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Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

j ourna l h om epa ge: www.elsev ier .com/ locate /co lsur fa

icelle formation of Tween 20 nonionic surfactant in imidazoliumonic liquids

ustyna Łuczak ∗, Anna Latowska, Jan Hupkaepartment of Chemical Technology, Chemical Faculty Gdansk University of Technology, ul. Narutowicza 11/12, 80-233 Gdansk, Poland

i g h l i g h t s

Micellar aggregation behavior of non-ionic Tween 20 surfactant was inves-tigated in eleven imidazolium ionicliquids.Structures of both cation andanion species affect amphiphileself-assembly ability.Addition of [BMIM][BF4], the ionicliquid with a good ability to sup-port Tween 20 micelle formation,facilitates micelle formation in[BMIM][TfO].Micellar solutions of Tween 20/ILwere analyzed as dispersion sys-tems according to effective-mediumapproximation theories.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 5 November 2014eceived in revised form 2 February 2015ccepted 4 February 2015vailable online 12 February 2015

eywords:ritical micelle concentration

midazolium ionic liquids

a b s t r a c t

Aggregation behavior of polyoxyethylene (POE)-type nonionic surfactant Tween 20 in imidazolium ionicliquids with varying chain length and different anions, such as tetrafluoroborate, hexafluorophosphate,bis(trifluoromethanesulfonyl)imide, and trifluoromethanesulfonate, was investigated by means of sur-face tension, conductivity and dynamic light scattering measurements. The role of the chain length, anionsize as well as interactions between ions forming ionic liquid, were discussed in terms of an ability to sup-port micelle formation. The ability of ionic liquids to induce and control self-aggregation of amphiphilicmolecules by introducing a mixture of ionic liquids with different anions was also presented. Thermody-namic parameters of Tween 20 micellization in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate

onductivityurface tensionhermodynamic parameters

were estimated using the dependence of critical micelle concentration on temperature. The normalizedconductivity data were analyzed basing on the effective-medium approximation theories and used fordetermination of the number of ionic liquid cations associated with oxyethylene units of Tween 20.The results reflect an influence of cation volume as well as strength of ionic liquid cation and anion

ion a

interactions, on its solvat

∗ Corresponding author. Tel.: +48 58 3471365; fax: +48 58 3472065.E-mail address: justyna.luczak@pg.gda.pl (J. Łuczak).

ttp://dx.doi.org/10.1016/j.colsurfa.2015.02.008927-7757/© 2015 Elsevier B.V. All rights reserved.

nd incorporation of ionic liquid molecules in the structure of the micelle.© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Ionic liquids (ILs) have attracted much attention due to theiruseful properties such as low volatility, non-flammability, highthermal stability, wide liquid range, and electric conductivity [1,2].

Physic

IaA[e

sttumbinasmmIb(IaaiB1[7wnm[ipptetasblItwmtao1tdaahtgictlnpnb

J. Łuczak et al. / Colloids and Surfaces A:

n addition, their physicochemical properties can be modified by suitable selection of cation, anion, as well as cation substituents.pplications of ILs in fields such as synthesis of organic compounds

3], nanomaterials preparation [4], separation processing [5,6], andlectrochemistry [7] have been extensively explored.

Nowadays, micellar aggregate formation of nonionic and ionicurfactants in ILs is intensively investigated [8–18]. From a prac-ical point of view, micellar ILs systems may give an opportunityo overcome ILs limitations to dissolve materials inherently insol-ble in them. Due to their nonvolatile nature, micellar IL solutionsay become more stable in high temperatures, than systems

ased on traditional solvents. Modification of ILs’ physicochem-cal properties by an addition of a surfactant may constitute aew class of reaction media, in parallel widening ILs applicationreas [19], thus broadening limited amount of solvents capable ofupporting surfactants self-assembly [20]. Up to date, 1-butyl-3-ethylimidazolium ILs with various anion species are most com-only investigated aprotic ILs in terms of surfactant self-assembly.

n one of the first works on surfactant self-assembly in ILs reportedy Anderson, micelle formation of some traditional surfactantsBrij 35, Brij 700, sodium dodecylsulfate SDS) was detected in twoLs, 1-butyl-3-methylimidazolium chloride [BMIM][Cl] and hex-fluorophosphate [BMIM][PF6] [8]. Moreover, Fletcher observedggregation behavior of common anionic, cationic, and non-onic surfactants (polyoxyethylene (POE)-type nonionic surfactantsrij-35, Brij-700, Tween-20, and Triton X-100) in low-viscosity-ethyl-3-methylimidazolium bis(trifluoromethanrsulfonyl)imideEMIM][Tf2N] [9]. Temperature dependent self-assembly of Brij6 in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]as a subject of Tang et al. research [18]. Tran proposedear-infrared spectroscopic method for determination of criticalicelle concentration (CMC) of various nonionic surfactants in

BMIM][PF6] and [EMIM][Tf2N] [10]. Amphiphile self-aggregationn all these ionic liquids has been interpreted in terms of solvo-hobic driving force, which is an analogue to hydrophobic effect,resent in aqueous solutions. Micellization in ILs was reportedo take place less spontaneously than in water. This may bexplained by not only interactions taking place between surfac-ants and ILs, which differ from those in aqueous systems, butlso molecular and aggregate packing constraints. Surface ten-ion measurements combined with 1H NMR technique, carried outy Wu, demonstrated that polyoxyethylene (20) sorbitan mono-

aurate (Tween 20) aggregates in [BMIM][BF4] and [BMIM][PF6]Ls, forming nanodroplets. The nanostructures are separated fromhe solution phase (at critical aggregate concentration 1—CAC1),hile those formed at CAC2 are analogical to the usual surfactanticelles, formed in aqueous solutions [16]. An influence of water

races on a microstructure of Triton X-100 micelles in [BMIM][PF6]nd [BMIM][BF4] was investigated by Li and coworkers. It wasbserved that an addition of small amounts of water to Triton X-00/[BMIM][PF6] system, results in bonding of water molecules tohe ethylene oxide (EO) units of the surfactant, and an appearance ofroplets in a form of microemulsion systems [21]. The results of Gaot al. revealed that micelles formed by Trition X-100 in [BMIM][PF6]nd [BMIM][BF4] have an irregular shape. Moreover, the surfactantas an ability to destroy ion pairs in the ILs due to the interac-ions between oxygen atoms in EO groups and positively chargedroups of the IL [22]. Inoue group investigated micellization behav-or of polyoxyethylene-type nonionic surfactants with varyinghain lengths, denoted as CnEm, in 1-butyl-3-methylimidazoliumetrafluoroborate [BMIM][BF4] [19,23]. It was observed that theogarithmic value of the CMC decreases linearly with an increasing

umber of carbon atoms in surfactant hydrocarbon chain—as it wasreviously shown for surfactants in aqueous solutions. Thermody-amic parameters of micelle formation in [BMIM][BF4], calculatedasing on a CMC-temperature dependence, revealed that at low

