Journal of Colloid and Interface Science 315 (2007) 620–626www.elsevier.com/locate/jcis

Synergistic interactions in the mixed micelles of cationic gemini withzwitterionic surfactants: The pH and spacer effect

Kulbir Singh, D. Gerrard Marangoni ∗

Department of Chemistry, St Francis Xavier University, P.O. Box 5000, Antigonish, NS, B2G 2W5 Canada

Received 28 March 2007; accepted 22 June 2007

Available online 30 June 2007

Abstract

Mixed micelle formation of binary cationic gemini (12-s-12, s = 4, 6) and zwitterionic (N -dodecyl-N,N -dimethylglycine, EBB) surfactantshas been investigated by measuring the surface tension of aqueous solution as a function of total concentration at various pH values from acidicto basic, under conditions of 298.15 K and atmospheric pressure. The results were analyzed by applying regular solution theory (RST), andMotomura’s theory, which allows for the calculation of the excess Gibbs energy of micellization purely on the basis of thermodynamic equations.The synergistic interactions of all the investigated cationic gemini + zwitterionic surfactants mixtures were found to be dependent upon the pH ofthe solution and the length of hydrophobic spacer of gemini surfactant. The evaluated excess Gibbs free energy is negative for all the systems.© 2007 Elsevier Inc. All rights reserved.

Keywords: Surface tension measurements; Mixed micelles; Synergistic interactions; Gemini surfactant; Zwitterionic surfactant

1. Introduction

Interactions between surfactant molecules in aqueous solu-tion have been extensively investigated as a result of their tech-nological relevance in interfacial systems allowing for a bettercontrol of stability, flotation and rheology of dispersed systemin different technological processes [1–3]. Many works havebeen developed in order to predict the ideal behavior of surfac-tants in aqueous solution regarding equilibrium surface tensionand adsorption at fluid interface [4–10]. Micellar systems con-sisting of mixtures of surfactants of variable structure are ofa high theoretical and industrial interest. The physicochemicalproperties of an aqueous solution constituted by two or moresurfactants often present a very different character in compar-ison to those formed by single surfactants [11]. Mixtures ofdifferent surfactant types often exhibit cooperative interactionsor synergism in their effects on the properties of a system [12].This synergism can be attributed to nonideal mixing effectsin the aggregates, which results in critical micellization con-centrations (cmc) and interfacial tensions that are substantially

* Corresponding author. Fax: +1 902 867 2414.E-mail address: gmarango@stfx.ca (D.G. Marangoni).

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.06.062

lower than would be expected on the basis of the propertiesof the unmixed surfactants alone. Synergism is often modeledin the literature by employing the regular solution theory (RST)[13] with a negative interaction parameter. RST has proven suc-cessful in accounting for the nonideal behavior of a number ofbinary surfactant systems but it does not adequately describethe behavior of monomers in a surfactant solution, activity co-efficient, and excess free energy in mixed surfactant systems[14]. Thermodynamic treatments have been developed to cal-culate micelle properties such as micellar composition [15–17].Molecular thermodynamics allows the calculation of the totalfree energy of micellization and the prediction of a broad spec-trum of solution properties [18]. Motomura’s theory is one suchtheory, which allows for the calculation of the free energy ofmicelle formation as a measure of molecular interactions.

Surfactants consisting of two hydrophilic and hydrophobicgroups have attracted considerable research interest over thelast decade. These are called gemini or dimeric surfactants[19–22] because they have a structure that can be viewed astwo monomeric surfactant units connected at or near the headgroups by spacer. This kind of architecture provides solutionproperties that are dependent upon the nature and size of thehead groups. A number of studies have reported their versa-

