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Micellizationandinterfacialpropertiesofalkyloxyethylenesulfatesurfactantsinthepresenceofmultivalentcounterions

ARTICLEinJOURNALOFCOLLOIDANDINTERFACESCIENCE·JUNE2003

ImpactFactor:3.37·DOI:10.1016/S0021-9797(03)00027-4·Source:PubMed

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RossitzaAlargova

InfinityPharmaceuticals

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JordanTPetkov

Unilever

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DimiterPetsev

UniversityofNewMexico

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Journal of Colloid and Interface Science 261 (2003) 1–11www.elsevier.com/locate/jcis

Feature article

Micellization and interfacial properties of alkyloxyethylene sulfatesurfactants in the presence of multivalent counterions

R.G. Alargova,a J.T. Petkov,b and D.N. Petsevc,∗

a The DEEPSTAR Group, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka-shi 237-0061, Japanb Colloid Science Unit, DP 2-4, Unilever Research Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, UK

c Center for Microgravity and Materials Research, Von Braun Research Hall, University of Alabama in Huntsville, Huntsville, AL 35899, USA

Received 26 June 2002; accepted 3 January 2003

Abstract

Alkyloxyethylene sulfates are a special class of surfactants that are unusually stable in the presence of multivalent counterionot as prone to precipitation as anionic surfactants without intermediate ethoxy groups in the molecule. However, formation oftheir structure, and the properties of monolayers of these surfactants exhibit very interesting and sometimes unexpected propertieon the nature of the ions dissolved in the solution. This paper presents a brief overview of our recent efforts to reveal the naturproperties, including some new results. We show that the strong binding of multivalent (and particularly trivalent counterions) tsphere-to-cylinder shape transition of the micelles and facilitates their further growth, even at very low ionic strength. The propsurfactant monolayers are coupled to those of the micelles in the bulk and are governed also by multivalent counterion binding.of multivalent counterions on the aggregation and structure formation in anionic surfactant solutions has both fundamental andimportance. 2003 Elsevier Science (USA). All rights reserved.

Keywords:Micellization; Micellar shape; Multivalent counterions; Micelles; Anionic surfactants; Alkyloxyethylene sulfate

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1. Introduction

Micellization and mono- or bilayer structure formatioof amphiphilic molecules (surfactants, lipids, block copomers) in water is a very interesting and important phenenon. Driven by the opposing hydrophilic and hydrophoforces, such molecules tend to form compact supramoular structures in the bulk or ordered layers at interfacThe origin of these opposing forces could be found instructure of the molecules, which typically consists of adrocarbon hydrophobic part linked to a hydrophilic one tmay be a charged or an uncharged group [1,2]. Formaof different amphiphilic molecular structures commonly ocurs in nature and has many practical and industrial apcations. The formation of living structures like bilayer lip

* Corresponding author. Present address: Department of ChemicNuclear Engineering, 209 Farris Engineering Center, The UniversitNew Mexico, Albuquerque, NM 87131, USA.

E-mail addresses:dimiter@cmmr.uah.edu, dimiter@unm.edu(D.N. Petsev).

0021-9797/03/$ – see front matter 2003 Elsevier Science (USA). All rights rdoi:10.1016/S0021-9797(03)00027-4

membranes in cells, the increasingly important problemcontrolled self-assembly of surfactants and surfactant-bstructures and templates at the nanometer scale, and evtrivial and routine everyday activities related to washing acleaning, all are governed by the same underlying phyand chemistry. Therefore, a detailed knowledge of theture of self-assembly of surface-active molecules has afundamental and yet practical significance.

Based on the nature of the hydrophilic group, surfactacould be specified as ionic (the hydrophilic groups caa charge in aqueous media) and nonionic (no chargespresent) [1]. Ionic surfactants on the other hand couldanionic (i.e., with negative charge) and cationic (positivcharged). The contribution of the surfactant head grouthe energy of micellization is much less in comparisonthe hydrophobic chain. However, its role is still essentialcause the nature of the hydrophilic head and the interacbetween the headgroups determine the size and the shathe surfactant aggregates [2]. Not surprisingly, many ofproperties of such surfactants in solution are determinethe amount and type of the background electrolyte prese

eserved.

2 R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11

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the bulk. In this paper we discuss some properties of ani(negatively charged) surfactants only.

Micelle formation of anionic surfactants in the presenof monovalent electrolyte has been studied in detail [3These studies showed that at high concentrations of 1:1trolyte (e.g., NaCl) spherical anionic micelles transform icylindrically shaped aggregates. The shape transformis also accompanied by a substantial increase of the mlar aggregation numbers. Studying the effect of multivacounterions, however, is much more difficult due to thethat normally such additives lead to surfactant precipitatOne way to overcome this problem is to use a special cof surfactants that contain a few oxyethylene groups in tmolecule. These groups stabilize further the surfactantsthey are much less prone to precipitation in the presencmultivalent counterions.