ochem. Eng. Aspects 471 (2015) 26–37 27

temperatures, micellization is an entropy-driven process, and anenthalpy-driven at elevated temperatures. In [BMIM][BF4], the con-tribution of entropic parameters to the overall micellization freeenergy was determined to be much lower than in aqueous solu-tions. As a continuation, the same group compared micellizationof CnEm amphiphiles in [BMIM][PF6], and observed an increaseof CMC, a drop of aggregation number, and weaker dependenceof CMC on the length of surfactant hydrocarbon chain [24]. Theresearchers also demonstrated that both cation and anion of theILs influence its ability to support micellization, and therefore mix-ing two ILs may induce self-assembly of surfactants [25,26]. Chenand coworkers observed charges screening effect when cationicand anionic surfactants (SDS/DTAB) were added to 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4]. These solutes mixwith the IL over the entire composition range revealing behavioropposite to the one in water, where precipitation of the surfactantmixture usually occur due to electrostatic attractions [27]. Someresearch was also carried out in order to investigate micellizationof long-chained ionic liquids in ionic liquids since it was previouslystated that ILs can be considered as surface active agents [28,29].For example, Li et al. discovered that 1-alkyl-3-methylimidazoliumbromides form spherical aggregates in [BMIM][BF4] being muchbigger than traditional micelles [30].

Apart from the aprotic imidazolium ionic liquids, many proticones were found to be capable of supporting amphiphile self-assembly [31,32]. Analogously, the driving force of self-assembledstructures formation was attributed to an entropic contribution tothe free energy of association—the solvophobic effect assisted byability of ILs to form hydrogen bonds. Most commonly reportedprotic ILs are ethylammonium nitrate (EAN) and various otherammonium derivatives [33–36]. As amphiphiles, both ionic andnonionic surfactants as well as imidazolium and pyrrolidinium ILswere applied [37–40]. For IL-EAN system, the basic character of thecounterions as well as the hydrophobicity of the alkyl substituentsof the heterocycling ring were found to influence the interactionsbetween IL amphiphile and EAN. Moreover, due to micelle for-mation, a partial conformational change in the alkyl chain, fromtrans to gauche structure, was observed [38]. Shi and cowork-ers based on the surface tension and 1H NMR results to concludethat ethylammonium cations of EAN are located around the headgroups of pyrrolidinium micelles whereas [NO3] counterions areadsorbed at the micelle surface [40]. Greaves and coworkers inves-tigated series of the C12En surfactants in ammonium derivativescontaining nitrate or formate anions, and described a correlationbetween the size of polar and nonpolar nanoregions formed in theILs and their ability to support micelle formation. It was statedthat ILs with hydroxyl functional groups in the alkylammoniumcation were found to have poor ability to dissolve surfactants, dueto negligible internal nanostructure segregation, and therefore noevidence of micelles formation was pointed. Protic ionic liquids,such as EAN, propylammonium nitrate, diethylammonium formateand triethylammonium formate, which have non-polar domains ina size range of 5.5–11.7 A, were found to have the highest abil-ity to promote micellization [32]. Moreover, decreasing surfactantethoxy chain length at constant ILs alkyl chain length results inmicelle sphere to rod transition [15]. Pluronic homologues seriesof nonionic surfactants as an example of surface active copoly-mers were also investigated for self-assembly in ionic liquids.Triblock structural composition of these compounds was foundto enable formation of micelles and lyotropic liquid crystals inprotic ionic liquids [41–43]. Wager and coworkers revealed theability of Pluronics to self-assemble into different-shaped micel-

lar structures with low curvatures, such as vesicles in the diluteregime, elongated (wormlike) micelles at intermediate concen-trations and low temperatures, as well as a nematic phase inhighly concentrated solutions, and multilamellar phases at high

28 J. Łuczak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37

N N n

X

+ O

OOH

OOH

OOH y

OOCH3(CH2)9CH2 z

On+x+y+z=20

A) B)

re [PF

tsimrpInc5ld

ffioahaiatiicsa

ltmwttmaaadaubtapts

2

2

(

X : PF6, BF4, Tf2N, TfO

Fig. 1. The structures of (A) 1-alkyl-3-methylimidazolium ILs, where X a

emperatures [42]. Also the same group discussed structural tran-ition of CTAB self-assembled structures in EAN, which turned withncreasing surfactant concentration from non-interacting spherical

icelles to micelles interacting via electrostatic repulsions, up toeversible, temperature-dependent isotropic (L1) to hexatic (Hex)hase transition [44]. Generally, it was observed that in protic

Ls amphiphilic compounds are able to form various, well orga-ized structures, such as micellar aggregates at lower surfactantoncentration, whereas at higher concentration (typically above0%) discrete cubic, normal hexagonal, normal bicontinuous cubic,

amellar, inverse hexagonal, inverse bicontinuous cubic and inverseiscrete cubic lyotropic liquid crystalline phases [20,41,45–50].

Even though the number of publications in this field increasesrom year to year, further systematic experiments are still requiredor better understanding of surfactants aggregation phenomenonn ionic liquids. Taking into account aprotic ionic liquids, mostf the investigations are restricted to TX-100 nonionic surfactantnd two ionic liquids, [BMIM][BF4] and [BMIM][PF6], despite theirigh viscosity, and susceptibility of the anions to hydrolysis incidic conditions and elevated temperatures [51]. Therefore, theres a need to choose more stable substituents to match with imid-zolium cations. Bis(trifluoromethanesulfonyl)imide [Tf2N] andrifluoromethanesulfonate [TfO] anions, due to their lower viscos-ty, and high thermal and electrochemical stability, are widely usedn the area of ILs research. It was already presented by Li andoworkers that fluorinated surfactants, which reduce surface ten-ion more efficiently than conventional amphiphilic compounds,re able to form micellar aggregates in [BMIM][Tf2N] [14].

In order to gain a better insight into interactions between ioniciquids and surfactants, we investigated aggregation behavior ofhe nonionic surfactant Tween 20, in selected ionic liquids. As

entioned before, micelle formation of Tween 20 in ionic liquidsas presented only for [BMIM][BF4] and [BMIM][PF6] [16]. To fur-

her elucidate the influence of ILs structure (alkyl chain length inhe imidazolium ring and anion type) on the ability to promote

icellar aggregate formation by Tween 20, we studied its self-ssembly in 1-ethyl-, 1-butyl-, and 1-hexyl-3-methylimidazolium,nd 3-methyl-1-octylimidazolium salts with [PF6], [BF4], [Tf2N],nd [Tf2O] anions (shown in Fig. 1). In addition, the parametersescribing adsorption and micellization processes were presentednd compared. The aim of this work is also to contribute to furthernderstanding of Tween 20 behavior in IL mixtures, and a possi-ility to regulate the ILs affinity towards the surfactant by mixingwo ILs with different anions. Therefore, we investigated how doesn addition of [BMIM][BF4] (ionic liquid with a good ability to sup-ort micelle formation) to [BMIM][TfO] (which has weak abilityo promote micellization) affects mixtures capability to promoteelf-assembly of Tween 20.

