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Synergistic interactions in the mixed micelles of cationic gemini with zwitterionic surfactants: The pH and spacer effect
Journal of Colloid and Interface Science 315 (2007) 620–626 www.elsevier.com/locate/jcis
Synergistic interactions in the mixed micelles of cationic gemini with zwitterionic 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) surfactants has been investigated by measuring the surface tension of aqueous solution as a function of total concentration at various pH values from acidic to basic, under conditions of 298.15 K and atmospheric pressure. The results were analyzed by applying regular solution theory (RST), and Motomura’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 of the 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 solution have been extensively investigated as a result of their technological relevance in interfacial systems allowing for a better control of stability, flotation and rheology of dispersed system in different technological processes [1–3]. Many works have been developed in order to predict the ideal behavior of surfactants in aqueous solution regarding equilibrium surface tension and adsorption at fluid interface [4–10]. Micellar systems consisting of mixtures of surfactants of variable structure are of a high theoretical and industrial interest. The physicochemical properties of an aqueous solution constituted by two or more surfactants often present a very different character in comparison to those formed by single surfactants [11]. Mixtures of different surfactant types often exhibit cooperative interactions or synergism in their effects on the properties of a system [12]. This synergism can be attributed to nonideal mixing effects in the aggregates, which results in critical micellization concentrations (cmc) and interfacial tensions that are substantially * Corresponding author. Fax: +1 902 867 2414.
E-mail address: [email protected] (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 properties of the unmixed surfactants alone. Synergism is often modeled in the literature by employing the regular solution theory (RST) [13] with a negative interaction parameter. RST has proven successful in accounting for the nonideal behavior of a number of binary surfactant systems but it does not adequately describe the behavior of monomers in a surfactant solution, activity coefficient, and excess free energy in mixed surfactant systems [14]. Thermodynamic treatments have been developed to calculate micelle properties such as micellar composition [15–17]. Molecular thermodynamics allows the calculation of the total free energy of micellization and the prediction of a broad spectrum of solution properties [18]. Motomura’s theory is one such theory, which allows for the calculation of the free energy of micelle formation as a measure of molecular interactions. Surfactants consisting of two hydrophilic and hydrophobic groups have attracted considerable research interest over the last decade. These are called gemini or dimeric surfactants [19–22] because they have a structure that can be viewed as two monomeric surfactant units connected at or near the head groups by spacer. This kind of architecture provides solution properties that are dependent upon the nature and size of the head groups. A number of studies have reported their versa-
K. Singh, D.G. Marangoni / Journal of Colloid and Interface Science 315 (2007) 620–626
tile synthesis as well as their wide range of physicochemical properties [23–31]. Gemini surfactants are superior to the corresponding conventional monomeric surfactants in number of aspects. They have much lower critical micelle concentrations (cmc) values, better lime-soap dispersing, and better wetting properties. In view of their high cost, gemini surfactants are likely to be used in combination with conventional surfactants. Recently, several research papers have appeared in the literature regarding their mixing behavior with conventional surfactants (mainly ionic and nonionic). These mixtures are considered to have practical applications in many surfactant formulations. On the other hand, there have been very few studies on mixtures of conventional zwitterionic surfactants and gemini surfactants. Zwitterionic surfactants have a dipolar head bearing both positive and negative charges and have many interesting solution properties [32–36]. In general, zwitterionic surfactants are mild to skin and eyes, have low toxicity, and display excellent water solubility, high foam stability, and excellent surface tension reducing properties. To date, zwitterionic surfactants have been generally much less studied than other classes of surfactants. Their behavior is often desirable in relation to ionic and nonionic surfactants but the electrically neutral nature of zwitterionic surfactants clearly distinguishes them from ionic surfactants. As solutions in pure water, their behavior has been found to reassemble that of “ionic surfactant with excess salt.” The biodegradability of such compounds and their reduced skin and eye irritation are also of great importance. In combination with other surfactants, these surfactants find their applications in, laundry detergents, shampoos, and other cosmetics products [37,38]. Due to the wide spread use of cationic/zwitterionic mixed surfactants systems both commercially and industrially, it would be very beneficial to develop a predictive theory for these kinds of mixtures [39]. The objective of this study was to investigate the mixed gemini/zwitterionic systems, and to observe the presence of cooperative interactions at various pH values ranging from acidic to basic.