In this paper we present an overview on the impof multivalent counterions on the self-assembly of anioalkyloxyethylene surfactants into micelles or monolayerinterfaces. These systems exhibit qualitatively differenthavior when compared to anionic surfactants in the presof monovalent electrolyte. That is why they represent funmental interest and have potential for various practicalplications. We show that the critical micellar concentrat(CMC) is determined mostly by the overall ionic strengof the background electrolyte solution and is not partilarly sensitive to the specific type of ions present. Thecellar structure and shape, however, depend strongly ocounterion charge numbers. We demonstrate that mullent counterions induce sphere to cylinder shape transiin micelles, even when present in very low concentrati(compared to shape transitions induced by 1:1 electrolThe transition in shape occurs when the number of anisurfactant molecules in micellar form is approximately eqto the amount of multivalent counterions multiplied by threspective charge number. In the vicinity of this appromately stoichiometric ratio, one may observe a numbeunexpected and counterintuitive phenomena like increathe solution surface tension upon increase of the surfaconcentration in the bulk or an increase of the micellarwhen the overall surfactant concentration decreases. Adiscussed above, there are not many studies of anionicfactant solutions containing multivalent counterions in retively high (comparable to the surfactant) concentrationsto stability problems. Therefore, the aim of the present pais not to exhaust the subject but rather to stimulate intein these systems. In the last section we formulate somthe questions that have to be answered next to furtheunderstanding of micellization and monolayer formationionic surfactants in the presence of multivalent counterand other additives (e.g., nonionic surfactants).

Besides the fundamental interest, solutions of ionicfactants in the presence of multivalent counterions occumany practical applications. Typical examples are desing shampoos or laundry detergents as well as environmtal applications (e.g., removal of toxic metals from was

-

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-

water streams by micellar-enhanced ultrafiltration [9,1Micelles with controlled size and shape could be usedtemplates for making nanoparticles for different nanotenology applications [11].

The experiments reported in the present review aresodium dodecyl dioxyethylene sulfate (SDP-2S). Asmentioned above, the presence of the oxyethylene grouthe molecule improves the stability of the anionic surfacagainst precipitation that may otherwise occur in the pence of multivalent counterions. The temperature of allperiments was 27◦C.

2. Micellization of sodium dodecyl dioxyethylene sulfatein the presence of trivalent (Al3+) counterions

2.1. Size and shape of the micelles

A detailed account of the micelle formation size aproperties is given in [12]. The main experimental methused there is dynamic light scattering (DLS), which msures the micellar diffusion coefficient,D [13]. The latter isthen related to the hydrodynamic radius,Rh, by the Stokes–Einstein relationship [14]

(2.1)D = kT

6πηRh

,

whereη is the solvent viscosity andkT is the thermal energyNote that, in the case of an anisotropic micelle,Rh hasthe meaning of an effective hydrodynamic radius as defiby Eq. (2.1). Interactions between the diffusing particmay lead to a concentration dependence of the diffucoefficient and the apparent hydrodynamic radius [which are linear for diluted dispersions [16,17]

D =D0(1+ λφ),

(2.2)

Rh = kT

6πηD0(1+ λφ)≈ kT

6πηD0(1− λφ) ∼Rh,0(1− λφ),

whereD0 andRh,0 are the values of the diffusion coefficieand hydrodynamic radius at infinite dilution (no effeof the interactions are present),φ is the micellar volumefraction, andλ is a parameter that accounts for the miceinteractions. Equation (2.2) predicts a linear decreasthe hydrodynamic radius with the micellar concentratif the micelles interact predominantly by repulsive forcand an increase inRh if the intermicellar interactions arattractive. The importance of the interactions on the appadiffusion coefficient (and hydrodynamic radius) has banalyzed in detail elsewhere [18]. The analysis showsfor diluted surfactant solutions the effects of interactionsthe apparent micellar size could be neglected. Most ofexperimental systems considered in this paper are withinconcentration range with a few exceptions discussed be

Figure 1a shows the apparent hydrodynamic radRh, of SDP-2S micelles. Curve 1 corresponds to an io

R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11 3

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Fig. 1. (a) Apparent hydrodynamic radius (Rh) of SDP-2S micelles vsξ .Curve 1 corresponds to an ionic strength of 0.024 M and a constant amof Al3+, CAl = 0.004 M; the surfactant concentration,Ct , is variable.Curve 2 is for the same ionic strength, constant amount of surfaCt = 0.008 M, and a variable concentration of Al3+. Curve 3 correspondto variation of the Al3+ at an ionic strength of 0.064 M andCt = 0.004M. The curves are only a guide for the eye. (Reproduced with permisfrom Langmuir 11 (1995) 1530–1536. Copyright 1995 Am. Chem. So(b) Schematic presentation of trimer formation.

strength of 0.024 M and a constant amount of Al3+,CAl = 0.004 M; the surfactant concentration,Ct , is variable.Curve 2 is for the same ionic strength, a constant amoof surfactantCt = 0.008 M, and a variable concentrationAl3+. Curve 3 corresponds to variation of the Al3+ at anionic strength of 0.064 M and constantCt = 0.004 M. Theoverall ionic strength for each curve is maintained consby adjusting it with the addition of appropriate amouof NaCl, where necessary. Therefore, SDP-2S micellesformed in a mixture of mono- and trivalent counterioHowever, it turns out that the Al3+ ions play a determiningrole for the micellar size and shape. This is clearly sby the behavior of the curves in Fig. 1, which showdependence ofRh on the dimensionless variable

(2.3)ξ = Ct − CMC

zMCM

.