. Materials and methods

.1. Materials

The nonionic surfactant used in this work was polyoxyethylene20) sorbitan monolaurate, high purity grade Croda (Tween 20 HD),

6], [BF4], [Tf2N], and [TfO] anions, and (B) Tween 20 nonionic surfactant.

and was used as received. The ionic liquids, 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide [BMIM][Tf2N], 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [BMIM][TfO], 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [BMIM][Tf2N], 1-butyl-3-methylimidazolium trifluorome-thanesulfonate [BMIM][TfO], 1-hexyl-3-methylimidazoliumtetrafluoroborate [HMIM][BF4], 1-hexyl-3-methylimidazoliumhexafluorophosphate [HMIM][PF6], 1-hexyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide [HMIM][Tf2N], 1-hexyl-3-methylimidazolium trifluoromethanesulfonate [HMIM][TfO],3-methyl-1-octylimidazolium tetrafluoroborate [OMIM][BF4],3-methyl-1-octylimidazolium hexafluorophosphate [OMIM][PF6],3-methyl-1-octylimidazolium bis(trifluoromethanesulfonyl)imide[OMIM][Tf2N], 3-methyl-1-octylimidazolium trifluoromethane-sulfonate [OMIM][TfO] with a purity of ≥99%, and 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] with a purityof ≥98%, were obtained from IoLiTec, Germany. Before use, ILswere degassed and dried under vacuum of 20 Pa (Thermo VT6025, Germany) for 24 h at temperature of 70 ◦C, to reducewater and volatile compounds content to negligible values [29].The water contents of these ionic liquids were determinedby coulometric Karl-Fischer titration (model 899 Coulometer,Metrohm). The water content in the ionic liquids was less than300 ppm.

2.2. Methods

Surface tension of IL solutions was measured at 25 ± 0.1 ◦Cusing a video based optical contact angle meter (OCA 15, Dat-aphysics) along with a software supplied by a producer, SCA22(pendant drop method), and a resolution of ±0.01 mN/m. Thetemperature was controlled by a thermostatic water bath (Poly-Science, AD07R-20). Before each experiment the instrument wascontrolled by measuring surface tension of deionized water. Errorcalculation was performed by Dataphysics software. The surfacetension results we determined for pure ILs were: 50.3 mN/mfor [EMIM][BF4], 40.6 mN/m for [EMIM][TfO], 35.9 mN/m for[EMIM][Tf2N], 44.9 mN/m for [BMIM][BF4], 44.3 mN/m for[BMIM][PF6], 34.9 mN/m for [BMIM][TfO], 34.4 mN/m for[BMIM][Tf2N], 39.2 mN/m for [HMIM][BF4], 39.1 mN/m for[HMIM][PF6], 31.9 mN/m for [HMIM][TfO], 31.3 mN/m for[HMIM][Tf2N], 32.9 mN/m for [OMIM][BF4], 33.6 mN/m for[OMIM][PF6], 30.6 mN/m for [OMIM][TfO], 30.1 mN/m for[OMIM[Tf2N] and are comparable with experimental data obtainedby other researchers [52–57].

Conductivity measurements were performed at a temper-ature of 25 ± 0.1 ◦C using conductivity meter equipped withan autotitrator (Cerko Lab System CLS/M/07/06, Poland) and amicroconductivity electrode (Eurosensor, EPST-2ZA, Poland). A

thermostatic water bath (PolyScience 9106, USA) was used to main-tain a stable temperature of the measurements. The conductivitycell constant k was determined with a range of aqueous solutionsof KCl (0.1–100 mmol/L).

Physicochem. Eng. Aspects 471 (2015) 26–37 29

dpLpiwptVLfivba

pca

�

A

wAcw

˘

s

�

wt11a

wG

�

f

�

w

3

3

Tlaobfim

Fig. 2. Surface tension isotherms (air–liquid interface) of Tween 20 in 1-ethyl-3-methylimidazolium salts: [EMIM][BF4] ( ), [EMIM][TfO] ( ), and [EMIM][Tf2N]( ) determined at 25 ◦C. For comparison, surface tension isotherm for aqueoussolutions ( ) is shown.

Fig. 3. Surface tension isotherms (air–liquid interface) of Tween 20 in 1-butyl-3-methylimidazolium salts: [BMIM][BF4] ( ), [BMIM][PF6] ( ), [BMIM][TfO] ( ),and [BMIM][Tf2N] ( ) determined at 25 ◦C.

J. Łuczak et al. / Colloids and Surfaces A:

The size of amphiphile micelles was measured by means ofynamic light scattering method (DLS). DLS measurements wereerformed with Malvern Zetasizer Nano (Malvern Instrumentstd, UK) equipped with a red laser (633 nm) and an avalanchehotodiode detector, operating in the backward mode (scatter-

ng angle of 173◦). The temperature of the samples was controlledithin ±0.1 ◦C by a Peltier-type electronic temperature controllerrovided by the apparatus. Before the measurements the solu-ions were filtered with a membrane filter (0.25 �m pore size).iscosities of the solutions were determined using BrookfieldVDV-III Programmable Rheometer (cone-plate viscometer; Brook-eld Engineering Laboratory, USA), controlled by a computer. Thealues of mean hydrodynamic diameters reported in this paper areased on the volume weighted size distribution. The DLS data werenalyzed by the CONTIN method, supported by the producer.

The surface excess concentration, � max, and the minimum areaer surfactant molecule, Amin, at the air–solvent interface were cal-ulated using surface tension measurement values, given by Eqs. (1)nd (2) [58,59]:

max = − 1RT

[d�

d ln c

]T,p

(1)

min = 1016

NA�max(2)

here R is the gas constant, T the absolute temperature, NA thevogadro’s number, � the surface tension, and c is the surfactantoncentration in the solutions. Surface pressure at the CMC, ˘cmc,as obtained by means of Eq. (3):

CMC = �o − �CMC (3)

Gibbs free energy of micelle formation, �Gm, of the nonionicurfactant is related with CMC by the following formula [58,59]:

Gm = RT ln xm (4)

here xm is the CMC expressed in mole fraction units. The densi-ies of 1280 g/dm3 for [EMIM][BF4], 1375 g/dm3 for [EMIM][TfO],200 g/dm3 for [BMIM][BF4], 1366 g/dm3 for [BMIM][PF6],298 g/dm3 for [BMIM][TfO], 1239 g/dm3 for [HMIM][BF4] werepplied [60].

The enthalpic contribution to the micellization process, �Hm,as determined basing on �Gm, temperature dependence, usingibbs–Helmholtz relation as shown in Eq. (5):

Hm = −T2 ∂(�Gm/T)∂T

(5)

The standard entropy, �Sm, of the micellization was calculatedrom Eq. (6):

Sm = (�Hm − �Gm)T

(6)

here T is the temperature.

