmc. A great number of studies have published on cmc determination for aqueous surfactant solutions by surface tension measurement [57–59]. Adding surfactants into solution leads to a continuous decline in surface tension. Above cm, even when micelles are increasingly formed, the surface tension remains constant. As shown in Fig. 4, cmc is the minimal concentration required to achieve the minimal surface tension. Other solution properties; e.g. solubility, undergo an abrupt change over a range around cmc.

Fig. 4. Concentration dependent properties of surfactant solutions undergo an abrupt change over a narrow cmc-region (adapted from [1,60]).

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In a homologous series, e.g. nonylphenol ethoxylates NPn, cmc increases gradually with increasing n, since hydrophilic heads become larger [1]. Decrease in cmc with longer alkyl chains leads to decrease in water solubility [61]. Cmc of technical-grade surfactants is usually given as a single value. In fact this value is purposed to represent a narrow range above which these physical properties of surfactant solution are changed abruptly [58]. A variety of techniques have been employed to determine cmc [62]. Research using tensiometry [58,63], capillary electrophoresis [64,65], calorimetry [66], conductometry [51,67] and spectroscopy [43,68] have been reported extensively. In particular, the two classic methods, surface tensiometry and conductometry, have been widely applied to analyse properties of aqueous micellar solutions. For non-polar organic solutions the both techniques are quite inapplicable [43]. The self-assembling in organic media takes place gradually. Compared to aqueous media, the change of physiochemical properties in organic media is not considered as a sudden-onset process [43]. Hence, it faces more difficulties in analysing precisely the critical concentration marking the occurrence of the reverse micelle formation. Reasonably, less systematic studies on reverse micellar systems have been done despite of their common applications [69]. For the non-ionic reverse micelles used in this study, there is no available data on cmc of the poly(oxyethylene)-5-nonylphenol ether in 1-dodecene. Thereby, an empirical visual observation using Gibbs’ phase triangle allows as an alternative method to determine the cmc, as outlined in detail in Section 3.6.2.

2.3 Thermodynamic models of micelle formation According to several literature, there are two common models describing the thermodynamics of micelle formation: (1) Phase separation model; (2) Mass action model [43]. In “phase separation” model, micelle formation is considered as a separated pseudo-phase and the cmc indicates the saturation of the amphiphile in unimeric state [41]. In thermodynamic equilibrium, the chemical potential of a surfactant in aqueous phase equals that in the micellar new phase at a certain critical concentration (cmc). Furthermore, this model is based on the assumption that the monomer activity stays constant above cmc. Consequently, the phase separation phenomenon can be approximately expressed by the free energy of micellisation per mole derived as follows:

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ln( )omG RT cmc (1)

where R is universal gas constant (8,314 J mol−1 K−1) and T is absolute temperature. Phase separation model is the simplest approach to analyse the variation of molecular properties with concentration [1,41]. The higher aggregation numbers (N ~100), the more accurate this model. The Eq. (1) points out, that molecular properties is a linear function of concentration above cmc. However, there is evidence that molecular properties changes gradually at cmc and the monomer activity does increase above cmc. Consequently, the micelle size and shape can be affected in some cases by free surfactant molecules above cmc [2]. The mass action model describes a more realistic picture of the change in monomer concentration above the cmc depending on total concentration. In case of non-ionic surfactants, mass action model assumes that one micelle (M) is formed by N monomers (D). It implies that the equilibrium between surfactant monomers and the micelles occurring in the solution can be written: ND M (2) with a corresponding equilibrium constant K, calculated by:

/ ( ) / ( )N Nm m L m LK a a c c (3)

whereas aL, am or cL, cm are the activity or concentration of monomers and micelles, respectively. Another advantage over phase separation model is that mass action model is also applicable for low aggregation numbers [2]. At high N, the both models deliver the same results in terms of the energetic balance of micelle formation [43].

2.4 Surfactant selection for chemical process Efficient use of surfactants requires a sound knowledge of basic physiochemical properties of the surfactants, e.g. their ability to solubilise solutes in themselves, their solubility in specific solvents, as well as their phase behaviours. Israelachvili et al. developed a fundamental theory of surfactant aggregation considering the shape and size of surfactants. The authors found that the mutual ratio of the size of hydrophobic and hydrophilic parts of the molecule affects the formation of micelles from amphiphilic molecules [70]. The statement is in good agreement with the study on solubility of nonylphenol ethoxylates NPn depending ethoxylation degree (n) at a constant temperature of 25°C [71]. The higher n, the

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higher the water solubility. Brix et al. realised that at the room temperature, low water-soluble surfactants prefer forming reverse micelles in non-polar solvents rather than high water-soluble surfactants do in polar solvents [71]. For instance, the nonylphenol NP (NPn with n = 0) possesses the hydrophobic part of the molecule which is apparently larger than the hydrophilic part. By contrast, with a very high ethoxylation degree, e.g., n = 100, NP100 appears more water-soluble and no micelle formation is seen [71]. Polyethylene-oxide chain, i.e. the head group of a surfactant, is globally hydrophilic although each ethoxylation unit possesses two methylene moieties.

Fig. 5. Effect of polyethylene-oxide chain length on solubility of surfactant in solvents [46].

Non-ionic surfactants have very low values of cmc and their micelles are larger than those of ionic surfactants. As an advantage over ionic surfactants, higher permeate flux can be obtained. Dunn et al. realised that solubilisation of solutes in non-ionic micelles is more efficient than in ionic micelles [72]. In the present study, non-ionic surfactants NP5 and NP9 (n = 5 and n = 9) are used for the MEUF experiments and membranes with a MWCO range of 5-30 kDa are chosen to ensure a high retention efficiency. In case of nonylphenol ethoxylates, their ethylene oxide chains are the most significant factor for the high solubilisation. NP5 is oil-soluble, while NP9 is a water-soluble surfactant. Fig. 5, a schematic representation obtained from Sasol’s product sheet, points out that the polyethylene oxide chain length has a significant impact on the change in surfactant properties such as the solubility. A range of NPn products with solubility ranging from water-insoluble to water-miscible can be obtained by changing the length of the hydrophilic polyethylene oxides. Furthermore, the broad selection platform works to the advantage of process integration. In the recent study on hydroformulation of 1-dodecene, non-ionic surfactants of the same type were utilised to compare their effect on reaction performance [37]. For example, NP9 exhibits higher reaction conversion and rate of yield than NP5, NP6 and NP7 under the same hydroformylation conditions. From the study of Brix et al. it follows that at room temperature and in hydrocarbon oils, oil soluble surfactants, e.g.

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NP5, can form reverse micelles more efficiently than water soluble surfactants, e.g. NP9 [71]. However, the micelle formation is an important factor for the retention efficiency of MEUF. Like the traditional UF, MEUF is carried out at room temperature. Temperature of reactions, e.g. 75°C for the hydroformylation [36], 50°C for the hydrogenation [73], are commonly higher than that of the process partner MEUF. Changing temperature leads to a significant change in the structure of microemulsion. Therefore, surfactant selection for chemical processes in multiphase micellar systems requires also a fundamental study on the surfactant phase behaviours. For the process design, the surfactant selection requires often a compromise, especially when surfactant types involving the best performance of the individual process stages are not the same one.

2.5 Ultrafiltration

2.5.1 Definition und classification Ultrafiltration is a pressure driven membrane process. Like every membrane process, ultrafiltration relies on the use of a selective barrier, i.e., membrane, to separate one or several particular species from a fluid stream [74]. A membrane can be either flat, or hollow fibrous in form, porous or dense in structure, as well as symmetric or asymmetric in geometry.

Classification of a membrane results from its specialised application on the one hand, or from diverse categories such as form, structure, geometry and manufacture, on the other hand. Fig. 6 shows the classification of various pressure driven membrane processes, e.g. filtration, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [74]. As can be seen here, the UF process is placed between MF and NF in regard to the range of applied pressure and of separation size. That means, UF requires a driving pressure ranging from 0.5 to 10 bar to concentrate or remove macromolecular solutes and colloidal particles [74] due to the average membrane pore diameter (2 to 100 nm) [75]. Consequently, UF-membranes are insufficient to reject ions, organic compounds with molecular weights lower than 500 Daltons [14] or molecular size smaller than 1 nm [76].

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Fig. 6. Classification of pressure driven membrane processes [74].

UF uses porous membranes whose structure is more asymmetric than those of MF membranes [75]. Such asymmetric membranes consist of two overlapping layers: a porous surface layer and a microporous substrate layer. While the surface layer, also called active layer or “skin” layer, plays a pivotal role in the separation performance, the substrate layer is responsible for the membrane’s mechanical stability. The surface layer can be considered as a permeable or semi-permeable thin film, made from various types of polymeric or inorganic material. Therefore, membrane producers typically provide information of the surface layer, e.g. material, molecular weight cut-off or average pore size, as membrane’s technical data.

2.5.2 Mode of operation A membrane acts like a selective barrier between two fluid phases. The upstream phase (feed) is a mixture of two or more components. The membrane permits the passage of certain components as downstream phase (permeate). At the same time, the other components are retained and concentrated on the feed side. The permeance across the membrane can occur by supplying energy, which provides a pressure difference p between feed and permeate side. The so-called

transmembrane pressure difference (TMP) can be considered as a force required to operate a membrane process. As mentioned in Section 2.5.1, TMP for UF process ranges from 0.5 to 10 bar.

Driv

en p

ress

ure

p[b

ar]

Particle or molecule size dP [µm]0.0001 0.001 0.01 0.1 1 10 100

0.1

10

100

1

200Reverseosmosis

MicrofiltrationFiltration

Ultra-filtration

Nano-filtration

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In regard of the main flow configuration, membrane processes are operated either on cross-flow or dead-end mode. In a cross-flow filtration the feed flows tangentially along the membrane surface, whereas concentrate (retentate) is removed from the same side. Thereby, the accumulation of solutes in the boundary zone adjacent to the membrane surface can be significantly reduced. Due to this advantage, cross-flow is an efficient operation mode with preference in industrial applications. In dead-end mode, the feed stream passes once orthogonally through the membrane. The equipment applied for dead-end filtration is easy in handling and requires less chemicals, especially when a membrane screening is conducted with expensive chemicals. Thus, dead-end mode is especially useful for laboratory scale. For that reason, it is chosen in the study in hand.

For the dead-end mode, TMP is defined as follows

F PTMP p p p (4)

where Fp is the applied pressure on the feed side and Pp is the back-pressure

on the permeate side. The value of Pp is zero if the permeate side is open to the

atmosphere.

Figure 7 represents schematically dead-end and crossflow as basic modes to operate membrane processes.

Fig. 7. Schematic representation of dead-end and cross-flow as two basic operation modes of UF process [74].

Permeate

Feed

Dead-end

Mem

bran

e

Permeate

Cross-flow

Feed

Mem

bran

e

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2.5.3 Pore scale model Various mathematical models have attempted with different success degrees to describe mechanisms of membrane transport [77,78]. The Hagen-Poiseuille law has proven as a very useful description models for a streamline flow through porous membranes [74].

Assuming that a porous membrane consists of straight cylindrical capillaries of equal size and the fluid flow are laminar, the Hagen-Poiseuille law uses numerous pore structure factors to predict the flow rate of the fluid permeating per unit membrane area J (L/m²h) as follows:

2

32capd p

JL

(5)

where p is the TMP (see Eq. (4)), is the dynamic viscosity of the permeating

fluid, is the membrane porosity, capd is the capillary radius and L is the capillary

length (the thickness of the ultrathin active layer).

The parameters given in the Hagen-Poiseuille represents schematically the pore-scale model (see Fig. 8).

Fig. 8. Pore-scale model for flux prediction of porous UF-membranes [78].

Feed

Permeate

L Active layer

Feed pressure pF

Permeat pressure pP

dcap

Mem

bran

e

Substrate

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It is notable, that the term 2

32Ld

characterises the specific membrane property. Its

value resembles the intrinsic membrane resistance Rm, if there is no fouling exists. Typically Rm is determined using pure water as feed.

Consequently, the Hagen-Poiseuille equation can be rewritten as

m

pJR

(6)

Under actual operating conditions, a certain fouling may occur due to interactions between membrane and solutes. Taking fouling (Rf) into account, the total membrane resistance becomes (Rm + Rf). Consequently, Eq. (6) can be rewritten as

( )m f

pJR R

(7)

At a constant TMP of 1 bar, J is defined as membrane permeability P ((L/(m²hbar)). The following equation describes the relation between J and P

JPp

(8)

Combining Eqs. (7) and (8), the membrane permeability can be expressed as follows

1( )m f

PR R

(9)

Besides permeability the efficiency of a UF process is referred to the retention coefficient, calculated as follows

F p

F

c cR

c (10)

where Fc is feed concentration and Pc is concentration of the separated

compound in permeate.