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tile synthesis as well as their wide range of physicochemicalproperties [23–31]. Gemini surfactants are superior to the cor-responding conventional monomeric surfactants in number ofaspects. They have much lower critical micelle concentrations(cmc) values, better lime-soap dispersing, and better wettingproperties. In view of their high cost, gemini surfactants arelikely to be used in combination with conventional surfactants.Recently, several research papers have appeared in the literatureregarding their mixing behavior with conventional surfactants(mainly ionic and nonionic). These mixtures are considered tohave practical applications in many surfactant formulations. Onthe other hand, there have been very few studies on mixturesof conventional zwitterionic surfactants and gemini surfactants.Zwitterionic surfactants have a dipolar head bearing both pos-itive and negative charges and have many interesting solutionproperties [32–36]. In general, zwitterionic surfactants are mildto skin and eyes, have low toxicity, and display excellent wa-ter solubility, high foam stability, and excellent surface tensionreducing properties.

To date, zwitterionic surfactants have been generally muchless studied than other classes of surfactants. Their behavioris often desirable in relation to ionic and nonionic surfactantsbut the electrically neutral nature of zwitterionic surfactantsclearly distinguishes them from ionic surfactants. As solutionsin pure water, their behavior has been found to reassemble thatof “ionic surfactant with excess salt.” The biodegradability ofsuch compounds and their reduced skin and eye irritation arealso of great importance. In combination with other surfac-tants, these surfactants find their applications in, laundry de-tergents, shampoos, and other cosmetics products [37,38]. Dueto the wide spread use of cationic/zwitterionic mixed surfac-tants systems both commercially and industrially, it would bevery beneficial to develop a predictive theory for these kinds ofmixtures [39].

The objective of this study was to investigate the mixed gem-ini/zwitterionic systems, and to observe the presence of coop-erative interactions at various pH values ranging from acidic tobasic.

2. Experimental

2.1. Materials and method

The cationic gemini surfactants, dialkylenebis(alkyldi-methylammonium bromide) (12-6-12 and 12-4-12) was syn-thesized according to the method reported elsewhere [19].These surfactants were purified by repeated crystallizationsfrom ethanol/acetone mixtures and finally from pure ethanol.The purity of these surfactants was judged from a plot of sur-face tension vs logarithm of surfactant concentration. Becausethere was no appreciable minimum visible in the surface ten-sion plots, the surfactants were determined to be relatively pure.The zwitterionic surfactant N -dodecyl-N,N -dimethylglycine(EBB) a 30% solution obtained from Flüka, was used as re-ceived. The surface tension measurements were carried outwith a Krüss (K-8) tensiometer using a platinum ring at con-stant temperature (298 ± 0.1 K). The measurement accuracy

was ±0.05 mN/m. The pH of the solutions was controlledwith sodium hydroxide (NaOH) and hydrochloric acid (HCl),and pH measurements were carried out with Digi-Sense digi-tal pH meter from Cole-Parmer Instrumental Co., USA triplydeionized water was used in the preparation of all solutions.All solutions were prepared by mass within the accuracy of±0.01 mg. The mole fractions were accurate to ±0.0001 units.The composition of the solutions was expressed in term of themole fraction (αi ) of the respective surfactant, defined by

(1)αi = [Si][Si] + [Sj ] ,

where [Si ] and [Sj ] are the molar concentrations of the sur-factants i and j in the mixed solutions. The cmc was takenas a point of intersection by fitting the pre-micellar and post-micellar data in linear equations as shown in Fig. 1. The errorsin cmc values were estimated to be less than 10%.

3. Results

3.1. Evaluation of nonideality from RST

The equilibrium surface tension values of pure 12-6-12,EBB, and their mixtures in various compositions are shownas a function of the logarithm of total surfactant concentra-tion, in Fig. 1. In order to determine the cmc, two linear fitswere used to each of the isotherm. The first line was fitted tothe interval of concentration characterized by a linear decreasein surface tension and the second to the region of concentra-tion with nearly constant surface tension. The cmc values forthe pure surfactants and the various mixtures have been eval-uated by linear fitting of pre-micellar and post-micellar data(Fig. 1). The experimental cmc values evaluated from surfacetension measurements at various pH conditions and the ideal

Fig. 1. Variation of the surface tension vs logarithm of concentration of pure12-4-12, pure EBB, and their mixtures in water at 25 ◦C. The solid line repre-sents the best fit to the experimental data.