. Results and discussion

.1. Surface tension measurements

In this work we aimed to investigate the surface activity ofween 20 nonionic surfactant in homologue imidazolium ioniciquids with 1-ethyl-, 1-butyl-, 1-hexyl-3-methylimidazolium,nd 3-methyl-1-octylimidazolium cations, and a varietyf anions, namely tetrafluoroborate, hexafluorophosphate,

is(trifluoromethanesulfonyl)imide, and trifluoromethanesul-onate. The surface tension isotherms determined for Tween 20n ionic liquids solutions are shown in Figs. 2–5. Surface tension

easurements, as a function of Tween 20 concentrations in ionic

Fig. 4. Surface tension isotherms of Tween 20 in 1-hexyl-3-methylimidazoliumsalts: [HMIM][BF4] ( ), [HMIM][PF6] ( ), [HMIM][TfO] ( ), and [HMIM][Tf2N]( ) determined at 25 ◦C.

30 J. Łuczak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37

Fig. 5. Surface tension isotherms for Tween 20 surfactant in 3-methyl-1-octyl-i[

lm

fuwiotgfocm

hg[w0flsssnobiw3c[ofiiwobcTbt

other anions. Therefore, solubility and CMC of Tween 20 in ionicliquids with [PF6] anion are higher. Moreover, [TfO] anion is biggerthan both [PF6] and [BF4], however the relative strength of hydro-gen bonds between cation and anion in [TfO] salts was determined

midazolium salts: [OMIM][PF6] ( ), [OMIM][BF4] ( ), [OMIM][Tf2N] ( ), andOMIM][TfO] ( ) determined at 25 ◦C.

iquids, were used to describe its adsorption at air–IL interface andicellization.Surface tension results revealed that Tween 20 nonionic sur-

actant is able to reduce surface tension only of some ionic liquidssed in the study. In the investigated set of compounds, ionic liquidsith two and four carbon atoms in the hydrocarbon chain of the

midazolium cation, containing [BF4], [PF6], and [TfO] anions, werebserved to promote Tween 20 micellization. For the binary solu-ions an increase of Tween 20 concentration in ILs results in a typicalradual decrease of surface tension, revealing adsorption of the sur-actant molecules at the air–ionic liquid interface. The appearancef a discontinuity in the �–log c curve, occurring at the critical con-entration, indicates that Tween 20 begins to self-assembly intoicelles. Obtained CMC values are presented in Table 1.It was observed that CMC values depend on the length of the

ydrocarbon chain in the cation, and the nature of the anion. Elon-ation of the alkyl substituent in the imidazolium cation fromEMIM] to [HMIM] results in an increase of CMC for ionic liquidsith [BF4] anions (from 0.002 mol/L observed for [EMIM][BF4] to

.14 mol/L for [HMIM][BF4]). Similar observation may be madeor [TfO] based salts, however only for [EMIM] and [BMIM] ioniciquids. For [EMIM][TfO], the effectivity of surfactant to decreaseurface tension was clearly visible. However, for [BMIM][TfO] theurface tension decrease of only 3 mN/m was observed, what mayuggest a potential for assembling, however this particular caseeeds further investigation by other methods. Further elongationf the hydrocarbon substituent in the imidazolium ring to six car-on atoms results in an inhibition of micelle formation, as shown

n Fig. 4. Untypical behavior was detected for [HMIM][BF4] salt,here transition point was observed near the concentration of

0 mmol/L, indicating possible aggregates formation. However, thisoncentration was found to be very close to the CMC measured forBMIM][PF6]. Moreover, Wu et al. described presence two typesf aggregates in [BMIM][BF4] and [BMIM][PF6], implying that therst transition point does not constitute typical micelles formed

n aqueous solution [16]. Taking into account these two aspects,e assume that critical micellar concentration in a form usually

bserved in water (or the other ILs collected in Table 1) was proba-ly not achieved. Ionic liquids with 3-octyl-1-methylimidazoliumation were observed to be unable to support micelle formation of

ween 20. Relationship between CMC values and a number of car-on atoms in the alkyl substituent of 1-alkyl-3-methylimidazoliumetrafluoroborates, [AMIM][BF4], is presented Fig. 6.

Fig. 6. Dependence of Tween 20 critical micelle concentration on the alkyl chainlength in 1-alkyl-3-methylimidazolium tetrafluoroborate at 25 ◦C.

For [BMIM] cation, CMCs of Tween 20 varied as follows:[BF4] < [PF6] < [TfO]. Similarly, the nonionic surfactant solution in[EMIM][BF4] revealed lower CMC than in [EMIM][TfO]. To com-pare, the characteristic dependence of surface tension on Tween20 concentration was not observed in salts with [Tf2N] anion(Figs. 2 and 3). It was found that the dependence of CMC valueson the alkyl chain length in the imidazolium cation and the aniontype are inversely proportional to the sequences observed for pureILs surface tension values (Table 1) [52]. Accordingly, an increase ofIL’s ions hydrophobicity (that is proportionally related to the sizeof the cation and the anion) limits its ability to promote micelleformation, due to increased solvophilicity of the solvent. The low-est CMC, and therefore the highest ability to support self-assembly,was observed for ILs with the shortest chain and [BF4] anion, due tohigher solvophobicity of Tween 20 in [EMIM][BF4], in comparisonwith other compounds. Solvation of Tween 20 functional groupsby ILs results mainly from an ability of surfactant and IL to formhydrogen bonds. In Tween 20/ILs systems, hydrogen bond accep-tors, being ILs counterions, as well as surfactant oxyethylene units,compete with C(2)-H atoms in the imidazolium cation for hydrogenbonds formation, as shown in Fig. 7 [61].

Considering the anion type, the bigger it is, the higher is the diff-usive nature of the negative charge, and as a consequence, the moredelocalized it becomes. Such property provides a decrease of hydro-gen bonds ability to form between cation and anion (leading alsoto a depletion of surface tension of pure salts) [52], and thus proba-bly stronger interactions amongst imidazolium cations/anions andEO units. This allows to assume that Tween 20 undergoes weakerinteractions with salts containing [BF4] than with compounds with

Fig. 7. Schematic representation of main interactions between imidazolium IL ionsand the Tween surfactant.

J. Łuczak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37 31

Table 1Summary of the surface properties of Tween 20 in ionic liquids determined at 25 ◦C.

System � IL [mN/m] CMC [mmol/L] �CMC [mN/m] � max × 106 [mol/m2] Amin [nm2] ˘cmc [mN/m] �Gad [kJ/mol] �Gm [kJ/mol]

Tween 20 in water 72.3 0.09 38.4 3.2 0.5 33.9 −43.7 −32.0Tween 20 in [EMIM][BF4] 50.3 1.6 32.6 1.7 1.0 17.0 −31.1 −20.6Tween 20 in [EMIM][TfO] 40.6 5.0 31.5 1.1 1.4 8.7 −24.3 −17.3Tween 20 in [BMIM][BF4] 44.9 9.0 33.4 1.4 1.1 11.5 −20.9 −12.8Tween 20 in [BMIM][PF6] 44.3 29.8 32.8 0.9 1.8 11.2 −24.6 −12.2Tween 20 in [BMIM][TfO] 34.9 148.0 32.2 0.3 5.0 2.6 −12.1 −8.4Tween 20 in [HMIM][BF4] 39.2 140.0 33.8 0.5 3.1 5.4 −18.6 −8.2

� surfaa gy of m

tyaishim

werssn[sda

hoplzlvattclco2[sobiilid

[wMigraf

Parameters describing adsorption and micellization are shown inTable 2.