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2.6 Micellar-enhanced ultrafiltration Micellar-enhanced ultrafiltration (MEUF) is an expanded version of the ultrafiltration process, which is based on the use of micellar aggregation to separate metal ions or small dissolved compounds from liquid solutions.

Fig. 9. Separation principle of o/w- and w/o-MEUF.

Most surfactants have a molecular weight less than approximately 500 Daltons [79], and therefore they pass through UF-membranes. The same effect is true for other compounds in nanometer scale, e.g. catalyst complexes from process water, dyes from textile effluent and nutrients from municipal wastewater. RO and NF are proven to accomplish the separation of such small substances. There are some reports on the application of NF [80,81] or RO [82] in homogeneous catalysis. In the conventional treatment, surfactants are highly rejected from aqueous streams by NF [83] as well as RO [84]. However, these processes require higher transmembrane pressures than UF [85]. For this reason, MEUF has been increasingly considered as an economic membrane process for separation of low molecular weight compounds.

Besides MEUF, there are a variety of separation methods based on surfactants such as e.g. extraction, flotation, surfactant adsorption, fractionation, surfactant precipitation and microemulsion formation [14]. Scamehorn and Harwell reported current trends and progress in the field of “surfactant-based separation” [14].

H2O

UF-

Mem

bran

e

H2O

surfactant Oil

„normal“ micelle

Oil

conventional MEUF (o/w-MEUF)

reverse MEUF (w/o-MEUF)

reverse micelle

Oil

H2O

UF-

Mem

bran

e

H2O

Flux Flux

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Upon reaching the so-called critical micelle concentration (cmc), surfactant molecules can self-assemble and form micelles. Owing to the surfactant’s natures, the target compounds are solubilised in the hydrophobic interior of the “normal” micelles (o/w-micelles), or in the hydrophilic interior of the reverse micelles (w/o-micelles). In this way, micelles and their solutes can be separated by a UF-membrane at surfactant concentrations above cmc.

Depending on the continuous medium, separation of “normal” micelles from water and reverse micelles from oily solutions by UF-membranes corresponds o/w-MEUF and w/o-MEUF, respectively. Fig. 9 shows schematically the separation principle of both MEUF types.

Table 1 Overview of published studies on MEUF for treatment of wastewater and process water.

Separation Typical application

Further published studies Separated

compound Surfactant Membrane (MWCO) Ref.

Heavy metals from wastewater in mining, mineral and metal industry

Cooper, cadmium,

nickel, zinc and lead

Lecithin, SDS

PAN (3 kDa), PVDF (3 kDa, 5 kDa) [86] [87–116]

Catalysts from chemical reactions

Rhodium complex NP9 PES (5kDa), RC (5

kDa) [15] [73,117–119]

Inorganic compounds from

groundwater, municipal and

industrial wastewater

Nitrate, chromate and ferric cyanide

CPC RC (3 kDa, 10 kDa) [120] [121–131]

Organic compounds from

chemical, petroleum

wastewater

Phenol and phenolic derivates

SDS, CTAB RC (10 kDa) [132] [17,114,115,133–

143]

Dyes from textile wastewater

Methylene blue SDS PS (10 kDa) [16] [85,144–148]

To the best knowledge of the author, o/w-MEUF has been established as a promising technique on laboratory scale to separate heavy metal, catalysts, inorganic, organic compounds and dyes from wastewater, as well as to recover catalysts from chemical reactions. Possibilities and application areas of MEUF are constant expanding. Table 1 summarises various applications of MEUF in wastewater and process water treatment. Furthermore, a great number of

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published studies show that MEUF has received increasingly attention. In 1922, the earliest investigation on MEUF of soap solutions was done by Mc Bain et al. [149]. For the reported experiments (see Table 1) a variety of cationic, anionic and non-ionic surfactants were used to prepare aqueous micellar solutions. According to reported experiments, membranes with MWCO to maximal 55 kDa were efficiently employed for MEUF [112].

2.7 Reverse micelle enhanced ultrafiltration In comparison to o/w-MEUF, the knowledge on w/o-MEUF (reverse MEUF) is very rudimental until now. Most of the published studies on w/o-MEUF have focused on AOT [21,150,151] and TTAB [20] in some organic solvents, e.g. iso-octane, cyclohexane and a mixture of heptane/octanol. Systematic data on AOT, e.g. phase behaviour and micellar structure, are available in numerous literature [152–155]. Owing on natural properties of AOT, the water content of AOT-systems can be varied over a wide range. Thereby, large micelle sizes are easily achieved to gain then a high retention during the reverse MEUF. By far, the most widely studied reverse micellar system is that formed by the ionic surfactant AOT [153]. Reverse micellar solutions are mostly used in biotechnology because the “water pool” in the core can serve as a host for hydrophilic enzymes. In this way they act like a microreactor for substrates which are preferentially soluble in non-polar media [153]. Numerous research have proven the important role of electrostatic interactions in aqueous micellar solutions. Using AOT in dodecane Hsu et al. found that ionic reverse micelles carry electronic charges in non-polar solvent [156]. Electrostatic forces bring these reverse micelles and counterions close to each other. On the one hand, reverse micelles comprised of ionic surfactants can spontaneously ionize. On the other hand, they enable surfaces to charge by stabilisation their counterions. Following these arguments, difference between ionic and non-ionic reverse micelles in non-polar solvents may be the stabilisation of multivalent counterions and the resulting membrane separation for these metal ions.

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Table 2 Overview of past research on reverse MEUF.

Surfactant Reverse micellar system

Reaction combined with reverse MEUF

Membrane (MWCO) Retained compound Reference

AOT 1) AOT/isooctane/ water Enzymatic lipolisis ZrO2

(10 kDa)

Chromobacterium viscosum lipase B (30 kDa)

[157]

AOT 1) AOT/isooctane/ water

Enzymatic transesterification

ZrO2 (15 kDa) Cutinase (22 kDa) [21,158]

MAOT 2) MAO/isooctane/ water Enzymatic lipolisis PS (10 kDa) Candida rugosa

lipase (33 kDa) [159]

TTAB 3) TTAB/heptane+octanol/ water

Synthesis of dipeptides

ZrO2 (10 kDa)

-chymotrypsin (25 kDa) [20]

Igepal CA 520 4)

Igepal CA 520/cyclohexane/water

No reaction PA (10 kDa) surfactants, micelles [18]

Marlipal 13/60 5)

Marlipal 13-60/cyclohexane/ water

Enzymatic ketone reduction

PA (10 and 20 Kda)

Enzymes FDH 6) (76 kDa) and ADH 7) (140 kDa), cofactor NAD+/NADH 8) (702 Da)

[160]

Marlipal 13/60 5)

Marlipal 13-60/cyclohexane/ water

Esterification PA (10 kDa) Candida rugosa lipase (33 kDa) [19]

1) Sodium bis(2-ethylhexyl)sulfosuccinate

2) Sodium bis(2-ethylhexyl polyoxyethylene)sulfosuccinate

3) Tetradecyltrimethylammonium bromide

4) Pentaethyleneglycol-monoisononylphenylether

5) Hexaethoxylene-tridecanol (C13EO6)

6) Alcohol dehydrogenase

7) Formiat dehydrogenase

8) Nicotinadenindinucleotide

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In the studies on reverse MEUF mentioned above, surfactant retention of more than 90% was achieved. To implement a continuous enzymatic reaction with a simultaneous UF most of the authors applied ceramic membranes (MWCO 10-15 kDa) for ionic surfactants [20,21,150], while Orlich and Schomäcker used polymeric membranes (polyamide, MWCO 10-20 kDa) with the non-ionic surfactant Marlipal O 13/60 [19,160]. Without combination with reaction, Schomäcker et al. investigated the w/o-MEUF utilising the ionic surfactant AOT [151] as well as the non-ionic Igepal CA520 [18] using the same polyamide membrane. In relation to the hydroformylation of 1-dodecene, rhodium complex are proposed to be solubilised in the hydrophilic interior of reverse micelles, which are dispersed in non-polar mixtures of tridecanal and 1-dodecene. The following table gives an overview of published studies on reverse MEUF.

2.8 Models for flux data evaluation From the filtration data, a relation between the decline in permeability and the surfactant concentration is concluded. Data evaluation is done using the Zhu and Gu’s adsorption model and Hermia pore blocking models (see Fig. 10).

Fig. 10. Flux-reducing mechanisms: (a) Adsorption of surfactant molecules based on the two-step model proposed by Zhu and Gu; (b) Pore blocking of micelles at the membrane due to Hermia’s blocking filtration models [194].

By using these two models the present study aims for understanding thoroughly the difference between w/o- and o/w-MEUF as well as to explain the reason of infeasible retention of reverse micelles from a non-polar organic solution.

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2.8.1 Hermia’s blocking filtration models Filtration blocking law proposed by Hermia is a useful method to evaluate the flux data of porous membranes. This law can be divided in four different mechanisms. According to Hermia there is either a deposition of a new layer onto the membrane surface – a cake filtration – or a pore blocking during filtration of a colloidal dispersion by porous membranes [38]. The latter mechanism can be divided further in three models: standard or internal blocking; complete blocking and intermediate or partial blocking. Physical blocking by deposition of organic solutes leads to a decline in the number of active membrane pores, and consequently a reduction [38]. Based on these blocking laws evidence was presented for the blocking by particles at surface or in pore walls of membranes in many instances of protein filtration [161] as well as MEUF of heavy metal-contaminated water [100,162]. Beside the ability to self-assemble surfactants have a preference for adsorption at interfaces [163], particularly at interface between membrane surface and solution in this study case. Mathematically, all fouling types can be expressed by the following general equation developed by Hermia.

2

2

md t dtkdV dV

(11)

where t is the filtration time and V is the cumulative permeate. As two model parameters, k and m are the mass transport coefficient and the filtration constant, respectively. As summarized in Table 1 each fouling type is defined by a certain value of m (m = 0; 1; 3/2; 2) with its respective transport equation and its schematic principle. To identify prevailing mechanisms, Hermia’s blocking laws have been a helpful tool. For three decades, they have been applied widespread in MF and UF of suspensions containing colloidal particles or macromolecules such as latex paint [40], glycerine [164], bentonite [165], protein [161,166]. Furthermore, they have been a subject in MEUF of trace amounts of metal ions and organics from aqueous media [128].

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Table 3

Schematic diagram of the four fouling models proposed by Hermia [38].

Fouling model m Schematic principle Transport equation

(1) Cake filtration 0

(12)

(2) Standard pore blocking

1.5

(13)

(3) Complete pore blocking

2

(14)

(4) Intermediate pore blocking

1

(15)

1( )( ) 2 o

t k V tV t J

1( ) 2 o

t k tV t J

( ) ( )oQ V t V kV t ( ) ( )o( ) ( )o( ) ( )Q V t V kV t( ) ( )Q V t V kV t( ) ( )( ) ( )o( ) ( )Q V t V kV t( ) ( )o( ) ( )Q V t V kV t Q V t V kV t( ) ( )Q V t V kV t( ) ( ) ( ) ( )Q V t V kV t( ) ( )

1 1 1( ) o

ktQ V t V

(15)1 1 1

( ) o

ktV t V( )V t V( ) oV t Vo

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Two decisive factors for these fouling mechanisms are the dimensions (i.e., size and shape) of particles as well as the pore size distribution of the utilized membrane. If membrane pores and particles possess a similar size, the particles obstruct the pores or walls of the membrane intensively. Thereby, a reduction of membrane pore size and subsequently a flux decline is caused [167]. In the second part of this work, attempt was made to identify the predominant fouling mechanism by applying Hermia’s laws to surfactant solutions with regard to surfactant concentration. From experimental flux data the four transport equations, Eqs. (12) – (15) are derived and graphically presented (see Table 1). The more appropriately a model can form a straight line, the more reasonable is the assumed model. In this way, the prevailing mechanism can be identified, and its parameters k and m can be determined.