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cmc’s (cmc*), calculated from Clint equation [40] (Eq. (2)),have been plotted against the mixing mole fraction for variousbinary surfactant mixtures (Figs. 2a–2f).

(2)1

cmc∗ = α1

cmc1+ (1 − α1)

cmc2,

where α1 is the mole fraction of surfactant 1 (i.e. EBB) in thetotal mixed solute, and cmc1 and cmc2 are the critical micelleconcentrations of pure components, respectively. The deviationof cmc from cmc* indicates nonideality in the mixed micelles.A nonideal mixing behavior is expected for the present binarymixtures due to various structural dissimilarities. All the figures

Fig. 2. Plot of cmc versus αEBB of 12-6-12 + EBB, (a) pH 2, (b) pH 7, (c) pH 10, and for 12-4-12 + EBB (d) pH 2, (e) pH 7, (f) pH 10, mixtures in pure water.Experimental cmc (points); predicted cmc* (lines).

K. Singh, D.G. Marangoni / Journal of Colloid and Interface Science 315 (2007) 620–626 623

show that the cmc values of each mixture vary nonlinearly withrespect to the change in bulk mole fraction, and that the cmcvalues are lower than the cmc* for all the binary mixtures. It isto be mentioned here that a lower value of cmc than the corre-sponding cmc* value indicates mixed micellization due to somesort of attractive interactions operating between the unlike com-ponents. Due to the presence of dimeric gemini head groups incombination with those of bulky zwitterionic head groups inthe Stern layer of the mixed micelles, a nonideal behavior is ex-pected in the mixed state. A quantitative interpretation of theresults can be carried out by considering the regular solutionapproximation. This theory allows for the calculations of themicellar mole fraction as well as the interaction parameter byusing equations, respectively,

(3)X2

1 ln(cmcα1/cmc1X1)

(1 − X1)2 ln(cmc(1 − α1)/cmc2(1 − X1))= 1,

(4)β = ln(cmcα1/cmc1X1)

(1 − X1)2,

where X1 is the micelle mole fraction of surfactant 1 (i.e. EBB)in the mixed micelles and β is the interaction parameter whichindicates the magnitude of interactions operating between theunlike components in the mixed micelle state. The micelle molefraction in the ideal state (Xideal) can also be evaluated by ap-plying Motomura’s theory [41], which is based upon excessthermodynamic quantities, equation

(5)Xideal = [(α1cmc2)

/(α1cmc2 + (1 − α1)cmc1

)].

Deviation in X1 from the corresponding Xideal values would in-dicate the presence of nonideality in the mixed micelles. Thisvariation between both X1 and Xideal values has been showngraphically in Figs. 3a–3f. In Fig. 3, X1 values are mostly lowerthan the Xideal values, with exception of 12-6-12 + EBB sys-tems at pH 7 (Fig. 3b). Here the X1 values are higher thanXideal values in the starting mole fractions and then come closeto Xideal, and there after X1 values are lower than Xideal. Thisresult is quite consistent with the previous findings of Singhand Bakshi for gemini/zwitterionic (10-2-10 + DPS) combi-nations under the same set of conditions [11]. The variationof X1 and Xideal in the 12-4-12 + EBB systems is more orless consistent with the 12-6-12 + EBB systems (Figs. 3d–3f).A lower X1 than that of corresponding Xideal value indicatesthat the mixed micelles are rich in gemini component, whileX1 value close to Xideal value indicates ideal mixing. A closeinspection of Fig. 3 indicates that a change in the pH of thesystem from neutral to acidic or basic allows the relativelylesser amount of zwitterionic component in the mixed micelles.This effect is very pronounced when the pH value increasesfrom 7 to 10 (Figs. 3a and 3b). A similar behavior is observedfor 12-4-12 + EBB mixed surfactant systems (Figs. 3d, 3e,and 3f).