As shown in the plot, the addition of [BMIM][BF4] to[BMIM][TfO] solvent changes the route of surface tension

IL, Surface tension of pure ionic liquid; CMC, critical micelle concentration; �CMC,rea per molecule at the interface; �Gad, free energy of adsorption; �Gm, free ener

o be higher than in [PF6] salts, as confirmed by ESI-MS-MS anal-sis [52]. Nevertheless, additional H-bonding between OE groupsnd IL’s anion may be formed due to the presence of oxygen atomsn [TfO] anion, which finally may decrease solvophobicity of theurfactant. In addition, lower CMC in ionic liquids with shorterydrocarbon substituents may be also related with lower viscos-

ty, thus resulting in higher diffusion coefficient of the surfactantolecules [62].In ionic liquids with [Tf2N] anion, the biggest ILs’ anions

e evaluated, no micelle formation was detected in appliedxperimental conditions. The [Tf2N]-based ionic liquids presentemarkably low surface entropy, and as a consequence a goodurface organization and highly structured liquid phase [55]. Thetructure of such ILs can be described as large, well organizedon-polar domains, which provide high solubility of surfactants63]. What is more, pure ILs with [Tf2N] anion have relatively lowurface tension, therefore its further decrease may take place withifficulties [22]. Absence of micellization in [BMIM][Tf2N] waslready reported by Patrascu for C14E8 surfactant [11].

Critical micelle concentration values of Tween 20 in all ILs areigher than in aqueous solutions, mostly due to higher solubilityf the surfactant in employed ILs, which leads to weaker solvo-hobic interactions, and also increased viscosity of the system,

imiting diffusion of the molecules [22]. Adsorption and micelli-ation parameters, characterizing behavior of Tween 20 in ioniciquids, were collected and compared in Table 1. Surface tensionalues at CMC, �CMC, are in the range of 31–33 mN/m, and follown order of [BF4] > [PF6] > [TfO], which is inversely proportional tohe CMC sequence and directly proportional to surface tension ofhe pure ILs. Surface pressure values, ˘CMC, were observed to varyonsiderably within ILs used in this study. For [BF4]-based ioniciquids, the surface pressure and the maximum surface excess con-entration at the air–IL interface, � max are higher than for ILs withther anions. Consequently, estimated area per molecule of Tween0 at air–IL interface, Amin, appeared to be lower for salts withBF4] anion. Taking into account the chain length, hydrocarbon sub-tituents with lower number of carbon atoms gave higher valuesf ˘CMC, � max, and then lower values of Amin. Therefore, it maye concluded that surfactant film is significantly less firmly packed

n ILs with bigger anions and longer chains, resulting in differentnterfacial properties than in other ILs. Moreover, the ability of ioniciquids to support micelle formation is accompanied with increas-ng surface pressure, maximum surface excess concentration, andecreasing area per surfactant molecule.

Critical micelle concentration values of Tween 20 inBMIM][PF6] presented by Wu et al. [16] are similar to ours,hereas CMC for [BMIM][BF4] system we obtained was lower.oreover, the values of �CMC, Amin, and � max for the non-

onic surfactant C14E6 in [BMIM][PF6], determined by Inoue

roup, was 33.2 mN/m, 0.88 nm2 and 1.89 × 10−6 mol/m2,espectively [24]. Our results for Tween 20 in [BMIM][PF6]ppeared to be quite similar to C14E6 in [BMIM][PF6], apartrom CMC values, which were lower for Tween 20 (for C14E6

ce tension at CMC; � max, maximum surface excess concentration; Amin, minimumicellization.

0.054 mol/L from surface tension, and 0.070 mol/L from 1H NMRmeasurements).

3.2. Surface tension study on Tween 20 in[BMIM][BF4]/[BMIM][TfO] mixtures with different composition

Inoue and Maema demonstrated that aggregation of nonionicC12E6 in ILs may be controlled by mixing two salts having thesame anion but differing by the imidazolium ring chain length [25].In their work, highly solvophilic 1-hexyl-3-methylimidazoliumtetrafluoroborate [HMIM][BF4] was mixed with [EMIM][BF4], beingimmiscible with C12E6. As a consequence, a mixture with an abil-ity to promote micelle formation was tailored [26]. For furtherexamination of this behavior, we evaluated a possibility to mod-ify IL’s properties by mixing two 1-butyl-3-methylimidazoliumcompounds with different anions. Therefore, we determined aninfluence of [BMIM][BF4] addition to [BMIM][TfO] ionic liquid, onthe ability of the mixture to support Tween 20 micelle formation.We chose ionic liquids with [BF4] anion, due to the highest abilityto prompt micellization of Tween 20 among ILs used in this study.As the second compound we chose [BMIM][TfO], an IL with a weakability to promote self-assembly of the surfactant (Fig. 3).

Surface tension isotherms for Tween 20 in[BMIM][BF4]/[BMIM][TfO] mixtures with different mass com-position is shown in Fig. 8. For comparison, surface tension dataobtained for [BMIM][TfO] and [BMIM][BF4] are also included.

Fig. 8. Surface tension isotherms of Tween 20 in pure and mixed 1-butyl-3-methylimidazolium salts, [BMIM][TfO]/[BMIM][BF4] with different mass ratios:[BMIM][BF4] ( ), [BMIM][TfO]/[BMIM][BF4] 5:5 ( ), [BMIM][TfO]/[BMIM][BF4] 7:3( ), [BMIM][TfO]/[BMIM][BF4] 9:1 ( ), [BMIM][TfO] ( ).

32 J. Łuczak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37

Table 2Summary of the surface properties of Tween 20 in [BMIM][TfO]/[BMIM][BF4] mixtures with different mass fractions.