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2.8.2 Zhu and Gu’s adsorption models As surface-active agents surfactants have been well known for their ability to adsorb or locate at interfaces when dissolving in a solvent at low concentration. The term “interface” in the filtration field means the boundary between feed and membrane’s active layer. Hence, significant chain in physical properties of the surface are observable. For several decades, mechanisms of surfactant adsorption at a solid/aqueous interface in a closed system have been researched intensely [168,169]. Beside four-region model proposed by Somasundaran and Fuerstenau the adsorption model established by Zhu and Gu has been known as a useful method to explain common features of adsorption isotherms. These models are employed for surface excess measurements based on solution deletion experiments [170]. The general isotherm equation of Zhu and Gu’s model presents quantitatively characteristic adsorption parameters, as well as three typical isotherm shapes from the experimental data: Langmuir (L-type), Sigmoid (S-type) and “two plateaus” shape (LS-shape) [171–173]. Regarding to membrane filtration this study for the first time uses the Zhu and Gu adsorption model. Its target is to approach adsorption isotherm from flux data under consideration of surface aggregation of surfactants. According to Zhu and Gu, surfactant adsorption takes place generally at the liquid/solid interface in two steps. In the first step, surfactant monomers leave the bulk phase to be adsorbed as single molecules in the first layer on the solid surface (i.e., membrane surface) due to a specific interaction (e.g., van der Waals) between these monomers and the surface. Basically, the first part of the adsorption at equilibrium can be described by the following reaction with its equilibrium constant K1 [171]: Surface site + Monomer ↔ Adsorbed Monomer (16) The second step originates from the character of surfactants. The surfactant adsorption increases significantly as surface aggregates, so-called hemimicelle, are formed through attractive or hydrophobic interaction between molecules already adsorbed and those to be adsorbed. At equilibrium this can be described by the following equation: Adsorbed Monomer + (n-1) Monomers ↔ Hemimicelle (17) The reaction Eq. (17) has an equilibrium constant K2, and n is the hemimicelle aggregation number. According to the mass action law, Zhu and Gu summarised all types of adsorption of surfactant at liquid/solid interfaces in form of the general isotherm equation as follows [171]:

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11 2

11 2

1( )

1 (1 )

n

n

K Kn

K K (18)

where is the amount of adsorbed surfactant at the bulk surfactant

concentration and is the limiting value of adsorption at high concentration.

For the adsorption of non-ionic surfactants from aqueous solutions on a porous surface (i.e., silica gels) the adsorption isotherm exhibits a sigmoidal shape (S-shape) as shown by several studies [172,174,175]. Moreover, Zhu and co-workers found that their two-step model with S-shape is also applicable to non-polar organic solvent [176]. For instance, this statement is consistent with calorimetric data according to the investigation on adsorption of decan-1-ol from heptane at solution/graphite interface [177]. In the present study, we apply the two-step model with S-shape proposed by Zhu and Gu [172] to our filtration experiments. The reason for its selection is that the used materials are similar to the ones of the mentioned research on adsorption of non-ionic surfactant at liquid/solid interfaces. NP5 is also a non-ionic surfactant and all porous UF-membranes exhibit a nominal pore size in a comparable nanometre range due to their MWCO of 10 – 30 KDa. The controlled-pore glass (CPG) materials used in the study of Dietsch and co-workers had nominal pore size ranging from 7.5 to 50 nm [175]. For the adsorption isotherm showing an S-shape, the general isotherm equation becomes [172]:

1

n

n

KK

(19)

where K is the adsorption constant that is given by Eq. (20), is the surfactant concentration, K and n are the specific adsorption parameters.

1 2K K K (20)

where K1 is quite small (≈1) in the case of S-shape. By taking natural logarithm of Eq. (19), this isotherm equation can be transferred in a linear form as following to determine the adsorption parameters

ln / ( ) ln lnK n

(21)

MATERIAL AND METHOD

27

3 Material and method The aim of this chapter is to describe the material and methods applied to experiments and furthermore the evaluation of filtration data. As material all chemicals used to prepare feed solutions and analytical solvents will be introduced in Section 3.1. Furthermore, tested membranes are listed in Section 3.2 with their technical specification. Section 3.3 focuses on the dead-end stirred cells as the applied filtration set-up, while the procedure of both typical experimental series are presented in Section 3.4. Analytical method outlined in Section 3.5 – 3.8 allows the determination of critical micelle concentration as well as the measurement of surfactant concentration and rhodium concentration. Thereby, the calculation of retention of micelles, reverse micelles and catalyst is carried out to estimate the separation membrane performance.

3.1 Chemicals

3.1.1 Surfactants In this study, poly(oxyethylene)-5-nonylphenol ether (NP5) and poly(oxyethylene)-9-nonylphenol (NP9) were applied as received without any further purification. Both of them are technical grade non-ionic surfactants of the product line Marlophen produced by Sasol Germany GmbH. Beside Marlophen, they have other commercial names, according to producers, such as Igepal, Tergitol or Ultranex.

Related to the ethoxylation group (n), NP5 has a lower water solubility, but higher oil-solubility than NP9. Their physical properties are subsequently outlined as follows and their general chemical structures are illustrated in Fig. 11:

Marlophen NP5 has a molecular weight of 441 g/mol and a density of 1.03 g/mL and the viscosity at 20°C is 340 mPas [47].

Marlophen NP9 has a molecular weight of 621 g/mol [46] and a density of 1.06 g/mL [178] and the viscosity at 25°C is 243 mPas [47].

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n

OO

H

Fig. 11. Chemical structure of the used non-ionic Marlophen NPn (NP5 with n=5; NP9 with n=9).

3.1.2 Solvents All solutions used in the study are prepared by weight. All values in per cent refer to mass fraction or mass concentration. In all experiments, the used water is always deionized and chemicals are applied without any purification.

Besides deionized water, pure 1-dodecene in synthesis quality is used as non-polar solvent to prepare reverse micellar solutions or binary mixtures NP5/1-dodecene. This organic solvent was purchased from Merck Darmstadt KGaA. It has a density ρs of 0.76 g/mL, and a molecular weight of 168.33 g/mol. Its moisture was measured by a Karl Fischer titrator [179] and we found its water content to be 0.0048 wt%.

Dodecanal is used as the product of the hydroformulation reaction, instead of the expensive tridecanal. Like 1-dodecene, dodecanal is also purchased from the same company. Its density amounts to 0.84 g/mL.

Acetonitrile (HPLC grade) purchased from Carl Roth GmbH + Co. KG. is used as analytical solvents for the UV/VIS-measurement (see Section 3.7.2.).

3.1.3 Catalyst-ligand-complex Rhodium used for the experiment of catalyst recovery was originally in the state of precursor Acetylacetonatodicarbonylrhodium (Rhacac(CO)2) purchased from Strem Chemicals Inc., Germany. The chemical structure of Rhacac(CO)2 is shown in Fig. 12.

Fig. 12. Chemical structure of the used Acetylacetonatodicarbonylrhodium Rhacac(CO)2.

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SulfoXantPhos (SX) is chosen for the purpose of reaction selectivity [37]. This hydrophilic bidentate ligand is obtained from Molisa GmbH Magdeburg, Germany. Its chemical structure is represented in Fig. 13.

O

PPh2 PPh2

SO3NaNaO3S

Fig. 13. Chemical structure of the used ligand SulfoXantPhos (SX).

In the first stage of this study (Sections 4.1.1 and 4.1.2.), experiments dealing with recovery of rhodium are conducted with feed solutions prepared as follows. Adding the precursor in a SX-containing solution under oxygen free condition results in a complex Rh/SX. In these experiments Rhacac(CO)2 and Sulfo-Xantphos were mixed together in water at a molar ration of 1:5 as (rhodium : SX). This metal complex is very soluble in water solvent. This solution is stirred overnight in a flask supplied with nitrogen, as shown in Fig. 14.

Fig. 14. The set-up to prepare rhodium-ligand-complex.

To achieve desired rhodium-concentrations (100 ppm – 200 ppm) regarding to conditions of hydroformylation reaction, this solution is added to a surfactant aqueous solution prior to the phase separation or MEUF.

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3.1.4 Rhodium-containing reverse micellar solutions As mentioned above, rhodium complex is water soluble. Therefore, it is hardly dissolvable in non-polar solvents such as long chain olefins and aldehydes applied in the present study. In a recent study on hydroformylation of 1-dodecene, there is evidence, that oil phase leaving the phase separator exhibits low rhodium concentrations, approximately in ppm range [180].

A rhodium-complex stock solution, as described in detail in Section 3.1.3, is then added to an aqueous surfactant solution either for filtration combined with phase separator in batch-mode or for filtration alone.

All non-polar organic solutions for the investigation on catalyst recovery are prepared by the phase separator or decanter prior to filtration. For this purpose, a model mixture of 50% organic components (1-dodecene and dodecanal with a mass ratio of 1:1), 50% aqueous surfactant solution containing rhodium-complex was made. At a certain critical temperature, phase separation occurred and new oil mixture was then extracted. In this way, an organic non-polar mixture containing modified rhodium can be obtained for the following w/o-MEUF.

For experiments with o/w-MEUF, aqueous micellar solutions containing rhodium complexes were either prepared directly as model solution without requirement of phase separation or extracted as water phase from the decanter.

3.2 Membranes Flat, circular membranes consisting of polymeric sheet or ceramic material are used for the filtration experiments. Membranes such as PAN10, PTFE10, TiO2, RC10, RC30, PA10, PES10 and PES5 with regard to material of the active layer and its molecular weight cut-off (MWCO) are examined in this research. First, their surface hydrophilicity established by measuring the water-membrane contact angle. The functional principle of the measurement is based on the static sessile drop method [181]. The measurement is digitally conducted by Contact Angle System (model OCA 15 Plus), a product of DataPhysics Instrument GmbH. Prior to each contact angle measurement, the membranes is already rinsed with deionized water and subsequently dried. Fig. 15 shows a photograph of a water drop on a tested membrane as well as the contact angle analysis for example.

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Fig. 15. A contact angle determined by a sessile water drop on a membrane surface (a self-shot photograph).

Table 4 Specification of the membranes used in the main experiments.

Membrane Producer Abbreviation MWCO/ Pore size

Active layer Contact angle (°)

ETNA10PP Alfa Laval

Nordic A/S PTFE10 10 kDa Polytetrafluoroethylenea 63.7

- PolyAn

GmbH PAN10 10 kDa Polyacrylonitrile 42.7

Inopor®UF

TiO2

Frauenhofer

IKTS TiO2 5 nm Titanium dioxideb 30.3

UC030 Microdyn-

Nadir GmbH RC30 30 kDa Regenerated cellulose 16.9

RC70PP Alfa Laval

Nordic A/S RC10 10 kDa Regenerated cellulose 10.2

a On polypropylene b On -Al2O3

Table 4 lists the technical data including composition, MWCO (for polymeric membranes) or nominal pore diameter dp (for ceramic membrane), and the measured contact angle as well as the producer of these membranes. Here the hydrophilicity of the membranes is displayed in declining order from PTFE10 to RC10. PES5 is once used to exam the possibility of rhodium retention by MEUF (see Section 4.1.). Hence, the contact angle of PES5 is not measured. However, its value is supposed to be in the same range as PES10 due to the same surface material (polyether sulfone) produced by Microdyn-Nadir GmbH.

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The tested membranes with a MWCO of 10 kDa or average membrane pore diameter (dp) of 5 nm are used to ensure that the formed micelles or reverse micelles can be completely retained due the micelles’ slightly larger size of 6-8 nm [31,182]. Both hydrophilic membranes made of regenerated cellulose (RC10 and RC30) were utilised only for the filtration of aqueous surfactant solution due to their chemical instability against 1-dodecene.

3.3 Experimental set-up The experimental set-up described here is applied for the three experimental types investigated and outlined in this study (Sections 4.1. – 4.3.). All filtration experiments were conducted using a batch stirred cell purchased from Schleicher & Schuell (SC 75). The test cell has a volume capacity of 75 mL and an effective membrane area Amembrane of 1.39∙10-3 m². It consists of a cylinder made of borosilicate glass and two plates (top and bottom) made of stainless steel. A flat, circular membrane is fixed between the cylinder and the bottom plate. Viton o- rings are used to seal off the main parts. On the permeate side of the UF set-up, an electronic balance is used in connection to a computer to monitor online the permeability calculated by Eq. (22). The whole set-up is displayed schematically in Fig. 16.

Fig. 16. Flow-sheet of the dead-end ultrafiltration test system (1: pressure gauge, 2: dead-end stirred cell, 3: permeate, 4: electronic balance) [182].

All experiments were carried out at 20°C, a stirring speed of 200 min-1 and under a nitrogen pressure ∆p of 1 bar for aqueous solution and 3 bar for 1-dodecene solution.

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3.4 Procedure of filtration experiment The procedure consists of two sets of experiments to investigate the UF of organic and aqueous surfactant solutions. The corresponding solvent prepared for feeds is either pure 1-dodecene (for Section 4.2.) or deionized water (for Section 4.3.).

The first experimental series, which includes several filtration steps at different surfactant concentrations, is illustrated in Fig. 17.

Fig. 17. Filtration steps of experimental series 1 [182].

The second experimental series named “rinsing” (see Fig. 18), consist of only one filtration step with every solution with a surfactant fixed concentration either below or above the cmc.

Fig. 18. Filtration steps of experimental series 2 [182].

For every experimental series, only one membrane rinsed first by filtration of pure solvent was used. After this pre-filtration, the membrane stayed in the test cell for further filtrations of surfactant solutions until the end of the experiment.