These results are further evaluated on the basis of the varia-tion of β values for all the present mixtures (Eq. (4)). In regularsolution theory, the component molecules are assumed to beof comparable volume, completely interchangeable, and the in-teraction energy is expressed as a sum of pair-wise, nearest

neighbor interactions. The β12 parameter is still useful even inthe presence of other existing theories in indicating the natureof the interaction. A better interpretation of the interaction pa-rameter as well as the shapes of the resulting aggregates canbe achieved by considering the molecular thermodynamic ap-proach [16,18,42] when the cmc values of unlike componentsare close enough to each other. However, this is not the casefor most of the present mixtures. β values calculated for all themixtures from Eq. (4), has been plotted against the bulk molefraction in Fig. 4a (12-6-12 + EBB, shown in supplementaryFig. S1), furthermore, average β(βavg) values are also plottedagainst the pH of solution in Fig. 4b. Fig. 4b demonstrates thatthe βavg values for all the mixtures are negative. A negativeβavg value can be attributed to mixed micelle formation dueto synergistic interactions. Therefore, all the present mixturesundergo mixed micelle formation due to attractive interactionsoperating between the gemini and zwitterionic components.However, a close inspection of Figs. 4a and 4b demonstratesthat the magnitude of synergism increases with an increase inbulk mole fraction of zwitterionic component for all pH val-ues, and maximum synergism exist under the basic conditions.Furthermore, the lower βavg value for the 12-4-12 + EBB sys-tems than the 12-6-12 + EBB (except at pH 10), indicates thatsynergism in the mixed micelles of gemini/zwitterionic surfac-tant mixtures decrease with an increase in the length of thespacer.

3.2. Evaluation of nonideality from Motomura’s theory

Quantitatively, deviation from ideal mixing in the micelles isdemonstrated by excess Gibbs free energy of micelle formation(gM,E), which was calculated by applying Eqs. (6)–(8), result-ing from the Motomura’s theory [43]

(6)gM,E = RT(XM

1 lnf M1 + XM

2 lnf M2

),

where f M1 and f M

2 are the activity of surfactant 1 (i.e. EBB)and surfactant 2 (i.e. gemini) in micelle evaluated by applyingequations

(7a)CX1 = C01f M

1 XM1 (for EBB),

(7b)CX2 = C02f M

2 XM2 (for gemini).

In this equation C is the cmc of the mixture, C01 and C0

2 arethe cmcs of pure EBB and gemini surfactant, respectively, XM

1and XM

2 are the micellar mole fractions in term of EBB andgemini surfactants, respectively. XM

2 for the gemini surfactantis evaluated through equation

(8)XM2 = X2 −

(X1X2

C

)(∂C

∂X2

)T ,P

.

The gM,E values calculated for EBB/gemini systems and 12-4-12 + EBB systems at various pH values have been plotted inFig. 4c (12-6-12 + EBB, shown in supplementary Fig. S1). AllgM,E values are negative, which means that mixed micelle for-mation by these two surfactants is a favorable process. Gharibiet al. [14] have observed a similar type of nonlinear variation in

624 K. Singh, D.G. Marangoni / Journal of Colloid and Interface Science 315 (2007) 620–626

Fig. 3. Micellar mole fractions, X1, Xideal determined from RST (points) and Motomura’s theory (lines), respectively, versus mole fraction of zwitterionic, αEBB,of 12-6-12 + EBB, (a) pH 2, (b) pH 7, (c) pH 10, and for 12-4-12 + EBB (d) pH 2, (e) pH 7, (f) pH 10, mixtures in pure water.

gM,E for the mixed surfactant systems of CTAB and Triton X-100. The variation of gM,E is very close to the variation of theβ values. Furthermore, the average excess Gibbs free energyof micelle formation (gM,E

avg ) have been evaluated and comparedwith the variation in βavg (Figs. 4b and 4d) and a very similartrend has been observed.