System Mass fraction CMC [mmol/L] �CMC [mN/m] � max × 106 [mol/m2] Amin [nm2] ˘cmc [mN/m] �Gm [kJ/mol]

[BMIM][TfO] 0 148 31.8 0.8 2.2 2.6 −10.1[BMIM][TfO]/[BMIM][BF4] 0.1 110 30.1 0.5 3.7 4.3 −9.2[BMIM][TfO]/[BMIM][BF4] 0.3 70 28.1 1.0 1.5 8.0 −10.1[BMIM][TfO]/[BMIM][BF4] 0.5 15 28.6 1.1 1.5 8.1 −14.2[BMIM][BF4] 1 9.0 33.4 1.4 1.1 11.5 −12.8

C surfaf

iwwh[i[pT[bTiitt

diipotafmtsba

FT

MC, Critical micelle concentration; �CMC, surface tension at CMC; � max, maximumree energy of micellization.

sotherms. Initial surface tension of the ionic liquid mixtures, asell as of the samples with low concentrations of Tween 20, risesith an increasing ratio of [BMIM][BF4] in the mixtures, due toigher surface tension value. Addition of only 0.1 mass fraction ofBMIM][BF4] changes surface properties of the mixture and facil-tates micelle formation in [BMIM][TfO] solutions. An increase ofBMIM][BF4] concentration in the mixtures enhances solvophobicroperties of the system, providing CMC decrease. Dependence ofween 20 CMC values in [BMIM][BF4]/[BMIM][TfO] mixtures onBMIM][BF4] mole fraction is presented in Fig. 9. The relationshipetween CMC values and [BMIM][BF4] mass fraction (includingween 20 in pure [BMIM][TfO]) was observed to be linear. Interest-ngly, CMC of Tween 20 in [BMIM][TfO]/[BMIM][BF4] 1:1 mixtures close to CMC value recorded for pure [BMIM][BF4] salt. However,he surface tension decrease provided by the surfactant is less effec-ive than in pure [BMIM][BF4] due to the presence of [BMIM][TfO].

The values of � max were observed to grow, whereas Amin toecrease with increasing mass fraction of [BMIM][BF4], also reflect-

ng more solvophobic interactions to take place, and therefore anncrease of surfactant concentration at the air–ILs interface. Onarallel, ˘cmc values also increase, underlining enhanced abilityf Tween 20 to reduce surface tension, as well as a tendency ofhe ILs mixture to undergo a better surface tension lowering withn increased amount of the solvophobic compound. The standardree energy of micellization decreases linearly with increasing

ass fraction of the more solvophobic component in the mix-

ure, [BMIM][BF4], revealing micellization process to become morepontaneous. The results suggest that surfactant CMC values maye modified by mixing two ILs with different anions as well asdjusting their amount in the solution.

ig. 9. Dependence of the critical micelle concentration A) and Gibbs free energy ofween 20 micelle formation B) on [BMIM][BF4] mass fraction in the IL mixtures.

ce excess concentration; Amin, minimum area per molecule at the interface; �Gm,

3.3. Temperature dependence of CMC and thermodynamicparameters of Tween 20 in [EMIM][TfO]

The surface tension method was also used to investigate thedependence of Tween 20 micellization in [EMIM][TfO] on the tem-perature. Thermodynamic parameters of this compound were notpresented in the literature yet. The variation of surface tensionisotherms determined in the temperature range of 15–35 ◦C isshown in Fig. 10. An increase of the temperature results in adecreased strength of hydrogen bonds and weaker van der Waalsinteractions, which are responsible for internal cohesive energy inionic liquids, justifying decrease of surface tension values of thefollowing solutions at different temperatures [53].

The dependence of the CMCs on temperature determinedfor Tween 20/[EMIM][TfO] system shows the U-shape relation(Fig. 11A) similar to the one observed for typical surfactants, as wellas for surfactant-type ionic liquids in aqueous solutions [28,64,65].According to the relations described for aqueous solutions, suchbehavior may indicate that in the temperatures below 20 ◦C micelli-zation is an endothermic process; however, with an increase of thetemperature the process becomes exothermic. The assumption wasconfirmed by the thermodynamic parameters, �Gm, �Hm, −T�Sm

calculated from the CMC vs temperature dependence, accordingto Eqs. (4)–(6) [19,23]. The temperature dependence of Tween 20nonionic surfactant parameters, �Gm, �Hm, −T�Sm, in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate is presented inFig. 11B. Entropy changes are given as −T�Sm term in order toclearly indicate its contribution to the free energy of micelle for-mation.

It was previously shown that interactions between cation and

anions in the IL lead to formation of highly organized structure ata nanomolecular scale [66,67]. Therefore, ionic liquids are consid-ered as a network of cations and anions interacting not only via

Fig. 10. Surface tension isotherms determined for Tween 20 in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate at different temperatures.

J. Łuczak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37 33

Fig. 11. A) Temperature dependence of CMC measured for Tween 20 in [EMIM][TfO] at different temperatures and B) thermodynamic parameters of micelle formation forT

ensotrtawappsmacbTateawaptt

tw

ttsctat

tcenttnma

ween 20 in [EMIM][TfO]: �Gm ( ), �Hm( ) and −T�Sm ( ).

lectrostatic forces but also forming extended hydrogen bondetworks [68,69]. An addition of a surfactant to this supramoleculartructure results in solvent–solute interactions, where counterionsf ILs, as well as oxyethylene (OE) units in the Tween 20, competeo preferentially interact with C(2)–H atoms in the imidazoliuming by hydrogen bonds formation (Fig. 7) [61]. An increase of theemperature may results in some changes among the interactionsffecting the final effect. What is more, the number of relativelyeak hydrogen and van der Waals interactions between surfactant

nd ionic liquid molecules becomes reduced. Thereby, the solvo-hobicity of Tween 20 in IL is increased, favoring micellizationrocess to occur at lower surfactant concentration. On the otheride, higher temperature influences also interactions between ILolecules in tridimensional ordered structure. It was revealed that

t elevated temperatures, van der Waals interactions between alkylhains of the imidazolium cations become reduced, and the saltsecomes dominated by electrostatic forces of the polar groups [70].he change of the interactions may result in the supramolecularggregate’s size reduction and ILs structural internal ordering, andherefore an increase of the structured solvent entropy. Since lowntropy of the solvent is the driving force of micelle formation,n increase of �Sm is unfavorable for micellization. In addition, theeaker interactions between IL’s molecules provide stronger inter-

ctions between IL and the surfactant impeding the self-assemblyrocess. Therefore, we may conclude that at lowered temperatureshe first effect described above is predominant, whereas at higherhe second one become more prevailing.

As mentioned before, at lower temperatures (up to about 20 ◦C)he Tween 20 micellization process is endothermic (�Hm > 0) andith increasing temperature becomes exothermic (�Hm < 0).

A positive contribution to �Hm results from the energy requiredo release the structural ionic liquid from the solvation layer aroundhe surfactant molecule which is necessary for hydrocarbons toelf-assembly into micelles. An opposite effect, being a negativeontribution to the total enthalpy of micellization, is related withhe transfer of Tween 20 molecules from IL’s solution to the micelle,nd a regeneration of the hydrogen bonding from between surfac-ant and solvent.

Free energy of micellization is negative over the whole range ofemperatures, indicating that micellization is a spontaneous pro-ess. The free energy depends on relative changes of enthalpy andntropy of the system and reflect CMC shifts. At lower temperaturesegative �Gm is controlled by highly, negative −T�Sm parameterhat transcends a positive �Hm contribution. At higher tempera-

ures, the relation of these two terms and their influence on theegative �Gm becomes conversed, therefore the driving force oficellization, �Sm, becomes positive at low temperatures, whereas

t elevated temperatures changes to an enthalpy-driven process.