Ct is the total molar concentration of SDP-2S and CMis the critical micellization concentration (usuallyCt �

CMC). Ct – CMC is the concentration of surfactant in tmicellar phase only. With CM we denote the molar concetration of the respective multivalent metal counterion (Al3+for Fig. 1a) andzM is its charge number (3 for Al). The varableξ then has the meaning of dimensionless concentraof surfactant in the micellar phase, but in general it depealso on the concentration of the multivalent counterioCurve 1 is for a constant concentration of Al3+ (0.004 M)and ionic strength equal to 0.024 M. The hydrodynamicdius exhibits a strong nonlinear dependence on the contration of surfactant in the micellar phase (orξ). This resultcould be explained if we assume that there is a changthe size (growth) and structure (i.e., shape) of the micellethe solution. Micellar shape transitions (sphere to cylindand subsequent growth are known to occur in anionic surtant solutions in the presence of high electrolyte concentions between 0.5 and 0.6 M [3,4,8,19,20]. The overall iostrength for the case depicted in Fig. 1a, however, is mlower, equal to 0.024 M. Clearly the presence of multivalcounterions (Al3+ in Fig. 1a) leads to a stronger tendenfor micellar growth than monovalent electrolytes. Even msurprising is the fact that the decreasing the surfactantcentration facilitates the micellar growth. At first glance tis an unexpected and counterintuitive phenomenon, wdoes not occur if only monovalent salt is present, irresptive of how high its concentration. One would expect tincreasing the surfactant concentration should increasemicellar size since more molecules are available to “bubigger micelles. However, the reason for micellar growis the presence of multivalent counterions (Al3+) and theirquantity relative to that of the surfactant in micellar forWith decreases in the surfactant concentration, the relaamount of multivalent counterions increases and facilitafurther micellar growth. The sharp size change occurs invicinity of ξ = 1. At this point the number of anionic sufactant molecules in the micellar phase is equal to the nber of available Al3+ ions multiplied by their charge numbeCt − CMC = 3CAl . All solutions used in our experimenwere at pH between 3 and 4 and under such conditionsof the Al counterions (> 98%) are in trivalent form; see [21and the discussion in Ref. [22]. Multivalent counterions teto bind strongly to micellar surfaces [10] and by doing tthey decrease the electrostatic repulsion between thefactant headgroups in the micelles, bringing them closegether. This leads to a decrease in the local curvature ancilitates a transition from spherical (greater local curvatuto cylindrical (lower local curvature) shape (see Fig. 1b).ξ < 1 the Al3+ ions are in excess in comparison with the sfactant anions in the micelles and that facilitates the groof rod-shaped micellar aggregates. Atξ > 1 there are lessAl3+ ions than surfactant anions in the micelles and thegregates are small and spherical. Hence, the key factthe observed micellar shape transition and growth is themation of a triplet of surfactant ions in the micelle that abound together by an Al3+ counterion (see Fig. 1b). Curvein Fig. 1a confirms this hypothesis. Here, the concentra

4 R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11

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of surfactant is constant (0.008 M). This trend of bindbetween surfactant molecules and Al3+ counterions in a 3:1ratio was also confirmed by independent ultrafiltrationperiments [22]. The concentration of Al3+ ions varies, buthe overall ionic strength is maintained constant, 0.024by adjusting it with appropriate amounts of NaCl. Thus,are able to trace the same range of values ofξ , althoughvarying a different experimental parameter (CAl instead ofCt − CMC). The two curves match perfectly in the viciniof ξ = 1. This proves that the important variable for inducshape transitions in micelles, when multivalent counterare present, is the molar ratioξ . A theoretical model baseon the notion of preferential surface binding of Al3+ as de-picted in Fig. 1b was developed [22] and was successapplied to explain experimental results. Some discrepanare observed between curves 1 and 2 at very low andvalues ofξ , far from ξ = 1. When the surfactant concentrtion is varied, keepingCAl constant, very low values forCt

are needed to be able to reach lowξ . This however leads tsurfactant deficiency and therefore inhibition of the micegrowth, although it might be facilitated by a favorable sfactant to multivalent counterion ratio. There is not enomaterial to supply the growing micelles, and the growinduced by the multivalent counterions, is opposed bytendency for greater entropy (translational and rotatiothat favors a greater number of smaller aggregates. Cleif the concentration of surfactant goes to zero, the micewill disappear. At the other limit (high surfactant concetrations) we observe smaller micelles in curve 1 (variasurfactant concentration) than in curve 2 (variable conctration of trivalent counterions). This is an example wherhigh surfactant concentration repulsive interactions likecluded volume start to become detectable, which leada decrease in the apparent hydrodynamic radius as dmined by dynamic light scattering; see Eq. (2.2). A mdetailed analysis of the importance of the micellar intertions is given in Ref. [22]. Curve 3 is obtained by vaing the amount of Al3+ counterions, keeping the concenttion of surfactant constant, similar to the case representecurve 2. The ionic strength in this case (curve 3), howevehigher (0.064 M), mainly due to the addition of NaCl. Suincreased ionic strength, compared to curve 2, leads to asloped curve aroundξ = 1. The reason for this effect is ththe monovalent Na+ ions, now in a much greater amoucompete with the trivalent Al3+ counterions in screening thmicellar surface charge, thus leading to less abrupt strucchange.

The CMC of ionic surfactants is mostly dependentthe ionic strength of the bulk solution and to a much lesextent on the nature of the specific counterions [1,23].main difference in the impact of ions with different charon the CMC is due mostly to their different contributito the overall ionic strength when added in the saconcentrations. In each set of our experiments we variedamount of multivalent and monovalent counterions in suway that the overall ionic strength remained constant.

-

s

l

ionic strength equal to 0.024 M we have CMC= 1.33 ×10−4 M, while for higher ionic strength, 0.064 M, CMC=0.78× 10−4 M.