Pre-filtration

Pure solvent

0 = 0

Filtration of surfactant solution

Surfactant solution

1

Surfactant solution

9

Surfactant addition

Experimental series 1: Increasing surfactant concentration

Filtration of surfactant solution

Surfactant addition

Surfactant addition

Post-filtration Filtration of surfactant solution

Surfactant addition

Pre-filtration

Pure solvent

Experimental series 2: Rinsing

Pure solvent

0 = 0

Surfactant solution

i < cmc or

i > cmc

Pure solvent

0 = 0

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In the experimental series 1, the surfactant concentration i of feed increases stepwise according to the schematic summary of Fig. 19. Its values cover in principle three ranges: (1) below cmc; (2) vicinity of cmc; (3) above cmc. The amount of surfactants bound to the membrane surface is negligible in comparison to the amount of surfactants in the bulk phase on the feed side. Therefore, the concentration of the feed is roughly constant during a fouling or adsorption process.

Fig. 19. Feed concentration of each step in the experimental series 1.

During each surfactant stage of both experimental series, the filtration is carried out at a constant transmembrane pressure difference Δp (1 bar for aqueous and 3 bar for organic solution, respectively) until the test cell is empty. stands for the density of the corresponding feed solution, i.e. either water or 1-dodecene.

During each filtration step, permeability (P) is recorded for the corresponding permeate mass Δmp over the time (t) and calculated using Eq. (22):

PTFE10

PAN10

TiO2

NP5-concentration

0

9 (%)

cmc

1

2

3

4

5

6

7

8

below cmc above cmc

PTFE10

PAN10

TiO2

RC30

RC10

0

9 (%)

cmc

1

2

3

4

5

6

7

8

below cmc above cmc NP5-concentration

Feed

N

P5/1

-dod

ecen

e Fe

ed

NP5

/H2O

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P

membrane

mPA t p

(22)

In Eq. (22) Amembrane is the membrane area, p is the applied pressure difference and is the density of the permeate. Also, its final steady state is noted to evaluate the relationship between surfactant concentration i and the corresponding permeability decline (PDi). PDi is defined by a decline in permeability (Ps,i) of a filtration stage i of a surfactant solution (i) in reference to the permeability of the pre-filtration (Pα) as follows (see Eq. (23)):

,s ii

P PPD

P (23)

Besides PDi, PRi can be used to describe the relative permeability regarding the value of pre-filtration (see Eq. (24)):

,s i

i

PRP

P (24)

In the second experimental series, the membrane which filtered already at a concentration either below or above cmc is treated by filtration of the corresponding pure solvent. Finally, the permeability of the post-filtration rises to be almost constant (Pω). Correspondingly, permeability recovery (PRi) describing the ratio of solvent permeability prior and after the filtration of surfactant solutions is determined by Eq. (25):

iPPRP

(25)

Fig. 20 summarises the procedures of the rinsing experiment (experimental series 2) schematically.

Fig. 20. Procedures of the experimental series 2.

0

(%)

i

Pure solvent Surfactant solution Pure solvent Feed

Feed concentration

Permeability P P

P

i

below cmc above cmc

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The surfactant concentration in permeate p is determined as an average over the whole filtration step. The rejection of surfactant RS and that of micelles RM are calculated as follows:

1 PS

F

R

(26)

1 PM

F

cmcRcmc

(27)

In the case of low critical micelle concentrations, which is usually valid for non-ionic surfactants, RS and RM differ only slightly.

3.5 Measurement of micelle and reverse micelle size

The size of a micelle is mainly dependent on the length of the hydrophobic chain. Therefore, it is expected for the same aqueous solution that the micelles of Marlophen NP5 have the comparable size as the micelles of NP9, which was determined to be 14 nm [189].

Reverse micelle size was measured by dynamic light scattering with a 2W Polytec laser and a Malvern Particle Sizer 4600. The data evaluation based on the cumulative method resulted in a reverse micelle size of 6-8 nm [182].

3.6 Determination of critical micelle concentration

In general technical grade surfactants comprise a series of pure homologous surfactants, apart from impurities. In contrast to pure surfactants, published experimental data of mixed surfactant solutions do not show a sharp break e.g. in a surface tension vs. logarithms concentration plot [57,185,186]. In other words the determined cmc value corresponds to a range, but not exactly a single value. This is also the case for the technical grade surfactants utilised in the present work.

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3.6.1 Marlophen in aqueous solution Critical micelle concentration of NP5 in water

Due its high hydrophobicity the cmc of NP5 in water amount to roughly 0.0028% which is much smaller than the one of another Marlophen, e.g. NP9, with higher ethoxylation degrees. There is no available product information on this physical property of NP5. Thus, its cmc is calculated averagely from the value confirmed in literature [187] and the one interpolated from a variety of well-described nonylphenol ethoxylates produced by Dow [178].

Critical micelle concentration of NP9 in water

In case of aqueous solvent, NP9 can be determined by a tensiometer system DCAT11 produced by DataPhysics Instrument GmbH. The solution is added stepwise with the surfactant and is measured for surface tension at each surfactant concentration at 20°C. The plot between the measured values and the concentrations contains two straight lines, whose turning point is the determined cmc (see Fig. 21). With 0.006% the founded value is accordance with the cmc values given in a technical data sheet of DOW [178].

Fig. 21. Cmc of NP9 in water is determined at 0.006% by surface tension measurement.

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3.6.2 Marlophen in organic solution Critical micelle concentration of NP5 in 1-dodecene

The cmc value of Marlophen NP5 in 1-dodecene (the continuous phase) investigated here is not available in the literature. Hence, it was measured using the turbidity method at 20°C.

Binary mixtures of 1-dodecene and surfactant with different surfactant concentrations were prepared. Subsequently, small quantities of water were added dropwise to the binary mixtures until these clear solutions turned turbid. Consequently, the cloud points, i.e. the concentration at which a single-phase (1) solution becomes a two-phase (2) solution, were plotted in a three-phase triangle according to Gibb’s rule [188]. Connecting all cloud points results in a line known as the turbidity line. The intersection of the turbidity line and the triangle edge of 1-dodecene/Marlophen NP5 yielded a cmc value of 0.83%. In Fig. 22, this cmc determination is illustrated schematically for Marlophen NP5 in 1-dodecene using Gibbs’ rule at 20°C.

Fig. 22. Cmc determination of NP5 in 1-dodecene based on Gibbs’ rule [182].

The formation of reverse microemulsions depends upon the composition of the surfactant, oil and water mixture. All ternary feed solutions used in this study were monophasic oil-rich microemulsions. Their compositions were selected according

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to chemical reaction mixture from the hydroformylation of 1-dodecene in a microemulsion [180]. The filtration point of the investigated reverse microemulsion is also plotted in the Gibbs’ phase triangle (see Fig. 22).

3.7 Surfactant concentration measurement The surfactant concentration is generally given as follows:

surfactant

surfactant oil water

mm m m

(28)

In Eq. (28), msurfactant, moil, and mwater are the mass of surfactant, oil and water, respectively.

UV/VIS-Spectrophotometer (PerkinElmer LAS GmbH, Germany) and 2 mm quartz cuvettes (Type-No. 100-QS, Hellma GmbH & Co. KG) are applied to measure the surfactant concentration for the both solvents. In Section 3.7.1 determination of NP5- and NP9 in water is presented, respectively. Section 3.7.2 shows the analysis of NP5 in 1-dodecene.

3.7.1 Marlophen in aqueous solution Concentration of NP5 in water The concentration of NP5 in water is measured at a wave length of 277 nm, where the benzene ring of the Marlophen adsorbs most strongly. As reference solvent water is used.

The calibration equation Eq. 29 shows a linear relationship between concentration of NP5 (in wt%) and light absorption A (in AU):

NP5 = 0.15∙A – 0.0007 (29)

It is noticeable, that the detection limit with this calibration is at a concentration of 0.0042%.

Concentration of NP9 in water Using the same analysis method for NP5, the Eq. (30) is applied as calibration equation to determine the concentration of NP9 (in wt%) in water.

NP9 = 0.20∙A – 0.0085 (30)

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3.7.2 Marlophen in organic solution Concentration of NP5 in 1-dodecene

In the case of UF of organic solutions (see Section 4.2) binary mixtures consisting of NP5 and 1-dodecene with varied surfactant concentration are used. Up to a surfactant concentration of 5%, permeate samples could be analysed without prior dilution. In such cases, the samples were analysed at 242 nm using 1-dodecene as the reference solvent. For concentrations above 5%, acetonitrile was used to dilute the samples and as the blank, and the measurement was carried out at a wave length of 277 nm.

The two resulting calibration equations showed a linear relationship (R² = 99.9%) between the concentration (in wt%) and the light absorption A (in AU) as follows:

NP5 = 2.217∙A, for NP5 < 5% (31)

NP5 = 0.170∙A, for NP5 ≥ 5% (32)

3.8 Rhodium concentration measurement Using inductively coupled plasma optical emission spectrometry (abbr. ICP-OES), samples can be analysed for its metal concentration. The measurement device is Varian 715-ES purchased from Varian Deutschland GmbH. For the ICP-OES measurement, we apply in this study a wave length of 369.236 nm to detect rhodium and distilled water as reference solution. The concentration of rhodium cRh (in ppm or mg/L) is determined by following calibration equation Eq. (33)

cRh = 0.0027∙A – 0.0935 (33) If the samples are aqueous, they are pre-treated to be acidic. Without digestion the sample of 5 g or more is ready for the rhodium measurement. The measurement accuracy amounts from 0.14 – 12.68% according to dilution factor.

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Fig. 23. Set-up of pressurized microwave to digest organic solvents as a preparation step for ICP OES analysis (from CEM GmbH [189]). Organic samples are not analysable by ICP-OES. Hence, they are pre-treated by digestion. In an amount of 0.1 – 0.25 g they are mixed in a 35 mL quartz vessel with an acid mixture. The mixture consists of 2 mL HNO3 and 3 mL H2SO4. After sealing with snap-on cap the vessel is inserted in a pressurized microwave (AD-1058). Using the program Discover SP-D installed on a computer the measurement can be conducted. The whole instrument is purchased from the company CEM GmbH, Germany. Its set-up is principally illustrated in Fig. 23. The digestion time (including cooling time at the end) takes about 30 min. After the digestion 1.0 mL distilled water and 0.8 mL H2O2 are carefully added to the sample. The mixture are well shaken repeatedly. To avoid existing decomposition products of H2O2 the mixture stands under a fume hood for many hours, before it is inserted into ICP-OES. In this case, the reference solution used for ICP-OES is mixture consisting of the digestion mixture and distilled water.

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4 Results and discussion Catalyst recovery by reverse MEUF of non-polar phase from the hydroformylation of 1-dodecene is the focus of the first stage of this research. Section 4.1 describes the result of catalyst separation by reverse MEUF (see Section 4.1.1) and by MEUF (see Section 4.1.2). To understand fundamentally different effects on retention efficiency according to the continuous phase of feed solutions, ultrafiltration of organic and aqueous binary mixtures (NP5/1-dodecene and NP5/H2O) at increasing surfactant concentrations is carried out with the same membrane types. For this purpose, section 4.2 reports on UF of NP5 from 1-dodecene, while section 4.3 involves the UF of NP5 from water. Finally flux behaviours are systematically compared for the non-polar and polar solvents. Next, mathematical models are applied in the second stage of this research to fit the data of permeability, i.e. flux. Section 4.4.1 reports on Hermia’s models which consider four different physical blocking mechanisms caused by particles or colloids. Section 4.4.2 deals with Zhu and Gu’s models, are based on the adsorption and surface aggregation as surfactant physicochemical properties at the interface between membrane and surfactant solution.

4.1 Catalyst recovery by micellar enhanced ultrafiltration

Precious catalysts such as rhodium, palladium and platinum are used with growing considerable attention for catalysed reactions. Micellar solutions, formed at surfactant concentrations above the critical micelle concentration (cmc), are a new class of reaction media which can be used in catalytic reactions [190–192]. The micellar structures can be used, e.g. to trap a homogenous catalyst complex and separate it for recovery. Because of the size of the micelles in the range of 1-10 nm, membrane filtration is a useful separation tool as shown in [15]. Some studies have been extensively investigated on MEUF for catalyst separation from aqueous streams for the last few years [15,73,117–119]. Most of the reports use o/w-MEUF in combination with a chemical reaction catalysed by such precious metals. Application of reverse MEUF (w/o-MEUF) is frequently found in biotechnology as for retention of enzymes and proteins [18–21,25,150,151,157–160]. From the process engineering point of view, reverse MEUF of hydrophilic catalysts works on the same principle as the selective separation and purification

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that also use reverse micelles in downstream processing of biotechnological products. In this traditional application area, reverse micelles contain microscopic water pools in their interior, where proteins or hydrophilic molecules can be solubilised while organic reactants and products remain in the bulk organic phase [27].

To expand the application of reverse MEUF toward chemical processes, the first stage of this research involves rhodium retention by reverse MEUF which is conceptually combined with the hydroformylation of 1-dodecene. Using the same chemical compounds and the same set-up, o/w-MEUF is also investigated to obtain a comparison with w/o-MEUF for the retention efficiency.