Similar correlation between thermodynamic parametersdetermined for micellization of polyoxyethylene monoalkylethers was observed by Inoue group for C12E6 solution in[EMIM][BF4]/[HMIM][BF4] mixture [26] as well as for CnEm in[BMIM][BF4] [19,23], and [BMIM][PF6] [24]. For the nonionicsurfactant Tween 20, the thermodynamic studies were presentedfor [BMIM][BF4] and [BMIM][PF6] by Wu et al. [16].

3.4. Dynamic light scattering of Tween 20 surfactant in ionicliquids

In order to confirm the presence of micelles and compare thesize and distribution of the aggregates in selected ionic liquids,dynamic light scattering measurements were performed. Intensityof the scattered light for pure solvents, and below CMC was below1 nm, whereas for concentrations of surfactant above the CMC anincrease of the scattered light was observed, indicating the pres-ence of micellar aggregates. Below the CMC some particles withdiameter close to 1 nm were detected due to the presence of theTween 20 surfactant mainly in the monomeric form. After reachingCMC the scattering intensity increased and much bigger aggre-gates were observed. The exemplary results of size distribution ofTween 20 micellar aggregates in ILs determined for several concen-trations are presented in Figs. 12a and 13a. Changes of scatteredlight intensity for micellar solutions as a function of hydrodynamicdiameter (Dh) are shown in Figs. 12b and 13b. The average Dh ofTween 20 micelles in water at CMC was determined to be about8 nm, what fits with the literature value [71]. For [EMIM][BF4] atsurfactant concentration of 1.6 mmol/L (CMC) the particles of 8 nmwere detected, however at a concentration of 9.6 mmol/L (6 × CMC)the particle size increased to 25–27 nm. The average size of themicelles measured in [EMIM][TfO] at CMC (about 5 mmol/L) wasmuch smaller—about 2 nm, and increased to 5 nm at a concentra-tion of 9 mmol/L (1.8 × CMC). The smaller size of the micelles maybe related with the higher CMC, resulting from relatively strongerinteractions between surfactant and IL, and therefore higher sol-ubility due to lower interactions between ILs cation and anion.Further increase of Tween 20 concentration resulted in a signifi-cant rise of the average Dh up to 100 nm (2 × CMC), and even to200 nm (3 × CMC). The average diameter of micelles determinedin [BMIM][BF4] and [BMIM][PF6] at CMC was found to be 80 and50 nm, respectively. For both ILs, increase in surfactant concentra-tion resulted in rise of the average diameter up to around 200 nm.This phenomenon may be related with the liquid crystalline phase

formation at higher surfactant concentration. In comparison, for[BMIM][TfO] the DLS measurements revealed only structures withsize of 1–1.3 nm regardless the range up to 4 × CMC. This may indi-cate micelles formation, however it is ambiguous and needs further

34 J. Łuczak et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 471 (2015) 26–37

0

5

10

15

20

25

30

0 2 4 6 8 10

D [n

m]

c [mm ol/L]

(A)(B)

0

3

6

9

12

15

18

0.1 1 10 100

Inte

nsity

by

volu

me

[%]

D [n m]

CMC

1.5 x CMC

2 x CMC

4 x CMC

6 x CMC

Fig. 12. Size distribution determined for Tween 20 in [EMIM][BF4] at 25 ◦C A), mean hydrodynamic diameter as a function of the surfactant concentration B).

0

40

80

120

160

200

0 50 100 150

D [n

m]

c [mm ol/L]

(A)(B)

0

10

20

30

40

50

0.1 1 10 100

Inte

nsity

by

volu

me

[%]

D [nm]

CMC

2 x CMC 3 x CMC 2.5 x CMC

A), m

iaitgta

3

wWioaTmtrrhmsccft[

(

Fig. 13. Size distribution curves determined for Tween 20 in [BMIM][PF6]

nvestigation. The size of micelles formed in [HMIM][BF4] solutiont CMC was found to be around 10 nm in diameter, and significantlyncreases above CMC to about 100 nm. Those observations confirmhat solvophobicity of Tween 20 molecules decrease with the elon-ation of ILs chain. The results from DLS measurements show thathe micellar diameter strictly depends on surfactant concentrationnd increases significantly with surfactant content increase.

.5. Conductivity of Tween 20/IL binary systems

As it was mentioned above, OE groups of Tween 20 interactith the imidazolium ionic liquids by hydrogen bonds and van deraals interactions [16,72]. For aqueous solutions, every OE group

s associated with 2–3 water molecules, as determined by a varietyf analytical techniques, such as conductivity, dielectric relax-tion spectroscopy, infrared spectroscopy and NMR studies [73].he amount of solvent molecules associated with the surfactantay be also estimated basing on effective-medium approxima-

ion theories (EMT) for suspensions of spherical particles with aandom distribution, treating properties of the suspended mate-ial as the whole solution [74]. Up to date, a variety of theoriesave been proposed, such as a model shown by Looyenga, Brugge-an, or Maxwell–Wagner [75]. According to these theories micellar

olution of Tween 20/IL may be described as a dispersion systemontaining particles (micelles), of conductivity �p, dispersed in aonducting medium (ionic liquid), of conductivity �IL, at a volumeraction of ˚· In this respect, the effective-medium approximationheories may be described by Bruggeman approximation equations

72,73,75]:

� − �p

�IL − �p

)(�IL

�

)1/3= 1 − (7)

ean hydrodynamic diameter as a function of surfactant concentration B).

where � is the conductivity of the system. In addition, Looyengaapproximation may be presented as follows:

� = [(�1/3p − �1/3

IL )� + �1/3IL ]

3(8)

whereas Maxwell–Wagner equation is:

� = �IL�p + f�IL − f�(�IL − �p)�p + f�IL + �(�IL − �p)

(9)

where f is a shape factor that is 2 for spherical particles, and 1.5 isfor randomly orientated thin, rodlike particles.

Conductivity measurements of Tween 20/IL systems were car-ried out in order to determine the number of imidazolium cationsassociated with one OE unit of the surfactant. The conductivitydependence determined for different volume fractions of Tween 20in 1-butyl-3-methylimidazolium ionic liquid solutions are shownand compared in Fig. 14.

The electrical conductivity of the systems, normalized to thatof ionic liquid solvents, as a function of the volume fraction ofsurfactant is shown in Fig. 15. It was observed that the normalizedconductivity depends on the ionic liquids structure what may beexplained by difference in their viscosities. In addition, increasinginteractions between ionic liquids and Tween 20 surfactant mayresult in decreased flexibility of the poly(ethy1ene oxide) chainsof the surfactant, as it was already revealed in the literature forpoly(ethy1ene oxide) polymers in water [73,76].

Normalized conductivity data were analyzed using threealready mentioned effective-medium approximation theories. Acomparison of the experimental results determined for Tween

20/[BMIM][X] systems and conductivity values calculated basingon Looyenga, Bruggeman, and Maxwell–Wagner equations are alsopresented in Fig. 15. It was observed that the plots representingmicellar systems fit most to the Bruggeman approximation over

J. Łuczak et al. / Colloids and Surfaces A: Physic

Fig. 14. Dependence of the conductivity of Tween 20/IL solutions on the surfac-tant content measured at 25 ◦C at selected ionic liquids: ( ) [BMIM][BF4], ( )[BMIM][PF6], ( ) [BMIM][TfO], ( ) [BMIM][Tf2N].