Summarizing the experimental observations descrabove, as well as data from the literature [3,4,8,19,we conclude that the growth of anionic micelles inpresence of multivalent counterions depends not onlythe surfactant concentration and overall ionic strengthalso and most importantly on the ratio of the surfactamount in the micellar phase over the amount of multivacounterions that is the quantityξ ; see Eq. (2.3). Sincthe ionic strength in general includes contributions frother ionic species besides the multivalent counterions, tthree parameters are independent. Figure 1a showsvaryingξ , the concentration of surfactant, or concentratof multivalent counterions leads to almost identical curvethe overall ionic strength is constant.

The results presented in Fig. 1a show that at highues ofξ SDP-2S micelles are small and spherical. It isteresting to examine the structure and shape of the lamicelles formed atξ < 1. The behavior of the micellar hydrodynamic radius upon dilution suggests the formatiorodlike micelles similar to the case of monovalent counrions [5,7]. We analyzed the shape of the large micelleexamining the ratio of the gyration radius over the hyddynamic radiusρm = Rg/Rh [12,22]. It was shown to bρm = 2, which is in the range between 1.35 and 4.01, whcorresponds to elongated rod-shaped micelles [24]. Thetual geometric parameters of the micelles could be direrelated to the diffusion coefficient and hence to the effechydrodynamic radius [12,25,26].

2.2. Thermodynamic analysis of the micellar growth

Knowing the geometrical parameters of the micelles frour DLS measurements and literature data for the pmeters of a single surfactant molecule [1,2], we can emate the aggregation number of the cylindrical micelledividing the volume of the hydrophobic core by the vume of a single hydrocarbon chain in a SDP-2S mcule. Thus, for the length of the hydrophobic tail eqto 1.672 nm, the volume of the hydrophobic and hydratail equal to 0.3502 nm3, and the total length of the hydrophilic group (two oxyethylene groups plus the charsulfate head) equal to 1.1 nm, we are able to calculate thgregation numbers [12,22]. Thermodynamic analysis ofcellization [2,5], developed for the case of a 1:1 electrolpredicts that the mass average aggregation number micshould increase linearly with the square root of the surfacconcentration,

(2.4)n̄M ≈ n0 + 2[K(X−XCMC)

]1/2,

wheren0 is the aggregation number of a spherical micewith minimal size,X is the total molar fraction of surfactanXCMC is the critical micellization molar fraction, andK is amicellization parameter [5]. All configurations withn < n0

R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11 5

theallest

t bye

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con-on

acebe-also

a

e

mi-of

t atlar-ons.

atac-cal

meen

and

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Fig. 2. Interpretation of data for micellar growth. The intercept andslope of the solid line correspond to the aggregation number of the smmicellen0 = 56 andKndl = 1.08×107. (Reproduced with permission fromLangmuir 14 (1998) 4036–4049. Copyright 1998 Am. Chem. Soc.).

are energetically unfavorable and are not proved to existhe experiment. Micelles withn > n0 are assumed to bspherocylinders.

The analysis of SDP-2S micelles in the presence of trlent counterions [22] showed that the above “ladder” mocould still be applied with some modification: the constK in Eq. (2.4) is not a true constant in a mixture of monand multivalent electrolytes, where the micellar growthinduced predominantly by the presence of multivalent coterions. It could be represented as a productK = KndlKdl,thus separating the non-double-layer and double-layertributions.Kdl is a function of the surfactant concentratibecause the background concentrations of Al3+ and Na+and correspondingly their adsorption to the micellar surfchange upon dilution of the samples. The competitiontween mono- and trivalent ions in the adsorption processbecomes important at low concentrations of Al3+ since thepredominant part of the ionic strength is maintained by N+ions. After accounting for these effects and calculatingKdl

at every point, one obtains a linear dependence ofnM on[Kdl(X − XCMC)]1/2 as shown in Fig. 2; for details of thanalysis see [22].

2.3. Solubilization properties

An important practical consequence of the enhancedcellar growth is the increase in the solubilization capacitythe micelles with respect to oils; see Fig. 3. It shows thaconstantsurfactant concentration the oil included in micelaggregates increases with the decrease inξ and behaves similarly to the micelle size measured at the same conditiThe largest SDP-2S micelles atξ < 1 solubilize almost 3times more oil in comparison with spherical ones formedξ > 1. The significant increase in the solubilization capity of SDP-2S micelles is due to the decrease of the locurvature. This allows for a greater internal micellar voluto be accessible for oil incorporation. As the ratio betwe

Fig. 3. Apparent hydrodynamic radius (Rh) of empty SDP-2S micelles(curve 1), oil-containing micelles in the presence of xylene (curve 2),solubilized amount of xylene (Voil/Vsolution) (curve 3) as a function ofξ .The parameters are the same as those for curve 2 in Fig. 1a. The curvonly a guide for the eye. (Reproduced with permission from Langmui(1995) 1530–1536. Copyright 1995 Am. Chem. Soc.).

Fig. 4. Solubilization capacity (Voil/Vsolution) (curve 1) and apparenhydrodynamic radius,Rh (curve 2), of oil-containing SDP-2S micellemeasured in the presence of counterions of different valence (Na+, Ca2+,and Al3+) at constant the surfactant concentration isCt = 0.008 M.The ionic strength is 0.024 M. The curves are only a guide for the(Reproduced with permission from Langmuir 11 (1995) 1530–1536. Cright 1995 Am. Chem. Soc.).

surfactant molecules and multivalent counterions decreathe charge screening between the head groups in the mincreases and the local curvature decreases.