4.1.1 Catalyst recovery by reverse micellar enhance ultrafiltration

Some batch experiments are at first conducted to investigate the feasibility of w/o-MEUF. For this purpose, organic solutions containing rhodium complex were obtained from the upper phase of the phase separator. The reverse MEUF of these solutions are carried out with ceramic and polymeric membranes. As the third process stage of hydroformulation, the reverse MEUF was conceptually proposed to recover rhodium from the upper phase leaving the second process stage, i.e. the phase separation. All these experiments show an unsuccessful rh-retention by reverse MEUF. Notably, membranes made from cellulose are proven to be chemically unstable to the organic mixtures of the reaction, i.e. tridecanal, dodecanal. Due to its minor retention efficiency w/o-MEUF is a different from o/w-MEUF, although both membrane processes work on the micelle formation as the basic principle. Furthermore, the negative results of reverse MEUF seem to be not in agreement with reported studies on w/o-MEUF [18,21,150,151,160]. For this reason, fundamental experiments with ternary mixtures NP5/1-dodecene/water are carried out. The feed solution used for the reverse MEUF is a microemulsion consisting of 9.0% NP5, 86.5% 1-dodecene and 4.5% water. In the organic medium reverse micelles are formed, so that water molecules are embedded in their interior. The hydrophilic rhodium complex is easily solubilised in these water droplets.

Fig. 24 illustrates the retention and permeability during the reverse MEUF using the three polymeric membranes PAN10, PES10 and PA10. For every filtration time corresponding to a permeate volume of 17 mL (approximately 34% of the

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initial feed volume) a permeate sample is collected to measure the retention efficiency.

Fig. 24. Negligible rhodium retentions by w/o-MEUF of a reverse micellar solution consisting of 9.0% NP5, 86.5% 1-dodecene and 4.5% water [182].

Both PAN10 and PES10 yield a slight and steady permeability decrease over time, while the PA shows a constant but significantly lower value. In comparison, the PES10 exhibits a sharper decrease in flux due to the interaction between the PES material and the hydrophilic heads of the surfactant. None of the tested membranes shows here a feasible permeability for a technical application due to their negligible retentions (0-7%). The result is in not in agreement with the functional principle of micellar-enhanced ultrafiltration, although the formed reverse micelles are larger than the membrane pore size. To date there are several investigations of UF of w/o-micellar solutions, which typically achieve a high micelle retention [18–21,150,151,160]. Already these experiments give evidence of the technical infeasibility of w/o-MEUF using technical non-ionic surfactant systems in a long-chain oil (the continuous phase).

None of the published investigations on ternary systems using ionic surfactants such as AOT [21,150,151] and TTAB [20], as well as non-ionic surfactants for example Marlipal O 13/60 [19,160] and Igepal CA520 [18] are able to be transferred to the ternary system 1-dodecene/water/NP5 used in this present study. Short-chain hydrocarbons like iso-octane [21,150,151], cyclohexane [18,19,160] or mixtures of heptane/octanol [20] were used as the continuous phase of the microemulsions in these studies.

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As a reason for the differing results compared to the mentioned studies, 1-dodecene, the long-chain hydrocarbon, may be responsible for the unexpectedly low retention of reverse micelles. For the same surfactant, the miscibility gap of a binary mixture containing a hydrophobic oil exhibits a higher upper critical point compared to a binary mixture containing a less hydrophobic oil [193]. Due to the significant repulsive interaction between Marlophen NP5 and 1-dodecene, the surfactant molecules approach the membrane surface.

For a subtle understanding of these issues and to develop a full understanding of a potential adsorption effect, further investigations of the permeability are required. Thus, in the following section binary solutions NP5/1-dodecene with increasing surfactant concentrations are investigated instead of ternary mixture in a detailed flux analysis.

4.1.2 Catalyst recovery by micellar enhance ultrafiltration In this section, the separation performance of MEUF of ternary micellar solution NP5/1-dodecene/water is demonstrated using following various polymeric membranes PAN10, PES10, PES5 and RC5. Above cmc, in this case at = 5.5%, surfactant molecules aggregate to micelles in water. In the o/w-microemulsion, oil droplets with a concentration of 0.2% are solubilised into the hydrophobic core of the micelles. As result Fig. 25 shows high micelle retentions in a range of between 85% and 100% for all membranes. Among these, PAN10 exhibits the best separation performance. This membrane shows a high micelle retention RM = 98% and the highest permeability P = 17 L/(m²∙h∙bar). On the contrary, PES10 has the worst filtration performance (P = 3 L/(m²∙h∙bar) and RM = 85%). Due to their lower MWCO, both PES5 and RC5 reach the highest retention (100%) with a permeability range between 3 and 4 L/(m²∙h∙bar). Having a high surface hydrophilicity, RC5 exhibits a very low permeability. These results indicate that the pore size effect for NP9 has a greater influence on the filtration performance than the effect of surface hydrophilicity. The high micelle retentions indicates that o/w-MEUF is a useful separation tool which is also applicable to retain rhodium from aqueous streams. Moreover, the concept of MEUF process has been efficiently used in a variety of areas such as metal finishing operations [86,87] , textile industry [147,148] or groundwater treatment [123,129]. All these studies give evidence that small solutes dissolved into micelle core are successfully rejected by MEUF.

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Fig. 25. High micelle retentions (85% - 100%) by o/w-MEUF of microemulsion (5.5% NP9, 0.2% 1-dodecene and 94.3% water) using various membranes.

Further o/w-MEUF-experiments involving two aqueous micellar solutions (A and B) are conducted (see Fig. 26). The feed A containing 112 ppm rhodium and 8% NP9 is prepared as a model solution. The feed B containing 161 ppm rhodium is originally the surfactant-rich lower phase extracted from a model hydroformulation mixture by a decanter [30]. The model mixture were prepared for the phase separation with respect to the typical composition of the hydroformylation (23% 1-dodecene, 23% dodecanal, 46% water and 8% NP9).

Fig. 26. High rhodium retention (91% - 93%) by o/w-MEUF using PES5.

For both feed solutions, o/w-MEUF shows a high retention efficiency (91% for feed A and 93% for feed B). It indicates that micelles are formed with a large size,

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47

so that the solubilised rhodium are successfully retained by PES5. Both experiments have permeability in the same range after more than 2.5 hours. In contrast to the feed A, the feed B has oil-droplets as dispersed phase. From the high achieved retention follows, that the hydrophilic rhodium complexes are well attached in the micelle, especially in the shell where head groups are located. That implies that they are not preferably embedded in the oily core of micelles. Due to this effect, the retention of rhodium for feed B is higher than feed A.

4.2 Ultrafiltration of surfactant organic solutions To identify the responsible phenomenon for this failure, the flux behaviour during UF of binary mixture consisting of NP5 and 1-dodecene is investigated in section 4.2. This section contains results under two experimental series: first, stepwise increase in surfactant concentration (see Section 4.2.1.); second, permeability recovery by rinsing with 1-dodecene as the same solvent used for preparation of feed solutions in this case (see Section 4.2.2.). Subsequently, to understand the fundamentals of the underlying physical phenomena during w/o-MEUF, as well as the difference in interactions between reverse micelles-membrane and normal micelle-membrane, the experiments are conducted with water instead of 1-dodecene.

4.2.1 Influence of surfactant concentration In this part of the investigation, the three commercially available and chemically stable membranes PTFE10, PAN10 and TiO2 are used for the experimental series 1. Their change of permeability with increasing concentration of Marlophen NP5 is illustrated in Fig. 27.

In the pre-filtration with pure 1-dodecene, the permeability always shows a maximum for all tested membranes. The final values for PAN10, TiO2 and PTFE10 are 120, 70 and 35 L/(m²∙h∙bar), respectively.

A sharp decrease in permeability occurs as soon as the membrane came into contact with the surfactant. The more surfactant is added to the feed, the lower the permeability value becomes. Its minimal value is approximately between 8 and 10 L/(m²∙h∙bar) for all membranes.

Contrary to the uncharacteristic change in flux of PTFE10 which will be discussed later, the flux of PAN10 and TiO2 tends to change in similar ways due to their hydrophilic surface property. Their permeability behaviour can be divided into three distinct sections: below, around and above the cmc. Hence, in the following

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48

the flux behaviour of these two membranes is jointly described and discussed. Their permeability reaches minimal values in the proximity of the cmc ( = 0.81% and 0.98% for PAN10 and TiO2, respectively). It can be clearly seen that their flux is reduced drastically by increasing the NP5 concentration, as long as the cmc is not exceeded. It is highly likely that the drastic reduction of permeability depends on the interaction between the membrane surface and the surfactant molecules. These may adhere preferentially to the membrane and caused pore blockages due to van der Waals and/or hydrophilic interactions between the surfactant and the surface. The decrease in permeability with increasing surfactant concentration gives evidence of the adsorption.

As soon as the surfactant concentration exceeds the cmc, however, the permeability increased progressively with the stepwise increased surfactant concentration. In the third region, well above the cmc, the observed increase in flux for PAN10 increases more strongly compared to TiO2. For example, at = 2.97% the permeability of PAN10 reaches a final value of approximately 70 L/(m²∙h∙bar), while TiO2 attained a minimum of 8 L/(m²∙h∙bar) at = 2.92%. With increased surfactant concentration above the cmc, surfactants tends to aggregate as reverse micelles. At this moment, two competitive phenomena take place simultaneously: The formation of reserve micelles and the adsorption of surfactants onto the membrane surface. Thus, with an increase of surfactant concentration, the association in the interfacial region of reverse micelles may induce a removal of adhered molecules from the membrane surface. Because parts of these surfactants moved towards the bulk phase of the feed solution, the permeate flux rose rapidly. A possible reason for the negligible retentions might be that desorbed molecules diffuse through membrane capillaries.

Despite a further increase of surfactant concentration, e.g., up to a concentration of 10.3% for TiO2, the flux cannot be maintained completely. A certain degree of attractive interaction between the membrane material and surfactants was observed. For that reason, surfactant molecules adhere to the membrane surface and then pass partially through the membrane capillaries. The minimal retention determined as a very low value of 12.9% involves PAN10 at = 1.52%, i.e. in the proximity of the cmc. In the cmc range ( = 0.98%), TiO2 exhibits a slightly higher but still negligible retention of 21%.

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Fig. 27. Permeability development during the stepwise increase of surfactant concentration for the three membranes PTFE10, PAN10 and TiO2 [182].

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50

The impact of surfactants on the permeability is also recognizable in the experiment with PTFE10, which follows, however, another pathway. In comparison to PAN10 and TiO2, PTFE10 shows more unspecific flux behaviour when increasing the surfactant concentration. At most of the concentrations ( = 0.14%; 0.32% and 2.94%-9.62%) its permeability exhibits a final value of approximately 10 L/(m²∙h∙bar), while the value at the other concentrations = 0.5%-2.03% (around the cmc) ranges between 15-25 L/(m²∙h∙bar). For the PTFE10, the permeability behaviour shows no steady state at each concentration. After every surfactant addition, the flux initially rises. That can partly be attributed to the fact that for each surfactant addition the cell had to be opened. Thus, the pressure is released which relaxes the membrane. This phenomenon is specifically observed for PTFE10. On rinsing with pure 1-dodecene, its permeability recovers substantially but do not remain constant, while the rate of recovery for TiO2 amounts approximately to 50%. Reasons for the uncharacteristic behaviour of PTFE10 compared to the others are its hydrophobic membrane surface and the corresponding specific interaction between the surface and NP5.

Fig. 28. Relative permeability as a function of surfactant concentration [182].

To summarize the above experiments, Fig. 28 shows the relative steady state permeability – or minimal values in cases where a steady state is unreachable – as a function of the feed surfactant concentration. As can be seen, in all cases the behaviour changes at concentrations around the cmc. The hypothesis about

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51

äthe interacting phenomena leading to this observation and to the insufficient membrane performance is summarized schematically in Fig. 29.

At concentrations above the cmc the permeability is in the lower range than below cmc. It is respected that the flux decrease relates ordinarily to a higher retention. However, the retention of reverse micelles is negligible above cmc indeed.

Fig. 29. Interacting phenomena like adsorption, desorption at membrane surface as well as the micellisation and surface diffusion at a concentration (a) below; (b) at the threshold of and above the cmc [182].