Fig. 15. Dependence of normalized conductivity values of (�/�IL) on the volumefractions (�) of the surfactant in Tween 20/IL binary solutions ( ) [BMIM][BF4],(E

tbao

(

bpsmwacw11

facilitates micelle formation in [BMIM][TfO]. A linear relationshipbetween CMC values as well as the Gibbs enthalpy and [BMIM][BF4]

) [BMIM][PF6], ( ) [BMIM][TfO], ( ) [BMIM][Tf2N], (1) Eq. (8), (2) Eq. (7), (3)q. (9).

he whole concentration range, therefore the interpretation wasased on Eq. (7). Since conductivity of the ionic liquid, �IL, as wells conductivity of the system, �, are much higher than conductivityf the micelles, �p, Eq. (7) can be transformed to a following form:

�

�IL

)2/3= 1 − �eff (10)

Due to extended hydrogen and van der Waals interactionsetween ionic liquids and Tween 20, IL molecules may be incor-orated in the structure of the micelle. Therefore, consideringolvation of the surfactant molecules in the micellar solution, weust take into account an effective volume of the micelle, �eff = ��,hich involves the effective fractional volume of the dry surfactant

nd the volume of the IL molecules. For ionic liquids, parameter �alculated basing on that assumption is therefore found to decreaseith the cation size. The average values are: 1.05 for [HMIM][BF4],

.2 for [BMIM][BF4], 1.2 for [BMIM][PF6], and 1.3 for [BMIM][TfO],

.5 for [EMIM][BF4] and 1.3 for [EMIM][TfO].

ochem. Eng. Aspects 471 (2015) 26–37 35

The parameter � may be further used to determine the numberof ionic liquid cations associated with oxyethylene units of Tween20 (nac), by means of the following equation [72]:

nac = cac

cOE= cac

20cTw= (� − 1)

VTwMTw

20VcMc

where cac is concentration of the associated cations, cOE the con-centration of OE units, cTw the concentration of the Tween 20, VTw,MTw the volume and molar mass of Tween 20, Vc, Mc are the vol-ume and molar mass of IL cations. Therefore, � corresponds to thefollowing amounts of ILs cations associated with one OE group:0.15 for [HMIM][BF4], 0.8 for [BMIM][BF4], 0.8 for [BMIM][PF6], and0.9 for [BMIM][TfO], 2.1 for [EMIM][BF4], and 1.3 for [EMIM][TfO].Lower number of the 1-alkyl-3-methylimidazolium cations associ-ated with every OE group calculated for the ILs with longer chainsmay be related to higher volume of the cationic moiety. Hence, theability of cation to interact with OE group is hampered by steric hin-drance. In addition, the weaker cation–anion interactions become,the higher is the ability of IL cation to interact with OE groupsand incorporate in micelles. For comparison, Lian et al. investigatedTX-100/[BMIM][PF6] system and used Bruggeman’s approximationto calculate � value, which was 1.49, whereas nac was found to be1.09. The differences between their results and ours are related witha higher number of OE units (20), higher molar mass and volumeof Tween 20 surfactant in comparison with TX-100 as well as pres-ence of the additional �–� interactions between TX-100 and IL thatare not formed with Tween surfactants.

4. Conclusions

Micellar aggregation behavior of nonionic Tween 20 sur-factant in eleven ionic liquids, 1-ethyl-, 1-butyl, 1-hexyl-3-methylimidazolium and 3-methyl-1-octylimidazolium saltswith hexafluorophosphate, tetrafluoroborate, trifluoromethane-sulfonate and bis(trifluormethanesulfonyl)imide anions was inves-tigated by means of surface tension and conductivity measure-ments. It was shown that ionic liquid structure, both cation andanion species, affects amphiphile self-assembly ability. Shorten-ing of the hydrocarbon substituent in the imidazolium cation aswell as selection of the anion with smaller size and less diffusenature, strongly facilitates micelle formation. In the investigatedset of compounds, mainly ionic liquids with two and four carbonatoms in the alkyl substituent of the imidazolium cation, and [BF4],[PF6], and [TfO] anions were observed to promote Tween 20 mice-llization. Micellar aggregate formation was not found in [OMIM]ionic liquids, suggesting that the solvophobic interactions betweenIL and surfactant are too weak. The presence of micelles in ionicliquids was confirmed by dynamic light scattering measurements.The results revealed that the micellar diameter strictly dependson surfactant concentration and increases significantly with sur-factant content increase. It was also shown that Tween 20 revealslower ability to reduce surface tension, and therefore surfactantfilm is significantly less firmly packed in IL than in water. Similarly,the maximum surface excess concentration at the air–IL interfaceis much lower in ionic liquid solutions than in water. The sur-face tension measurements were also applied to investigate thetemperature dependence of the CMC of Tween 20 in [EMIM][TfO].The micellization process was found to be endothermic at lowertemperatures and changes to exothermic with the increase in tem-perature.

Addition of [BMIM][BF4], the ionic liquid with a good abilityto support micelle formation, changes the surface properties and

mass fraction in the ionic liquids mixture was observed, how-ever, the surface tension decrease due to the addition of Tween 20

3 Physic

svma

aiaaccecli

uacaiwo

A

(g2

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

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[

[

[

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[

[44] C.R. López-Barrón, N.J. Wagner, Structural rransitions of CTAB micelles in a

6 J. Łuczak et al. / Colloids and Surfaces A:

urfactant was not as effective as in pure [BMIM][BF4]. This obser-ation reflects the possibility to tailor surfactant CMC values byixing ionic liquids with different anions as well as adjusting their

mount in the solutions.Micellar solutions of Tween 20/IL systems were also analyzed

s dispersion systems according to the effective-medium approx-mation theories. At higher concentrations of the surfactant in ILs,

good fitting with Bruggeman approximation was detected, actu-lly following the assumptions of this model. However, for loweroncentrations„ the fit of normalized conductivity data becomesloser to Looyenga theory. The number of IL cations associated withvery oxyethylene unit depends on the ionic liquid structure (bothation and anion). The weaker cation–anion interactions and theower volume of the cation, the higher is the ability of IL cations tonteract with oxyethylene groups and intercalate the micelle.

This work provides a further insight into the role of ionic liq-id structure (alkyl chain in the imidazolium cation and type ofnion) in the ability to support the self-assembly of amphiphilicompounds. Amphiphile aggregates have a potential to be used as

matrix in a wide range of applications, including formation ofnorganic nanostructures, separation processes, organic synthesis,

etting, lubrication, encapsulation, and controlled release amongthers [20].

cknowledgement

Financial support was provided by National Science CenterNCN) within program OPUS 3, grant entitled: Study on aggre-ation behavior of nonionic surfactants in ionic liquids, no.012/05/B/ST4/02023.

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