4. Discussion

A collective interpretation of the present results, evaluatedfrom regular solution theory, indicates nonideality for all thebinary mixed systems and synergistic interactions are observedunder different pH conditions (Fig. 3). It is well known, how-

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Fig. 4. Plot of (a) β versus mole fraction of zwitterionic, αEBB, for 12-4-12 + EBB, (b) βavg versus pH of solution for all combinations, (c) gM,E determined from

Motomura’s theory versus mole fraction of zwitterionic, αEBB, for 12-4-12 + EBB (lines 1, 2, 3 correspond to pH 7, 2, 10, respectively), and (c) gM,Eavg versus pH

of solution for all combinations.

ever, that combinations of a zwitterionic surfactant based on abasic carboxylate group that may be protonated and an anionicsurfactant may exhibit strong synergism [44,45]. The reasonfor this is that the zwitterionic surfactant will be protonated,i.e. transferred into a cationic surfactant, in the micellizationprocess. One way to express this is that formation of mixedmicelles between a cationic and an anionic surfactant is so en-ergetically favored that it drives protonation of the carboxylategroup although the bulk pH is much higher than the pKa forthe unimer. In the present study an attempt has been madeto study the effect of pH induced protonations/deprotonationof carboxylate/carboxylic group of EBB on the mixed micelleformation between EBB and cationic gemini surfactants, addi-tionally, the effect of spacer has also been studied. The headgroup of zwitterionic surfactant studied here carries both a qua-ternary ammonium group and a carboxyl group as well. At lowpH, i.e. in acidic conditions, EBB has only a positive charge lo-calized on the quaternary ammonium group due to protonationof carboxylate group. Hence, it acts more like a cationic surfac-

tant and imparts synergistic behavior with gemini surfactants.While under basic conditions, EBB acts as a zwitterionic sur-factant because of the deprotonation of carboxylic group, andagain synergism is imparted in the mixed micelles, but to agreater extent. The latter behavior has been observed for pre-viously studied zwitterionic/cationic surfactant systems [11].In all cases mixed micelles are rich with gemini component(Fig. 2b) and it is the incorporation of zwitterionic surfactantmonomers into the micelles of gemini surfactants which leadsto the synergistic effect. Apart from this, increase in spacerlength also affects synergistic interactions, most likely due tothe increase in hydrophobicity at the level of head group andthe modification in the distance between the head groups ofamphiphiles in the micellar phase. Negative Gibbs free energyof micellization for all the mixtures indicates that mixed mi-celle formation is thermodynamically favorable. Furthermore,a close variation of βavg, and g

M,Eavg vs pH clearly indicates that

strongly interacting surfactant has greater feasibility of mix-ing.

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5. Conclusions

Synergistic mixing behavior of cationic gemini and carboxy-late based zwitterionic surfactant can be modified under differ-ent conditions of pH, and further modification is also possiblefrom the change in the spacer length of gemini component.Maximum synergistic effect has been observed under basic con-ditions, where EBB acts like a true zwitterionic surfactant. In allcases mixed micelles are rich in gemini component and incor-poration of zwitterionic surfactant monomers into the micellesof gemini surfactant is most likely the cause of synergistic ef-fect. A collective interpretation by regular solution approxima-tion and Motomura’s theory indicates that strongly interactingsurfactant molecule has greater feasibility of mixing. These ob-servations may be of interest in practical applications of geminiand zwitterionic surfactant combinations.

Acknowledgments

The research grants from NSERC (Research Grant and Re-search Capacity Development), the Atlantic Innovation Fund,StFX University, and Newpark Resources, are thankfully ac-knowledged.

Supplementary material

The online version of this article contains additional supple-mentary material.

Please visit DOI: 10.1016/j.jcis.2007.06.062.

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