The importance of the counterion valence for the solulization process is explicitly shown in Fig. 4 where the subilization capacity of SDP-2S micelles, in the presenceAl3+, is compared to the ones measured in the presenNa+ and Ca2+ only. The amount of solubilized oil (curve 1increases from mono- to divalent and further to trivalent iin accordance with the corresponding increase in the etive micellar size (curve 2).

The solubilization capacity is related to the cleaningtion of surfactant solutions. Hence, the addition of an apppriate amount of multivalent counterions could improveperformance of the composition.

6 R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11

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2.4. Influence of nonionic surfactant additives onthe size of SDP-2S micelles formed in the presence ofAl3+ counterions

The data presented above demonstrate that the ratioξ isan important parameter that governs the sphere-to-rodsition in the micellar shape. It occurs aroundξ = 1 where theamount of Al3+ is enough to saturate all the surface charof a micelle through formation of trimers. Hence, any perbation of the trimer formation on the micellar surface woeventually inhibit the micellar growth. A possibility for sucperturbation is the addition of small amounts of noniosurfactants [27]. In this section we briefly present somedata for mixed anionic/nonionic micelles in the presenctrivalent metal counterions (Al3+). We used two types ononionic surfactants with different sizes of the hydrophhead groups: (i) polyoxyethylene-4 dodecyl ether (C12E4)that has a hydrophilic head with length (1.5 nm [28]) comrable to that of SDP-2S (1.1 nm) and (ii) polyoxyethyle10 dodecyl ether (C12E10) with a much larger head grou(2.1 nm [28]). Pure C12E4 has very low solubility in wateat room temperature and does not form micelles, but inpresence of SDP-2S its solubility increases dramatically

-

to formation of mixed micelles. The micelles formed by puC12E10 at the conditions studied (the same temperatureAl3+ concentration) are small (mean radius around 3.5and spherical. The distinct behavior of these two surfacthaving the same length of the hydrophobic chain is duthe quite different size of the oxyethylene chain servinga polar head in both cases. Hence, one can expect diffeffects on the micellar properties when C12E4 and C12E10

monomers are introduced into SDP-2S micelles. This is cfirmed by our results. Figure 5a shows the micellar size msured at 0.2 molar fraction of the nonionic monomers insurfactant mixture as a function ofξ . At all values ofξ stud-ied C12E4 leads to the formation of even bigger mixed agregates compared to pure SDP-2S micelles, while incase of C12E10 the trend is opposite. The behavior of C12E10

is more understandable since the larger hydrophilic groare expected to increase the repulsion between the surfaheadgroups, introducing a steric component. It is also poble that the large hydrated ethoxy groups of C12E10 hinderthe adsorption of trivalent counterions on the micellar sface (see Fig. 5b), thus preventing the formation of trimBoth effects would lead to smaller micelles.

nonionicformed

(a) (b)

(c) (d)

Fig. 5. (a) Apparent hydrodynamic radius,Rh, of SDP-2S micelles: pure (solid circles) and mixed with C12E10 (open boxes) or C12E4 (open triangles) inthe presence of Al3+ at 0.024 M ionic strength. SDP-2S concentration is 0.008 M for all curves. In the case of mixtures the molar fraction of thesurfactant is 0.2.ξ is varied through the amount of trivalent ions. The curves are only a guide for the eye. (From Ref. [27]). (b) A part of a mixed micellein the presence of C12E10. (c) A part of a mixed micelle formed in the presence of C12E4. (d) Separating of SDP-2S/Al3+ trimers by C12E4 monomers.

R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11 7

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Addition of C12E4 probably does not prevent the inteaction of ionic surfactants in the micelle with Al3+ counte-rions. A speculation of how this interaction occurs is shoschematically in Fig. 5c. In such case a two-component mture of trimers and C12E4 molecules forms into the micellabody. The mixing leads to entropy gain but it is also favoby the fact that the trimers have dipole moments and tto repel each other. The nonionic C12E4 dilutes the trimersformed in the micelle. Therefore, mixing with C12E4 willhelp separate the trimers (see Fig. 5d), which is favoraThe idea of trimers formation in pure anionic micelles wused in developing the thermodynamic model in Ref. [2The success of the model in describing the system coulregarded as an indication that the trimer formation is a psible physical picture. Whether it occurs in mixed micellhowever, is an open question. Actually, mixed micellesthe presence of multivalent counterions require furtherthorough investigation to be fully understood.

2.5. Micellization of SDP-2S in the presence of mono-, dand trivalent counterions: a comparison

At the end of this section we compare the micellizatof SDP-2S micelles in the presence of various electrolin an attempt to elucidate further the role of the valeand the specific type of the counterion. It is shownRefs. [12] and [18] that the transition in the micellshape in the presence of Ca2+ and/or Na+ occurs at muchhigher overall ionic strength than in the case of Al3+.The results summarized in Fig. 6 show the hydrodynaradius of micelles formed in the presence of 1:1, 2:1,3:1 electrolytes (various chlorides) at constant surfacconcentration. [27]. The dashed line presents the vof Rh below which the micelles are small and sphericFor each curve the ionic strength is varied, increasingconcentration of a given salt, as opposed to the data in Fi

Fig. 6. Apparent hydrodynamic radius,Rh, of SDP-2S micelles measureas a function of the total ionic strength (I0) created by various chloridesSurfactant concentration is 0.008 M for all curves. The curves are onguide for the eye. (The curves for Na+, Ca2+, and Al3+ are reproducedwith permission from Langmuir 11 (1995) 1530–1536. Copyright 1995 AChem. Soc.; the rest of the curves are from Ref. [27]).

where the total ionic strength was maintained constant wvarying the concentrations of the different ionic species.