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was ±0.05 mN/m. The pH of the solutions was controlled with sodium hydroxide (NaOH) and hydrochloric acid (HCl), and pH measurements were carried out with Digi-Sense digital pH meter from Cole-Parmer Instrumental Co., USA triply deionized 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 the mole fraction (αi ) of the respective surfactant, defined by αi =
[Si ] , [Si ] + [Sj ]
(1)
where [Si ] and [Sj ] are the molar concentrations of the surfactants i and j in the mixed solutions. The cmc was taken as a point of intersection by fitting the pre-micellar and postmicellar data in linear equations as shown in Fig. 1. The errors in 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 shown as a function of the logarithm of total surfactant concentration, in Fig. 1. In order to determine the cmc, two linear fits were used to each of the isotherm. The first line was fitted to the interval of concentration characterized by a linear decrease in surface tension and the second to the region of concentration with nearly constant surface tension. The cmc values for the pure surfactants and the various mixtures have been evaluated by linear fitting of pre-micellar and post-micellar data (Fig. 1). The experimental cmc values evaluated from surface tension measurements at various pH conditions and the ideal
2. Experimental 2.1. Materials and method The cationic gemini surfactants, dialkylenebis(alkyldimethylammonium bromide) (12-6-12 and 12-4-12) was synthesized according to the method reported elsewhere [19]. These surfactants were purified by repeated crystallizations from ethanol/acetone mixtures and finally from pure ethanol. The purity of these surfactants was judged from a plot of surface tension vs logarithm of surfactant concentration. Because there was no appreciable minimum visible in the surface tension 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 received. The surface tension measurements were carried out with a Krüss (K-8) tensiometer using a platinum ring at constant temperature (298 ± 0.1 K). The measurement accuracy
Fig. 1. Variation of the surface tension vs logarithm of concentration of pure 12-4-12, pure EBB, and their mixtures in water at 25 ◦ C. The solid line represents 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 various binary surfactant mixtures (Figs. 2a–2f). α1 (1 − α1 ) 1 = + , ∗ cmc cmc1 cmc2
(2)
where α1 is the mole fraction of surfactant 1 (i.e. EBB) in the total mixed solute, and cmc1 and cmc2 are the critical micelle concentrations of pure components, respectively. The deviation of cmc from cmc* indicates nonideality in the mixed micelles. A nonideal mixing behavior is expected for the present binary mixtures 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
show that the cmc values of each mixture vary nonlinearly with respect to the change in bulk mole fraction, and that the cmc values are lower than the cmc* for all the binary mixtures. It is to be mentioned here that a lower value of cmc than the corresponding cmc* value indicates mixed micellization due to some sort of attractive interactions operating between the unlike components. Due to the presence of dimeric gemini head groups in combination with those of bulky zwitterionic head groups in the Stern layer of the mixed micelles, a nonideal behavior is expected in the mixed state. A quantitative interpretation of the results can be carried out by considering the regular solution approximation. This theory allows for the calculations of the micellar mole fraction as well as the interaction parameter by using equations, respectively, X12 ln(cmc α1 /cmc1 X1 ) = 1, (1 − X1 )2 ln(cmc(1 − α1 )/cmc2 (1 − X1 )) ln(cmc α1 /cmc1 X1 ) β= , (1 − X1 )2
(3) (4)
where X1 is the micelle mole fraction of surfactant 1 (i.e. EBB) in the mixed micelles and β is the interaction parameter which indicates the magnitude of interactions operating between the unlike components in the mixed micelle state. The micelle mole fraction in the ideal state (Xideal ) can also be evaluated by applying Motomura’s theory [41], which is based upon excess thermodynamic quantities, equation Xideal = (α1 cmc2 ) α1 cmc2 + (1 − α1 )cmc1 . (5) Deviation in X1 from the corresponding Xideal values would indicate the presence of nonideality in the mixed micelles. This variation between both X1 and Xideal values has been shown graphically in Figs. 3a–3f. In Fig. 3, X1 values are mostly lower than the Xideal values, with exception of 12-6-12 + EBB systems at pH 7 (Fig. 3b). Here the X1 values are higher than Xideal values in the starting mole fractions and then come close