At < cmc, the surfactant adsorption reduces the flux considerably. The permeability decline here is much stronger than it is at ≥ cmc. Hence, the adsorption below cmc is much stronger for all membranes in organic solutions than above cmc. Moreover, small surfactant molecules pass through membrane capillaries below cmc despite of adsorbed amounts at the membrane surface. There has been no evidence on the possibility of reversibility of the adsorption mechanism. At ≥ cmc, desorption plays a major role due to the formation of reverse micelles, especially for PAN10. It is observed that the permeability of PAN10 recovers above cmc with increasing surfactant concentration. However, a membrane diffusion of surfactants takes place to some extent while reverse micelles are not being retained. However, the result gives us no further information about the reversibility of surfactant adsorption, which still occurs to a certain degree at feed concentrations above the cmc. Another question remains

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open: Is the flux behaviour reproducible when the surfactant concentration of the binary solution starts with a value above the cmc? To answer these questions, rinsing experiments are done and desribed in Section 4.2.2.

4.2.2 Permeability recovery To answer the questions above, rinsing experiments are performed with the PTFE10, PAN10 and TiO2 membranes. Prior to every rinsing step, the filtration of the binary mixture (surfactant/1-dodecene) is operated according to the procedure of the experimental series 2 (see Fig. 17). The following filtration steps are carried out for every membrane: first, a pre-filtration with pure 1-dodecene; second, surfactant is added to 1-dodecene to filtrate a binary mixture (NP5/1-dodecene); third, the membrane is rinsed with pure 1-dodecene again. The experiment is divided into two experiment types with a feed concentration either below or above the cmc. The aim of the rinsing experiments is to observe the recovery of the permeability, as well as to estimate the intensity of the surfactant adsorption.

Rinsing experiment after UF at a concentration below cmc

A sharp decline in permeability is observed for every membrane, especially when the binary mixture (surfactant/solvent) is first ultrafiltered. The permeability is decreased by 85% for PAN10, 92% for TiO2 and 57% for PTFE10. The reductions occur as soon as each of these membranes comes into contact with the surfactant solution below the cmc ( = 0.74% - 0.76%). The reason for the sharp decline in permeability presumably is an adsorption as a consequence of the interaction between membrane and surfactant molecules (see Fig. 29 (a)) which block the membrane pores.

Fig. 30 shows that for every membrane type, the rinsing step is proven as inefficient, once the membrane is already used for the filtration at a concentrate below the cmc.

For PAN10, the permeability declines by another 60% despite rinsing with pure 1-dodecene. Subsequently, it continues relatively constantly over time. The reason for the reduction in permeability may be that a part of the surfactant molecules adhere already to the membrane. The mentioned concentration = 0.74% is significantly lower than the cmc. Hence, the rinsing with pure 1-dodecene is unsuccessful because of the remaining surfactant molecules, which are still persistent adsorbed at or in the membrane.

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Fig. 30. Rinsing of PTFE10, PAN10 and TiO2 membrane after filtration with a surfactant concentration below the cmc [182].

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For TiO2, the strongest reduction (by about 92%-95%) occurs when the surfactant concentration ( = 0.76%) is below the cmc. The interaction between membrane and surfactant regardless of filtration duration is apparently stronger than the other two membranes. Rinsing with the pure solvent leads to a sevenfold increase of the steady state flux. Finally, a permeability recovery of about 37% is achieved. When another rinsing attempt is repeated subsequently, the steady-state value increased in a stepwise manner. However, the improvement is unnoticeable. For instance, a permeability recovery of 50% is possible after TiO2 is rinsed three times

For PTFE10, a sharp increase is observed as soon as the surfactant is added to 1-dodecene, which was in contrast to PAN10. However, the permeability decreases sharply over time. Its final value of the filtration of the binary solution amounts to about 57%, i.e. is lower than the final value of the pre-filtration. Unexpectedly, the permeability is not improved by rinsing with 1-dodecene, but reduced by 47%. When the rinsing step is repeated another time, the flux only increased by 50% in comparison to the previous rinsing step. It is worthy to note that a permeability recovery of approximately 34% is achieved by this second rinsing.

Rinsing experiment after UF at a concentration above cmc

A permeability decline is observed for each membrane as soon as it comes into contact with the surfactant solution (Fig. 31). The flux behaviour of each membrane during the filtration of the binary solution (surfactant/1-dodecene) at a surfactant concentration above the cmc ( = 1.88% - 2.01%) is rather similar to the case of surfactant concentration below the cmc. For PAN10 and TiO2, the respective permeability decline of 47% and 83% is distinctly lower than below the cmc. In contrast, a decrease in permeability of 72% for PTFE10 here is higher than below the cmc. Possibly, micellisation occurs preferentially and simultaneously it inhibits the surfactant adsorption at the membrane surface or within membrane capillaries. This argument is confirmed by a substantial permeability recovery as a consequence of rinsing with 1-dodecene. For PAN10, a maximum flux permeability of approximately 93% is obtained while for PTFE10 and TiO2 its value amounts to only 83% and 67%, respectively. In summary, there is hardly a difference in the permeability of PTFE10 in the various regions of the surfactant concentration.

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In contrast to PTFE10, the interaction between the membrane TiO2 and surfactants above the cmc occurs not as significantly as it does below the cmc. The permeability is reduced by 83%. Making use of the rinsing with 1-dodecene improves the flux fairly well. Still, it is not completely recovered.

At a concentration above the cmc, the permeability is regained much more than below the cmc. It implies that the adsorption effect is a reversible phenomenon once the membrane is already in contact with surfactant solution at ≥ cmc. The interaction between membrane and surfactants appears to be weaker than the formation of reverse micelles. A detailed description of these involved effects was given in the UF-experiment with varying concentration and illustrated in Fig. 29 (b).

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Fig. 31. Rinsing of PAN10, TiO2 and PTFE10 membrane after filtration with a surfactant concentration above the cmc [182].

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57

4.3 Ultrafiltration of surfactant aqueous solutions In this section, experiments are repeated using NP5-containing water solutions under the same procedure conditions so that effects of polar solvent on micelle retention and permeability behaviours could also be evaluated. This experimental part is an extension of the earlier investigation on “ultrafiltration of surfactant organic solutions” (see Section 4.2). Moreover, it aims to fill the knowledge gap in the mechanism effecting on the infeasibility of reverse MEUF.

4.3.1 Influence of surfactant concentration According to the procedure described in Section 3.4 it is shown that in the case of experimental series 1, the retention of micelles and the reduction in permeability can be determined simultaneously at any concentration level. For all tested membranes the micelle retention values are summarised in Table 5. The values range from 84% to 100%.

Table 5 Micelle retention achieved in the experimental series 1 [194].

Feed concentration F/cmc

Micelle retention RM (%)

PTFE10 PAN10 TiO2 RC10 RC30

6 1.5 100 100 100 100 100

7 2.4 100 96 100 100 100

8 3.2 100 100 100 100 100

9 4.4 100 84 100 100 100

For concentrations above cmc (using feeds F6 to F9) highest retention is achieved with PTFE10, TiO2, RC10 and RC30. The high retentions achieved are generally in agreement with several studies on UF of aqueous micellar solutions [183,184]. This result is assigned to the formation of micelles in aqueous solutions at concentrations above the cmc. The size of micelles is larger than the membrane pore size, so that the micelles are retained to a high amount. Actually, the result of NP5/H2O is opposite to the minor retentions of reverse micelles in the non-polar solvent NP5/1-dodecene. Contrary to the other membranes PAN10 shows slightly smaller retention of micelles (RM = 84% and 96%). In spite of small MWCO (10 kDa) PAN10 has a broad pore size distribution [195]. Hence, a certain amount of micelles pass through PAN10 due to its large pores.

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Fig. 32. Permeability, calculated from the initial flux, as a function of the surfactant concentration during filtrations of experimental series 1 with various membranes (cmc at 0.00274%) [194].

Based on the experimental data the permeability is expressed as function of the surfactant concentration at a constant transmembrane pressure difference (1 bar). For every concentration level, each point refers to the final steady flux. Generally, two distinct groups due to the permeability behaviours are realised in Fig. 32. Group a is characterised for its moderate decline in permeability, as long as the cmc is not exceeded. By contrast, group b shows no significant reduction for all concentration levels. Considering the surfactant concentrations, the flux behaviour of every group can be divided into two ranges:

a) high permeability with a sharp flux decline in the range below the cmc; b) low flux with a slight flux decay with increasing concentration.

For group (a), including PAN10, TiO2 und RC30, permeability starts at a level between 200 – 250 L/(m² h bar). Subsequently, permeability declines strongly at concentrations below cmc and levels off at cmc. The strongest decline occurs at the highest concentration above cmc (i.e., F9), resulting in 83% to 94% reduction of the initial pure water permeability. Overall, PAN10 undergoes the most extreme

permeability decrease reducing it to 6% of the original permeability, e.g., from 250 to 16 L/(m² h bar). In contrast group (b), including PTFE10 und RC10, starts out at moderate permeability of 37 and 43 L/(m² h bar), which decreases marginally and results in permeability decline of 24% and 36%, respectively, of the initial pure water permeability at the highest concentration 9 (i.e., F9). A comparison of the permeability with the measured contact angle shows that flux

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59

decline depends not only on the hydrophobic character of the membrane because PTFE10 with the highest contact angle has not the highest flux decline. At a surfactant concentration of 0, the permeability of PAN10 was about 5 times higher than that of PTFE10 although both have the same MWCO. This could be due to a broader pore-size distribution of PAN10. PAN10 is commercially not available and was produced only for scientific research without optimising its pore structure during the synthesis.

Remarkable for all membranes in both groups is a comparatively similar range of permeability at 9 which might be explained by the adsorbed surfactant layer.

4.3.2 Permeability decline

This section presents the concentration-dependent flux behaviour of all tested membranes. The different conditions include the choice of the feed solvent (i.e., polar or non-polar) and the experimental procedure (i.e., experimental series 1 or 2). All analyses are based on permeability decline, which can be generated from permeability data as a function of the surfactant concentration (ref. Fig. 28 and Fig. 32). First, flux behaviour of w/o and o/w-MEUF is compared at different surfactant concentrations. MEUF of aqueous solution and MEUF of non-polar solution [182] show similar general patterns. RC10 and RC30 membranes extend the present study on aqueous solutions due to their chemical stability in this medium. Previously, these two membranes had been disregarded due to their chemical instability in 1-dodecene solution [182]. When filtering aqueous surfactant solutions, flux of all membranes already decreases dramatically below cmc. However, it remains at a constant, but minimal level above cmc. For both solvents, the flux declines substantially below cmc-level. Up to this level, adsorption of surfactant monomers at the membranes occurs, which reduces the diameter of membrane capillaries strongly. According to Doulia et al. [7], concentration polarisation is insignificant for flux reduction below cmc. By comparison, the flux behaviour is different for every examined membrane when using 1-dodecene (Fig. 33). For the both solvents the flux behaviour of TiO2 membrane in dependence of the surfactant concentration exhibits a similar progress at a glance. On the contrary to the result of TiO2, the hydrophobic membranes PTFE10 and PAN10 show a permeability recovery

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close to or above cmc presumably due to desorption, when 1-dodecene is used instead of water. An essential question is whether permeability decline depends on the type of solvent or the execution of the experimental procedure. Hence, a comparison of permeability decrease between experimental series 1 and 2 is outlined in Table 6. In this instance, two concentration levels are compared: 4 (at below cmc) and9 (at above cmc). At the surfactant concentration 4, permeability decrease in the experimental series 1 is stronger than in the experimental series 2, while at9 the value remains similar. Reason for the latter may be that flux-limiting processes such as adsorption which reduce the pore size of the membrane have already reached saturation at a high concentration like 9. Above cmc the extent of permeability decline is independent of the membrane surface having been in contact with surfactant molecules and of the amount of surfactant adsorbed.

Fig. 33. Comparison of flux behaviour between polar and non-polar solution in the experimental series 1: (A) NP5/H2O [194] und (B) NP5/1-dodecene [182].

The behaviour at 4 differs strongly between the two experimental series, because filtration steps from feed F1 until F3 was conducted in the series 1 before the start of filtration of F4. In other words, the number of existing adsorption places is lower than in experimental series 2. According to Paria’s theory, head groups of individual surfactant monomers preferably adhere to hydrophilic surfaces. In the present study the adsorption of NP5 on hydrophilic membranes follows an attractive interaction between the polyoxyethylene chain and the membrane surface. A similar observation is stated in several published works using silica surface [163]. Consequently, more and more surfactant molecules start to occupy vacancies of the membrane surface

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and their head groups displace neighbouring molecules - even hydrocarbon chains - from the surface. At a particular concentration surfactant molecules form a layer at the surface so that the tail groups are oriented directly to the solvent [169]. Subsequently, if the membrane exhibits capillaries with a nominal diameter, which is large enough to offer space for more than one layer, either a second layer is adsorbed on the first one or adsorbed clusters are formed on the surface (see Fig. 34a). As a result, adsorbed surfactants that obey both structural models with hydrophilic head groups in the outer layer make a hydrophilic surface more hydrophilic. However, a negative effect of the double layer as well as of the surfactant clusters is a reduction in the membrane pore diameter. As an example of a hydrophilic surface, this effect can be observed at RC membranes. RC10 shows a lower permeability than RC30 at the first filtration step of pure water due to its narrower pore diameter. This result is in good agreement with Hagen-Poiseuille’s law. Owing to the stepwise increase in the surfactant concentration, the permeability of RC30 is declining at every concentration level. This is especially true at concentrations below cmc where the flux of RC30 declines more strongly. However, only a negligible change is observed for RC10. At 9 the permeability of RC30 shows a minimal value, which is similar to the one of RC10. This implies for hydrophilic membranes such as RC with a MWCO range from 10 kDa to 30 kDa that the pore narrowing is the dominant effect in comparison to the mechanism of hydrophilicity enhancement. For a hydrophobic membrane, the surface aggregates consist either of a monolayer or semi-spherical clusters. In contrast to a hydrophilic membrane, here the tail groups are adsorbed to the surface. Hence, head groups arrange in the outer coat so that they are in direct contact to the aqueous medium (see Fig. 34b).