It is evident that beyond certain values of the iostrength almost all curves show an abrupt increase inmicellar size. These values of the ionic strengths areferent for different salts used and depend not only onvalence but also on the specific type of metal ions presFor NaCl the transition occurs at about 0.5–0.6 M as isserved for the case of sodium dodecyl sulfate [1,3,5,8,19In the presence of K+ large micelles are formed at loweionic strength, similarly to the results reported in Ref. [For NH+

4 the situation is intermediate between the two alline ions. More complex is the situation with divalent couterions: the curves are not as steep as those obtained foand 3:1 electrolytes and Ca2+ counterions lead to the formation of rodlike micelles at 0.024–0.030 M ionic strengwhile in the case of Mg2+ the transition value ofI0 is closeto that in the case of K+ but still lower than that for Na+.In the case of trivalent ions the shape transition occurs asame value of the ionic strength and the curves for Al3+ andCr3+ differ only slightly.

Clearly the difference in the micellar growth in the preence of monovalent or divalent counterions only cannoexplained completely by a simple electrostatic argumThe differences between ions of the same valence are reto their individual properties of size, hydration, polarizabity, ability for specific chemical interaction with surfactaheads, etc. Missel et al. conducted an extensive study oinfluence of alkali metal ions on the size of SDS micellesThey have shown that the degree of cylindrical growthmarkedly dependent on counterion identity and varies wthe inverse of their hydrated radii mainly through electrostic effects. By virtue of their finite size different counteons could affect the electrostatic energy of micellizationa different manner. The counterion size determines thetance of closest approach to the micellar surface and hit is related to the thickness of the Stern layer and thesorption energy of ions. The counterions can be partiallyhydrated upon adsorption in the Stern layer, which decrethe distance to the negatively charged surfactant head. Inrespect the strongly hydrated ions will be less effectivepromoting micellar growth (cf. Mg2+ and Ca2+). The inter-action between counterions and charged surfactant heamainly Coulomb electrostatic, but at small distances, vfor adsorbed ions, van der Waals interactions may becsignificant. Since they depend on the individual properof the interacting objects, they can also contribute todifferentiation of the counterion effect. For instance, larions have larger polarizabilities. The interplay of all thefactors is probably responsible for the behavior of thedrodynamic radius of SDP-2S micelles presented in FigIn general, the results suggest that the increase in the cterion charge initiates formation of large micelles at lowionic strengths and therefore it plays a primary role inmicellar growth in anionic surfactant solutions. For trivale

8 R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11

the

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counterions, however, the charge seems to outweigh all ofactors discussed above.

3. Monolayers of sodium dodecyl dioxyethylene sulfatein the presence of trivalent (Al3+) counterions

The adsorption of Al3+ on cylindrical micelles is highethan that on spherical ones (see Fig. 10 in Ref. [22])therefore one can expect that trivalent counterionsbind to the SDP-2S monolayers at flat interfaces, evehigher concentrations than in the case of curved surfaThe properties of anionic surfactant monolayers formethe air/solution interface are sensitive to the presenccounterions and monomer/micelle transition (CMC). Itinteresting to study how the factors responsible fortransition in the micellar structure and shape affectproperties of a monolayer of the same surfactant.

Surface tension measurements using the Wilhelmy pmethod at the same conditions as those for the DLS meaments of the micellar size are presented in Fig. 7 whereratio ξ is changed: (i) by increasing SDP-2S concentrawhile keeping the Al3+ concentration unchanged (Fig. 7and (ii) by varying the amount of added Al3+ at constansurfactant concentration (Fig. 7b). Curve 1 presents thehavior of the hydrodynamic radius of SDP-2S micelles msured in each point. Independent of howξ is varied, a changin the surface tension (curve 2) is observed in both cand it is close to the value ofξ where the micellar shaptransition occurs. The surface tension is higher in the reof small spherical micelles and lower in the region of lacylindrical micelles. Forξ � 1, the amount of Al3+ in thebulk is enough to saturate all of the adsorption places aable in the SDP-2S monolayer and hence the degree oferage of the subsurface with trivalent ions is high and leto a lower and constant value of the surface tension inlimits of the experimental error (±0.1 mN/m).

The increase in the surface tension,σ , shown in Fig. 7a isunusual becauseξ is changed by adding more surfactantthe solution. Normally the increase in surfactant concention reduces the surface tension. An exception is the rein the close vicinity of the CMC where a minimum in thdependence surface tension vs bulk surfactant concentrmay form and that is usually attributed to the presence ofpurities [30,31]. If the system is free from such impuritithe surface tension decreases very slowly and finally leoff for bulk concentrations that exceed the CMC [32,3The total surfactant concentration in our case is much hithan the CMC and an abrupt increase of about 2 mN/cm isa surprisingly big change. The observed variation of surtension withξ can be explained if we consider two mainfects: (i) competitive adsorption of Al3+ and Na+ ions inthe SDP-2S monolayer at the solution surface; (ii) comption between the micellar surface and the air/solution inface for the adsorption of Al3. The theoretical analysis [23shows that the change in the surface tension with the su

r

.