to Xideal , and there after X1 values are lower than Xideal . This result is quite consistent with the previous findings of Singh and Bakshi for gemini/zwitterionic (10-2-10 + DPS) combinations under the same set of conditions [11]. The variation of X1 and Xideal in the 12-4-12 + EBB systems is more or less consistent with the 12-6-12 + EBB systems (Figs. 3d–3f). A lower X1 than that of corresponding Xideal value indicates that the mixed micelles are rich in gemini component, while X1 value close to Xideal value indicates ideal mixing. A close inspection of Fig. 3 indicates that a change in the pH of the system from neutral to acidic or basic allows the relatively lesser amount of zwitterionic component in the mixed micelles. This effect is very pronounced when the pH value increases from 7 to 10 (Figs. 3a and 3b). A similar behavior is observed for 12-4-12 + EBB mixed surfactant systems (Figs. 3d, 3e, and 3f). These results are further evaluated on the basis of the variation of β values for all the present mixtures (Eq. (4)). In regular solution theory, the component molecules are assumed to be of comparable volume, completely interchangeable, and the interaction energy is expressed as a sum of pair-wise, nearest
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neighbor interactions. The β12 parameter is still useful even in the presence of other existing theories in indicating the nature of the interaction. A better interpretation of the interaction parameter as well as the shapes of the resulting aggregates can be achieved by considering the molecular thermodynamic approach [16,18,42] when the cmc values of unlike components are close enough to each other. However, this is not the case for most of the present mixtures. β values calculated for all the mixtures from Eq. (4), has been plotted against the bulk mole fraction in Fig. 4a (12-6-12 + EBB, shown in supplementary Fig. S1), furthermore, average β(βavg ) values are also plotted against the pH of solution in Fig. 4b. Fig. 4b demonstrates that the βavg values for all the mixtures are negative. A negative βavg value can be attributed to mixed micelle formation due to synergistic interactions. Therefore, all the present mixtures undergo mixed micelle formation due to attractive interactions operating between the gemini and zwitterionic components. However, a close inspection of Figs. 4a and 4b demonstrates that the magnitude of synergism increases with an increase in bulk mole fraction of zwitterionic component for all pH values, and maximum synergism exist under the basic conditions. Furthermore, the lower βavg value for the 12-4-12 + EBB systems than the 12-6-12 + EBB (except at pH 10), indicates that synergism in the mixed micelles of gemini/zwitterionic surfactant mixtures decrease with an increase in the length of the spacer. 3.2. Evaluation of nonideality from Motomura’s theory Quantitatively, deviation from ideal mixing in the micelles is demonstrated by excess Gibbs free energy of micelle formation (g M,E ), which was calculated by applying Eqs. (6)–(8), resulting from the Motomura’s theory [43] g M,E = RT X1M ln f1M + X2M ln f2M , (6) where f1M and f2M are the activity of surfactant 1 (i.e. EBB) and surfactant 2 (i.e. gemini) in micelle evaluated by applying equations CX1 = C10 f1M X1M
(for EBB),
(7a)
CX2 = C20 f2M X2M
(for gemini).
(7b)
In this equation C is the cmc of the mixture, C10 and C20 are the cmcs of pure EBB and gemini surfactant, respectively, X1M and X2M are the micellar mole fractions in term of EBB and gemini surfactants, respectively. X2M for the gemini surfactant is evaluated through equation X1 X 2 ∂C M . X2 = X 2 − (8) C ∂X2 T ,P The g M,E values calculated for EBB/gemini systems and 12-412 + EBB systems at various pH values have been plotted in Fig. 4c (12-6-12 + EBB, shown in supplementary Fig. S1). All g M,E values are negative, which means that mixed micelle formation by these two surfactants is a favorable process. Gharibi et al. [14] have observed a similar type of nonlinear variation in
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Fig. 3. Micellar mole fractions, X1 , Xideal determined from RST (points) and Motomura’s theory (lines), respectively, versus mole fraction of zwitterionic, α EBB , 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.
g M,E for the mixed surfactant systems of CTAB and Triton X100. The variation of g M,E is very close to the variation of the β values. Furthermore, the average excess Gibbs free energy M,E of micelle formation (gavg ) have been evaluated and compared with the variation in βavg (Figs. 4b and 4d) and a very similar trend has been observed.