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Fig. 34. Layer formation models with the orientation of head groups of non-ionic surfactant to aqueous medium under influence of: (a) hydrophilic membrane surface; (b) hydrophobic membrane surface (adapted from [163]).

Table 6 Comparison of permeability decline (PD) during UF of aqueous surfactant solutions obtained for experimental series 1 and 2 [194].

Membrane

Permeability decline PD [%]

Experimental series 1 Experimental series 2

UF of NP5/H2O

F4 F9 F4 F9

PTFE10 27 36 21 37

PAN10 71 93 48 83

TiO2 38 83 19 78

RC30 25 83 9 76

RC10 0 24 3 16

4.3.3 Permeability recovery

According to the procedures of the experimental series 2, this part of the study aims to exam how efficiently adsorbed surfactant molecules can be removed from the membrane by consecutive reflushing. Permeability recovery (PR) is determined by permeability measurement with the pure solvent before, as well as after the filtration of the surfactant solution with concentrations below the cmc (e.g., F4) or at above cmc (e.g., F9). Fig. 35 gives an overview of the PR behaviour with regard to the following major aspects: (1) the hydrophilicity of the

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membrane surface; (2) the surfactant concentration of the solution filtrated prior to the rinsing step; (3) the solvent of the feed; (4) the permeability decline. Although, the study part deals with the filtration of aqueous surfactant solutions only, the results of the previous experiment using 1-dodecene as solvent is plotted additionally to gain a deeper understanding of solvent influence on the flux recovery. In case of water as solvent for all membranes, PR generally tends to increase in surface hydrophilicity, except for PAN10. At surfactant concentrations below cmc the PR of all membranes achieves higher values (≥ 86%) with increasing hydrophilicity compared to experiments performed with surfactant concentrations above the cmc. When rinsing with water for each membrane, the flux after filtration of F4 is better recovered than in case of F9. This implies that with higher surfactant concentration rinsing becomes less efficient: The more micelles are present in the feed solution, the less efficient it is to rinse the adsorbed surfactants from the membrane surface. In an aqueous medium the micelles are aggregates with hydrophilic heads, which are oriented towards the water. Therefore, they adhere to membrane surface more insistently than the surfactant monomers do. For a non-aqueous solvent such as 1-dodecene, PR seems to be directly inverse to the results of water at the comparable concentration levels. In particular below cmc, when applying 1-dodecene as solvent, every membrane exhibits a smaller PR value (between 5 % and 54%) than in case of water (≥86%). Above cmc PTFE10 and TiO2 have the same PR, in 1-dodecene and water, respectively. As in case of water, PAN10 is again an exception here. Across the spectrum of all membranes, this one accounts to a maximum of PR for 1-dodecene (98%), whereas its PR hits a minimum for water (15%) (see Fig. 35). It is realised that the stronger the decline in permeability during the filtration of aqueous surfactant solutions is, the lower the permeability recovery becomes. The trend of this relation between PD and PR is recognisable for different surface hydrophilicity as well as feed solvents. A remarkable example is PAN10, which displays this behaviour during the filtration of NP5/water as well as during the filtration of NP5/1-dodecene above cmc. In case of aqueous solution, this membrane exhibits a maximal PD (83%) and a minimal PR (15%). On the contrary, PAN10 reaches a minimal PD (43%) and maximal PR (98%) in case of 1-dodecene. That means, after the filtration at concentrations above cmc rinsing with pure water hardly shows any efficiency for PAN10, but with pure 1-dodecene its PR is easily achievable.

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Similar to the observations for PD, PR of the typical hydrophilic material RC displays a difference between large and small MWCO. The effect of different pore sizes is investigated for RC in this study, because the other tested membranes are only available with one pore size or one MWCO. Based on Hagen-Poiseuille’s law, the larger the membrane pores are, the higher the flux through these pores becomes [39]. Despite of its larger pore size, the membrane RC30 exhibits a higher PD and therefore a lower PR than RC10, for all examined concentrations. For an unchanged flux, shear stresses in pores of a membrane with a larger MWCO are lower than in membrane with smaller MWCO [39]. Due to the lower shear stresses and the larger pore size, surfactants prefer to access the larger pores of RC30. As a result of the hydrophilic interaction, they are easily adsorbed within membrane capillaries so that their hydrophilic polyethylene oxide segments orient towards the pore wall to form surface aggregates there.

Fig. 35. Permeability decline (PD) from UF of surfactant solutions: (A) NP5/H2O (< cmc), (B) NP5/H2O (> cmc), (C) NP5/1-dodecene (< cmc) and (D) NP5/1-dodecene ( >cmc) and permeability recovery (PR) after rinsing with pure solvents [194].

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Consequently, the rinsing effect for RC30 is worse than for the same membrane material with a smaller MWCO. Especially, above cmc the PR value of RC10 and RC30, respectively, is less than below cmc. Increasing PD may have been caused by concentration polarisation as a result of retained micelles. Jönsson et al. delivered experimental evidence for comparable pore effects in porous membranes made from cellulose acetate with different MWCO and the non-ionic surfactant Triton X-100 [10]. Their PD behaviour observed is in a good agreement with these results. Field and co-workers put forward a summarising table of PD at concentrations below cmc for a variety of membranes and non-ionic surfactants [39]. Their work presents an overview of a general trend in flux decline for respective membrane material by MWCO, which ranges from 6 to 500 kDa. The trend in PD is recognisable with one continual direction, i.e., higher or smaller, in an ascending MWCO. However, a general direction of the trend cannot be predicted, as soon as the adsorption behaviour of the applied surfactant changes. Some illustrating examples are offered in Fig. 36 giving a variety of trends in PD according to MWCO.

Fig. 36. Direct and inverse proportional relationship between PD and MWCO (6-500 kDa) in accordance to the type of applied surfactants and membrane materials (PS: Polysulphone; PVDF: Polyvinylidene fluoride; CA: Cellulose acetate), adapted from [39].

Surfactant type (< cmc) PD MWCO

Non-ionic surfactant

Anionic surfactant

Cationic surfactant

Triton X-100

Potassium oleate

Sodium dodecyl- benzenesulphonate

Hexadecyltrimethyl- ammonium bromide

PS PVDF CA

PS PVDF

PS PVDF CA

PS PVDF CA

PD MWCO

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4.4 Application of models for flux data evaluation

4.4.1 Hermia’s blocking models Hermia’s blocking models are applied for all membranes in this study [38]. This section presents the results for PAN10 as significant example. Hereby, flux data for increasing surfactant concentrations (Feeds F0 to F9) are analysed and presented for each of the four fouling models by Hermia (Fig. 37). Cake filtration model (1) and standard pore blocking model (2) fit the data very well, if micelles are formed (i.e. above the cmc). Below the cmc the solution contains only surfactant monomers, both models become inapplicable. At a glance, complete pore blocking model (3) and intermediate pore blocking model (4) seem to be unsuitable according to their slopes, which are completely different from the one described in Table 1.

Neither PAN10, nor any of the other membranes PTFE10, PAN10, TiO2, RC10 and RC30 do exhibit a predominant mechanism under the assumptions of Hermia’s models. In other words, there is none of Hermia’s models able to fit the experimental series 1 for each membrane. For this reason, Hermia’s laws are not applicable for surfactant solutions, although these are a special kind of colloidal dispersions [41]. Hermia’s blocking laws describe physical mechanisms for solutions that typically contain colloid and particulate matter. Such description lacks precision since there are general differences between surfactant-containing and colloidal solutions. An amphiphile exhibits otherwise distinct characteristics than particulate matter. Noteworthy, surfactant molecules are much smaller than particles which have been a focus in past research based on Hermia’s laws. Moreover, surfactants associate in a dynamic and thermodynamic driven process to form micelles at a critical concentration. Thus, they have a different structure compared to particulate compounds. At a solid/liquid interface formation of surface aggregation occurs in an analogous way as the formation of micelles in the bulk phase. The reason for both phenomena is an entropy-driven process [196].

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Fig. 37. Hermia’s four blocking models applied to the data obtained for the filtration of NP5/H2O with PAN10 at 1 bar, 20°C and 200 min-1 [194].

As Hermia’s models lack the ability to explain the adsorption behaviour of surfactants on membrane surface, Zhu and Gu’s adsorption isotherm equation is considered in the next step of the present with regard to of surface micellisation, an important property of surfactant at a solid interface.

4.4.2 Zhu and Gu’s adsorption models Hagen-Poiseuille‘s law, as mentioned in Section 2.5.3, works on the assumption that the flow of a fluid permeating cross straight capillaries in the surface active layer of membrane is laminar. Combing the Hagen-Poiseuille equation (Eqs. (5) and (8)), the permeability P of a pure solvent through the capillaries is calculated as

2

32capd

PL

(34)

In Eq. (34) is the porosity, dp is the pore diameter, is the viscosity and L is the capillary length.

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Adding surfactant to the solution leads to a decline in permeability at every concentration level. Surfactant monomers are much smaller than dcap, the capillary diameter at the beginning. The more surfactants are present in feed, the more they may accumulate within the capillaries. When the interior space of each capillary becomes equally narrower, the corresponding hydraulic diameter is reduced to a value of dh. Hence, the permeability of a surfactant-containing fluid through these narrowed capillaries Ps can be analogously expressed as

2

32h

sdP

L

(35)

Fig. 38. Zhu and Gu’s adsorption model in combination with Hagen-Poiseuille’s law [194].

Figure 38 represents schematically a narrowed capillary as a consequence of the surfactant adsorption layers inside of the membrane capillaries a tube-shaped cover is formed with a specific diameter (dcap -dh).

According to Eqs. (23), (34) and (35) the permeability decline PD at a specific surfactant concentration can be obtained as

2 2

2cap h

cap

d dPD

d (36)

Following this approach, a relationship between the adsorption isotherm and the adherent surfactant mass at the capillary walls may be put forward as:

2 2

4 cap hd d L (37)

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Fig. 39. Determination of n and K by using Eq. (40) based on the combination of the two-step adsorption model with S-shape proposed by Zhu and Gu and the PD from UF of surfactant solutions [194]: (A) NP5/H2O (< cmc), (B) NP5/H2O (≥ cmc), (C) NP5/1-dodecene (< cmc) and (D) NP5/1-dodecene (≥ cmc).

In Eq. (37), L is the capillary length and is the density of permeate. The relation

between and its limiting value at saturation level max may be expressed by Eq.

(38) using the original diameter of membrane capillaries dcap and the reduced diameter as consequence of adsorption dh:

2 2

2max

cap h

cap

d dd

(38)

By combining Eqs. (36) and (38) it is shown that the degree of surface coverage

max

has the same meaning as permeability decline PD.

max

PD (39)

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Moreover, the isotherm equation Eq. (11) proposed by Zhu and Gu can be re-written in terms of filtrations:

ln / (1 ) ln lnPD PD K n (40)

In Eq. (40), K and n are the specific adsorption parameters, is the surfactant concentration and PD is the permeability decline (or flux decline). According to

this equation, the plot of ln / (1 )PD PD against ln presents a straight line with

the slope n and the intercept lnK. By using Eq. (40) n and K can be calculated from experimental UF of surfactant solutions under the assumption of the S-shape adsorption model and Hagen-Poiseuille’s law.