-

-

n

-

(a)

(b)

Fig. 7. (a) Apparent hydrodynamic radius,Rh, of SDP-2S micelles (curve 1and surface tension,σ (curve 2), vsξ at constant Al3+ concentrationCAl = 0.004 and 0.024 M ionic strength (surfactant concentrationvaried). The solid line (curve 2) presents the theoretical fit of theaccording to Eq. (3.1). (Reproduced with permission from Langm13 (1997) 5544–5551. Copyright 1997 Am. Chem. Soc.). (b) Appahydrodynamic radius,Rh, of SDP-2S micelles (curve 1) and surfatension,σ (curve 2), vsξ at constant surfactant concentrationCt = 0.008and 0.024 M ionic strength (CAl is varied). The solid line (curve 2) presenthe theoretical fit of the data according to Eq. (3.1). (Reproducedpermission from Langmuir 13 (1997) 5544–5551. Copyright 1997 AChem. Soc.).

tant or counterion concentration is given by

(3.1)

dσ = −kT Γs

(d ln CMC+ θNd ln cNB + θA

3d ln cAB

),

whereΓS is the surfactant adsorption,θN = ΓN/Γs is thedegree of relative adsorption of sodium ion in the Stlayer, θA = ΓA/Γs is that for Al3+, andcNB and cAB arethe bulk concentrations for Na+ and Al3+, respectively. Thecritical micellization concentration, CMC, is a functionthe ionic strength and therefore only one of the differenton the right-hand side is independent. Details of the mare given in Ref. [23].

The solid lines in Fig. 7 present the best fits ofexperimental data. The adjustable parameters for the fithe energies of adsorption for Na+ and Al3+, ΦN = 8.1 andΦA = 13.1kT . The adsorption energy of Al3+ is greater,

R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11 9

.1)

theMCown

the

d astanthe

. 7b.to

asein,tive

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9

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tthe

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iredsentrioncialysis

thehisthethus

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which is consistent with its greater charge. Equation (3predicts an increase in the surface tension withξ because inboth situations presented in Fig. 7,cAB decreases, whilecNBincreases with the increase inξ .

The factors causing the increase inσ affect mostly theelectrostatic part of the free surface energy as far assurfactant concentration studied is much higher than Cand SDP-2S adsorption reaches its maximum. It is shexperimentally in Fig. 8 that the replacement of Al3+ withNa+ in the SDP-2S solution leads to an increase insurface potential,)V , with the parameterξ . These newresults were measured using the vibrating plate methoa difference between the surface potential of a surfacsolution and the reference potential of pure water. Texperimental conditions are the same as those for Fig)V has the lowest value in the region correspondingthe formation of largest micelles and begins to increwith ξ after the micellar shape transition region. Agathe explanation is a gradual neutralization of the negainterfacial charge by adsorption of Al3+.

The properties of SDP-2S monolayers formed at anwater interface in the presence of a mixture of tri- amonovalent counterions exhibit a similar trend. Figurepresents new results for the interfacial tension,γ , measuredat the solution/hexadecane interface at constant surfaconcentration of 0.008 M (ξ is varied by the amount of addeAl3+ counterions). The data were obtained by a sesdrop method. The observed behavior ofγ is in qualitativeagreement with that of the surface tension for the simsystem shown in Fig. 7b. Even the detected increase intension is about the same (≈ 2 mN/m). Note, however, thathe overall magnitude of the tension is much lower foroil/solution interface than for the air/solution interface.

The presence of multivalent ions influences alsoadsorption kinetics of the surfactant, making it fastercomparison to the case of monovalent electrolyte at the sionic strength [34]. Interesting effects have been obsefor the foams grown from a micellar solution of SDP-2

Fig. 8. Apparent hydrodynamic radius (Rh) of SDP-2S micelles (curve 1and surface potential,)V (curve 2), vsξ at constant surfactant concentrtion Ct = 0.008 and 0.024 M ionic strength. The curves are only a gufor the eye.

t

Fig. 9. Apparent hydrodynamic radius,Rh, of SDP-2S micelles (curve 1and interfacial tension,γ (curve 2), vsξ at constant surfactant concentratioCt = 0.008 and 0.024 M ionic strength in the aqueous phase. The oil pis hexadecane. The curves are only a guide for the eye.

in the presence of Al3+ [35]. It was shown that the foamgrowth rate has a maximum around the shape transpoint (ξ = 1) where the dynamic surface tension exhibitminimum. This observation was explained by the fact tmicelles formed in the transition region are less stablelifetime of micelles is shorter) and easily release monomsupplying more effectively the newly formed surfaces wsurfactant.

4. Fundamental and practical implications and futureresearch directions

A deeper understanding of the self-assembly of surfacsystems is an important step for their implementationmodern technologies and applications. Knowing the facthat govern the formation of amphiphilic structures providtools for manipulating and guiding the process in desdirections down to the nanometer scale. In the prereview we analyze the importance of the metal countetype for the properties of micellar structures and interfamolecular monolayer of anionic surfactants. The analshows the following:

1. Small amounts of multivalent counterions facilitatemicellar sphere-to-rod shape transition and growth. Tis due to the strong binding of these counterions tomicellar surface and decreasing the local curvature,favoring cylindrical instead of spherical shape.