4. Discussion A collective interpretation of the present results, evaluated from regular solution theory, indicates nonideality for all the binary mixed systems and synergistic interactions are observed under 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) g M,E determined from M,E versus pH 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) gavg of solution for all combinations.
ever, that combinations of a zwitterionic surfactant based on a basic carboxylate group that may be protonated and an anionic surfactant may exhibit strong synergism [44,45]. The reason for this is that the zwitterionic surfactant will be protonated, i.e. transferred into a cationic surfactant, in the micellization process. One way to express this is that formation of mixed micelles between a cationic and an anionic surfactant is so energetically favored that it drives protonation of the carboxylate group although the bulk pH is much higher than the pKa for the unimer. In the present study an attempt has been made to study the effect of pH induced protonations/deprotonation of carboxylate/carboxylic group of EBB on the mixed micelle formation between EBB and cationic gemini surfactants, additionally, the effect of spacer has also been studied. The head group of zwitterionic surfactant studied here carries both a quaternary ammonium group and a carboxyl group as well. At low pH, i.e. in acidic conditions, EBB has only a positive charge localized on the quaternary ammonium group due to protonation of 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 surfactant because of the deprotonation of carboxylic group, and again synergism is imparted in the mixed micelles, but to a greater extent. The latter behavior has been observed for previously 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 surfactant monomers into the micelles of gemini surfactants which leads to the synergistic effect. Apart from this, increase in spacer length also affects synergistic interactions, most likely due to the increase in hydrophobicity at the level of head group and the modification in the distance between the head groups of amphiphiles in the micellar phase. Negative Gibbs free energy of micellization for all the mixtures indicates that mixed micelle formation is thermodynamically favorable. Furthermore, M,E a close variation of βavg , and gavg vs pH clearly indicates that strongly interacting surfactant has greater feasibility of mixing.
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5. Conclusions Synergistic mixing behavior of cationic gemini and carboxylate based zwitterionic surfactant can be modified under different conditions of pH, and further modification is also possible from the change in the spacer length of gemini component. Maximum synergistic effect has been observed under basic conditions, where EBB acts like a true zwitterionic surfactant. In all cases mixed micelles are rich in gemini component and incorporation of zwitterionic surfactant monomers into the micelles of gemini surfactant is most likely the cause of synergistic effect. A collective interpretation by regular solution approximation and Motomura’s theory indicates that strongly interacting surfactant molecule has greater feasibility of mixing. These observations may be of interest in practical applications of gemini and zwitterionic surfactant combinations. Acknowledgments The research grants from NSERC (Research Grant and Research Capacity Development), the Atlantic Innovation Fund, StFX University, and Newpark Resources, are thankfully acknowledged. Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2007.06.062. References [1] J.C.T. Kwak, Polymer–Surfactant Systems, Dekker, New York, 1998. [2] R.M. Hill, in: K. Ogino, M. Abe (Eds.), Mixed Surfactant Systems, in: Surfactant Sci. Ser., vol. 46, Dekker, New York, 1993, chap. 11. [3] S. Ghosh, S.P. Moulik, J. Colloid Interface Sci. 208 (1999) 357–366. [4] E.H. Lucassen-Reynders, J. Lucassen, D. Giles, J. Colloid Interface Sci. 82 (1981) 150–157. [5] M.J. Rosen, X.Y. Hua, J. Am. Chem. Soc. 59 (1982) 427–431. [6] M.J. Rosen, Surfactant and Interfacial Phenomena, second ed., Wiley, New York, 1989, chap. 11. [7] M.J. Rosen, Langmuir 7 (1991) 885–888. [8] D.N. Rubingh, T. Jones, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 176– 182. [9] I. Reif, P. Somasundaran, Langmuir 15 (1999) 3411–3417. [10] P. Wydro, M. Paluch, Colloids Surf. A Physicochem. Eng. Aspects 245 (2004) 75–79.