The adsorption parameters n and K are determined for all tested membranes by corresponding correlations shown in Fig. 39. The values (n, lnK) in two concentration regions (below and above cmc) are summarised for NP5/H2O in Table 7, as well as for NP5/1-dodecene in Table 8. These values characterise the adsorption behaviours of surfactants onto various membranes regarding the hydrophilicity degrees, the MWCO, the pore size distribution of the membrane active layer as well as the polarity of the feed solution. The hemimicelle aggregation number n indicates the structural arrangement of surfactants adsorbed at the membrane surface. The intensity of the surfactant adsorption is described by the adsorption constant K. The higher K, the stronger surfactants are adsorbed onto the membrane surface. For the system NP5/H2O at surfactant concentrations below cmc, the following order of the adsorption constants is obtained: PAN10 > RC30≈RC10 > TiO2 > PTFE10. Below cmc, only the adsorption of NP5 monomers on the different membrane surfaces or in the membrane pores has to be considered. When we take PTFE10 out of this series, because it is not only hydrophobic but also lipophobic [197], the tendency correlates with the contact angle. However, the situation changes at surfactant concentrations above cmc, because of the micelles which are formed. While the lowest value is obtained for PTFE10 and the lnK value for the RC30 membrane remains almost the same, we can see a significant change in the adsorption constant for the RC10 membrane. If the adsorption of the NP5 on the membrane surface is a predominant step, we would expect the same value for the RC10 and RC30 membrane as obtained below cmc. This indicates that also the pore size and pore size distribution should be considered. RC30 has much larger pores and a broader pore-size distribution than RC10; therefore, adsorption inside the membrane pores of RC10 is difficult

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for concentrations above the cmc. Using the same membrane materials such as PAN10 and RC30 made by the same producers as in the present study, Schwarze showed that PAN10 has a broader pore size distribution than RC30 [195]. This information is in good agreement with the results of PAN10 and RC30 for any surfactant concentrations. PAN10 has the maximal lnK value, which is slightly higher than the one of RC30. It follows that PAN10 shows a stronger adsorption affinity than RC30 and the influence of the pore size distribution on the adsorption intensity is more critical than the surface hydrophilicity. For the system NP5/1-dodecene, only 3 membranes which were stable in 1-dodecene were investigated. In contrast to the aqueous system, for concentrations below cmc the highest adsorption is observed for the TiO2 membrane, followed by the PAN10 membrane. Also in NP5/1-dodecene, lowest adsorption is found for the PTFE10 membrane. Moreover, a strong desorption (negative lnK value) is found for this membrane. For the same system at concentrations above the cmc, desorption was obtained for the PAN10 membrane, while much lower adsorption for the TiO2 membrane occurred. The PTFE10 membrane showed an increased adsorption. The results for n determined from the plots in Fig. 39 are also given in Table 7 and Table 8 for each membrane to complete a picture of the surfactant aggregation as the surfactants adsorb on the membrane surface. Different adsorption behaviours between the concentration region below and above cmc are also recognisable here. When n>1, the hemimicellisation (“hmc”) known as surface micellisation occurs. This phenomenon happens at PAN10 in the both solvents for example. When 0<n<1, each adsorbed molecule occupies more than one surface site (i.e., multi-site adsorption, abbr. “msa”). This occurs at PTFE10 in aqueous, as well as in non-aqueous surfactant solutions. For n=1, the general isotherm equation turns to be the Langmuir equation (i.e., L-shape).

Table 7 Adsorption isotherm parameters determined from PD for NP5/H2O [194].

Membrane n (< cmc) lnK ( < cmc) n (≥ cmc) lnK (≥ cmc) PTFE10 0.13 0.42 0.18 0.96 PAN10 1.34 15.43 1.07 12.28 TiO2 0.50 5.23 0.70 7.89 RC30 1.14 11.71 1.02 10.87 RC10 1.22 11.25 0.35 1.98

RESULTS AND DISCUSSION

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Table 8 Adsorption isotherm parameters determined from PD for NP5/1-dodecene [194].

Membrane n (< cmc) lnK (< cmc) n (≥ cmc) lnK (≥ cmc) PTFE10 -0.73 -4.06 0.33 2.06 PAN10 1.28 7.96 -1.30 -4.90 TiO2

RC30 RC10

2.35 - -

12.80 - -

0.15 - -

2.85 - -

Notably the value range of n and K determined in the present filtration experiments is in a similar magnitude in some published works obtained by gravimetric method [171,173,177,198]. Contrary to the present study, the aforementioned studies focus their investigation on adsorbents, e.g., silica gel, silica glass and carbon black etc. Probably as a first study, the author applies the two-step adsorption model with S-shape derived by Zhu and Gu [172] to achieve an explanation of different effects of o/w- and w/o-MEUF. In the concentration region ≥ cmc, K for aqueous solutions media is higher than for 1-dodecene solutions, except for PTFE10. The difference in K value can be attributed to the solubility of NP5 in the respective solvent. Due to the low ethoxylation number (n = 5), NP5 is better soluble in 1-dodecene than in water; and NP5 micelles adsorbs at membrane in water more intensively than in 1-dodecene. Thus, the non-polar solvent facilitates the removal of reverse micelles from the interface during the membrane flushing. By contrast, water inhibits the flushing effect. The explanation for different K in different media is consistent with the study on reverse micellar system Igepal CO-520/cyclohexane/water by dynamic light scattering [69]. In aqueous solutions, micelles can be retained very well, even completely by all tested membranes (see Section 4.1.2). However, retention of reverse micelles from a non-polar solution, e.g., 1-dodecene, is technically impossible [182]. The adsorption intensity for 1-dodecene should be substantially smaller than for water. Consequently, reverse micelles do not prefer adsorption on the membrane surface. Moreover, according to the result of PD (see Fig. 35) it is observed for 1-dodecene that the adsorption intensity above cmc is much weaker than below cmc. A mass transport of surfactants is attributable to the very minor retentions of the reverse micelles. Reverse micelles as well as reverse hemimicelles are larger than the membrane pores. Therefore, they cannot take part in the diffusion. Hence only monomer surfactant molecules experience the mass transport through the porous membranes. It implies that reverse micelles are on the one

RESULTS AND DISCUSSION

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hand desorbed away from the surface. On the other hand, reverse micelles dissociate into monomer molecules. For PAN10, the micellisation of reverse micelles in 1-dodecene seems on the one hand to be thermodynamically less stable than formation of “normal” micelles in water. On the other hand, reverse micelles pass through the larger pores of PAN10, because this membrane has a broad pore distribution.

CONCLUSION

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5 Conclusion This study aimed to deliver the responsible mechanism for the technical infeasibility of reverse MEUF. Change in flux and retention were investigated by applying two models: Hermia’s blocking models and Zhu and Gu’s adsorption models. Fundamental research revolves around contrasting MEUF and reverse MEUF using various membranes in aqueous and organic solutions.

As a basis the study focused on a feasibility investigation on reverse MEUF for catalyst recovery from the oil phase of the hydroformylation. For this purpose various UF-membranes that were commercially available have been tested. They showed negligible rhodium retentions. The catalysts that are embedded in reverse micelles transgress the membrane pores. Consequently, reverse MEUF was proven as unsuitable concept for catalyst recovery from the oily product phase of the hydroformylation of 1-dodecene. When using water as solvent to prepare micellar solutions, instead of 1-dodecene, traditional MEUF is the applicable separation method. As a result a high retention of micelles occurs, as well as a high retention of the solubilised rhodium is achieved.

This difference between reverse and traditional MEUF is the core question of the following parts of this study. It can be traced back to a number of factors, for example the solvent used to prepare the (reverse) micellar solutions and the membrane surface with its corresponding interaction with surfactants.

Two types of surfactants solutions were prepared for the UF-experiments: NP5/1-dodecene and NP5/H2O. For each solution, the concentration of surfactant NP5 was increased stepwise in two regions: the first one with 0 < < cmc and the second one with ≥ cmc. Thus, in order of increasing hydrophilicity membranes PTFE10, PAN10, TiO2 were used for the UF of NP5/1-dodecene due to their high stability to the reaction mixtures. Additionally, in UF of NP5/H2O, the same membranes, as well as RC-membranes (RC10 and RC30) were examined, so that the effects such as hydrophilicity of membrane surface and the pore size distribution could be investigated.

In summary, the tested membranes show similar trends in permeability decline as response to surfactant concentration. However, a significant difference in the flux behaviour is observed, when the whole concentration region is divided at cmc into two parts (i.e. above and below cmc). In order to deliver detail, this thesis compares effects of different solvents and helps to understand more about flux-

CONCLUSION

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reducing mechanisms such as cake-layer formation and pore blocking. Main contribution of this work is an overview of flux (permeability) behaviour either in water or in 1-dodecene. This thesis shows as a result, that with stepwise increasing concentrations of the non-ionic surfactant NP5 the permeability of UF-membranes decreases continuously and approaches finally a plateau at a certain concentration above the cmc. In this work, for every solvent and every membrane, a strong decline degree is recognisable especially in the concentration range below cmc. Here, the strongest tendency is emphasised for the organic solvent. The sole exception is PAN10. In 1-dodecene, the permeability decline (PD) of this membrane shows no plateau after reaching the maximum at the cmc. It means, above cmc there is desorption, which enhances the permeability of PAN10 in 1-dodecene significantly. By contrast, there is no desorption during the filtration with increasing concentration in aqueous surfactant solutions.

In order to evaluate the flux data and to investigate the responsible mechanism for the permeability decline in the respective solvents, this study applied two models: (1) Hermia’s models; (2) Zhu and Gu’s models. The first one is concerned with physical mechanisms like blocking of membrane. The second one considers the accumulation of surfactant monomers and their adsorption layers inside of the membrane capillaries.

This part of the study shows as result, that all fouling models established by Hermia’s filtration blocking laws fail to describe the flux data. Hermia’s models have been widely used for MF and UF of suspensions containing colloidal particles or macromolecules. However, these compounds are not comparable with surfactants. Surfactants are much smaller and have the ability to associate in solutions to micelles or to aggregate at membrane surfaces.

Consequently, the study points out that the flux-reducing mechanisms observed in this work are not caused by a mechanical blocking of the membrane capillaries. By combining Zhu and Gu’s adsorption models with S-shape and Hagen-Poiseuille‘s law, a relationship between permeability decline PD and adsorption constant K is mathematically expressed. Using hemimicelle aggregation number n, it is possible to draw a conclusion of the aggregation form of surfactants during the adsorption onto membranes. In the research field of membrane filtration, this work applies Zhu and Gu’s adsorption models for the first time, to the best knowledge of the author, to explain the adsorption mechanism of surfactants from oil, as well as from water onto UF-membranes. Additionally, different impacts

CONCLUSION

76

such as the surface hydrophilicity, pore size and pore size distribution of membranes are also observed with the help of Zhu and Gu’s models.

Taking PTFE10 out of the series of increasing hydrophilicity membranes PTFE10, PAN10, TiO2, RC10 and RC30, because this membrane is not only hydrophobic but also lipophobic, the adsorption tendency correlates with the contact angle. For NP5/H2O below cmc, an order of increasing adsorption intensity PTFE10<TiO2<RC30≈RC10<PAN10 is gained due to the comparison of lnK by using Zhu and Gu’s models. At ≥ cmc, as soon as the micellisation takes place, the permeability behaviour changes. According to lnK, the most significant change was realised for the hydrophilic membrane RC10, while PD of RC30 remained almost the same in any concentration region. RC30 has much larger pores and a broader pore-size distribution than RC10; therefore, adsorption inside the capillaries of RC10 is difficult at ≥ cmc. Despite its lower MWCO (10 kDa), PAN10 showed a stronger permeability decline than RC30 (MWCO 30 kDa), however, in a similar trend. The reason is that PAN10 shows a broader pore size distribution than RC30. From these results, such membranes with broad pore size distributions as PAN10 and RC30 are not recommendable to use for UF of surfactant solutions at all concentrations.

In summary, the permeability decline above cmc is stronger for NP5/H2O than for NP5/1-dodecene. While only adsorption occurs in aqueous solvent, both mechanisms are found in 1-dodecene. For example, at PAN10 adsorption and desorption take place in 1-dodecene at concentrations below cmc and above cmc, respectively.

Zhu and Gu’s models application is an interesting tool for membrane screening suggested by this study. For instance, hydrophilic membranes, e.g. RC10, are unsuitable for the filtration of surfactant molecules from any solvents due to their high lnK. The membrane surface’s hydrophilicity plays a dominant role. This effect shows the strongest degree of PD when the oily solution contains monomeric surfactants.

Adsorption in membrane capillaries or at membrane surface has a negative impact on the retention of reverse micelles. There are less reverse micelles in the bulk solution, because they prefer accumulating as surfactant molecules at membrane. The formation of reverse micelles in a non-polar solvent is inhibited, even though surfactants are added to the solution. When adsorption reaches its saturation, the surfactants remaining in the bulk solution do not participate in the adsorption onto membrane. These molecules pass through the membrane

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capillaries. Diffusion of surfactant monomers caused by adsorption or desorption should cause the technical infeasibility of reverse MEUF. For this reason, reverse MEUF, which was originally expected to work successfully as a recycling process of the hydroformylation of 1-dodecene, is infeasible to recover catalyst. Figure 40 summarises all interactive mechanisms which may be responsible for the technical infeasibility of reverse MEUF.

Fig. 40. Interactive phenomena as responsible effects on the negative filtration performance of reverse MEUF [194].

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