2. The effect of the multivalent counterions of the aphiphilic structure has significant fundamental and prtical consequences. Addition of such ions in approate amounts increases the solubilization capacity ofSDP-2S micelles and hence improves the cleaningtion of the surfactant solution. Formation of amphiphaggregates in living organisms (like cell membrancould also be affected by the multivalent counteriothat are usually present in such systems. Varying the

10 R.G. Alargova et al. / Journal of Colloid and Interface Science 261 (2003) 1–11

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tio of surfactant and counterion concentrations is a vconvenient tool for controlling and directing the seassembly of anionic surfactants at molecular scale.

3. Addition of multivalent counterions has also an impon the tensions of air/surfactant solution surfacesoil/surfactant solution interfaces.

4. The properties of anionic/nonionic surfactants mixtuin the presence of multivalent counterions strondepend on the size of the nonionic hydrophilic hegroup.

Solutions of anionic alkyloxyethylene sulfates mayhibit some unexpected and counterintuitive properties wmultivalent counterions are present. Note that such coterintuitive behavior usually occurs in the vicinity ofξ =(Ct − CMC)/zMCM ∼ 1 (M stands for multivalent metacounterion). For example, micelles may decrease in sizeincreasing the overall surfactant concentration, which isopposite of what anionic surfactants would normally dothe presence of monovalent electrolyte. The surface tenof surfactant solution (above CMC) may also increase wfurther addition of surfactant into the solution. These exples illustrate the qualitative difference between the effthat monovalent and multivalent counterions may havethe system.

Fine-tuning of the ratioξ = (Ct − CMC)/zMCM allowsfor controlled assembly of micelles with desired size ashape or interfacial surfactant monolayers with given dsity. Such structures could find application for productof nanoparticles and templates nanostructures [11]. Foample, rod-shaped micelles could be further polymerizeobtain stable rod-shaped nanometer-sized particles withsired size. Another practical consequence of these stuis the possibilities they suggest for surfactant formulatifor cleaning purposes. Since amounts of multivalent coterions tend to increase the oil solubilization capacity ofsolution, it may be expected that controlled addition of sions would lead to better cleaning action. Multivalent coterions are also present in the tap water in certain regand alkyloxyethylene sulfates dissolved in a proper contration may turn out to be very beneficial for shampoos, ladry detergents, etc.

Further understanding of alkyloxyethylene sulfate/muvalent counterion systems discussed in this review coulachieved by studying in more detail the properties of molayers. The initial data for the surface tension and potepresented here has to be extended to cover a wider ranexperimental conditions. Also, one may employ fluorescand scattering techniques to probe directly the structurthe interface. Particularly interesting is the situation whmixtures of anionic (alkyloxyethylene sulfate) and noniosurfactants form the monolayer in the presence of multlent counterions. Such studies could be correlated withproperties of mixed micelles. The properties of the latterstill far from being fully understood. For example, it is n

-s

f

clear why the addition of nonionic surfactant with a smhead group would lead to an increase in the micellar siz

Another possible direction of research is to examineability of oil/water/surfactant/multivalent salt mixturesform emulsions and microemulsions. Our preliminary msurements suggest that the addition of multivalent courions to the oil/water/SDP-2S system may substantiallycrease the interfacial tension. At certain conditions oneexpect formation of a microemulsion. It has been shownvarying the salt concentration may have a profound effecmicroemulsions stabilized by ionic surfactants [36]. Thstudies, however, were restricted to monovalent electroonly. It would be very interesting to investigate what propties a microemulsion may exhibit in the presence of a mvalent electrolyte and especially in the vicinity ofξ ∼ 1.

Acknowledgments

We appreciate the help of Dr. I. Surtcheva for obtainsome of the data in Fig. 9. R.G.A. is indebted to PP. Kralchevsky and Prof. K. Danov for their invaluable hin the theoretical interpretation of the data for the micegrowth and surface tension. This review is partially baon studies (Refs. [12,18,22,23,27]) supported in the pasColgate-Palmolive Company.

References

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[2] J. Israelachvili, Intermolecular and Surface Forces, Academic PNew York, 1995.

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[4] N.A. Mazer, in: R. Pecora (Ed.), Dynamic Light Scattering, PlenuNew York, 1985.

[5] P.J. Missel, N.A. Mazer, G.B. Benedek, C.Y. Young, M.C. CarJ. Phys. Chem. 84 (1980) 1044.

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[11] J. Plestil, H. Pospisil, J. Kriz, P. Kadlec, Z. Tuzar, R. CubLangmuir 17 (2001) 6699.

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[27] R.G. Alargova, Thesis, University of Sofia, Sofia, 1998.[28] M. Rosch, Nonionic Surfactants, Marcel Dekker, New York, 1967.[29] S. Hayashi, S. Ikeda, J. Phys. Chem. 84 (1980) 744.[30] J.T. Davis, E.K. Riedal, Interfacial Phenomena, Academic Press,

York, 1963.[31] K.J. Mysels, Langmuir 2 (1986) 423.[32] S.J. Renfeld, J. Phys. Chem. 71 (1967) 738.[33] E. Kissa, Fluorinated Surfactants and Repellents, Marcel Dekker,

York, 2001.[34] I.U. Vakarelsky, C.D. Dushkin, Colloids Surf. A 163 (2000) 177.[35] C.D. Dushkin, T.L. Stojchev, T.S. Horozov, A. Mehreteab, G. Bro

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