[11] M.S. Bakshi, K. Singh, J. Colloid Interface Sci. 287 (2005) 288–297. [12] A.A. McLachlan, D.G. Marangoni, J. Colloid Interface Sci. 295 (2006) 243–248. [13] D.N. Rubingh, in: K. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1, Plenum, New York, 1979, p. 337. [14] H. Gharibi, S. Javadian, R. Behjatmanesh, J. Colloid Interface Sci. 285 (2005) 351–359. [15] K. Motomura, M. Yamanaka, M. Aratono, Colloid Polym. Sci. 262 (1984) 948–955. [16] S. Puvada, D. Blankschtein, J. Phys. Chem. 96 (1992) 5567–5579. [17] S. Puvada, D. Blankschtein, J. Phys. Chem. 96 (1992) 5579–5592. [18] A. Shiloach, D. Blankschtein, Langmuir 14 (1998) 1618–1636. [19] R. Zana, M. Benrraou, R. Rueff, Langmuir 7 (1991) 1072–1075. [20] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1991) 1451–1452. [21] M.J. Rosen, Chem. Technol. 23 (1993) 30–33. [22] R. Zana, Curr. Opin. Colloid Interface Sci. 1 (1996) 566. [23] X. Wang, J. Wang, Y. Wang, H. Yan, P. Li, R.K. Thomas, Langmuir 20 (2004) 53–56. [24] P. Qualiotto, G. Viscardi, C. Barolo, E. Barni, S. Bellinvia, E. Fisicaro, C. Compari, J. Org. Chem. 68 (2003) 7651–7660. [25] G. Sugihara, A. Miyazono, S. Nagadome, T. Oida, Y. Hayasi, J. Ko, J. Oleo Sci. 52 (2003) 449–461. [26] Cl. Oelschlaeger, G. Waton, S.J. Candau, M.E. Cates, Langmuir 18 (2002) 7265–7271. [27] T. Yoshimura, K. Kawashima, Y. Koide, H. Shosenji, K. Esumi, J. Oleo Sci. 51 (2002) 221–227. [28] T. Tatsumi, W. Zhang, Y. Nakatsuji, K. Miyake, K. Matsushima, M. Tanaka, T. Furuta, I. Ikeda, J. Surfact. Detergents 4 (2001) 271–277. [29] G. Bai, H. Yan, R.K. Thomas, Langmuir 17 (2001) 4501–4504. [30] L. Grosmaire, M. Chorro, C. Chorro, S. Partyka, R. Zana, Colloid Polym. Sci. 246 (2002) 175–181. [31] K. Esumi, M. Miyazaki, T. Arai, Y. Koide, Colloids Surf. 135 (1998) 117. [32] K. Tsubone, N. Uchida, J. Am. Oil Chem. Soc. 67 (1990) 149–153. [33] L.M. Camillo, B. Sesta, M.G. Bonicelli, G.F. Ceccaroni, Langmuir 6 (1990) 728–731. [34] B. Sesta, J. Phys. Chem. 93 (1989) 7677–7680. [35] K. Tsubone, N. Uchida, H. Niwase, K. Honda, J. Am. Oil Chem. Soc. 66 (1989) 829–833. [36] H. Hidaka, M. Moriya, M. Takai, J. Am. Oil Chem. Soc. 56 (1979) 914. [37] N.C. Christov, N.D. Denkov, P.A. Kralchevsky, K.P. Ananthapadmanabhan, A. Lips, Langmuir 20 (2004) 565–567. [38] K.D. Danov, S.D. Kralchevska, P.A. Kralchevsky, K.P. Ananthapadmanabhan, A. Lips, Langmuir 20 (2004) 5445–5453. [39] A. Shiloach, D. Blankschtein, Langmuir 13 (1997) 1486–1495. [40] J.H. Clint, J. Chem. Soc. Faraday Trans. 1 (71) (1975) 1327–1334. [41] K. Motomura, M. Aratono, in: K. Ogino, M. Abe (Eds.), Mixed Surfactant System, Dekker, New York, 1993. [42] R. Nagarajan, Langmuir 1 (1985) 331–341. [43] M. Aratono, M. Villeneuve, T. Takiue, N. Ikeda, H. Iyota, J. Colloid Interface Sci. 200 (1998) 161–171. [44] B.Y. Zhu, M.J. Rosen, J. Colloid Interface Sci. 99 (1984) 435–442. [45] M.J. Rosen, B.Y. Zhu, J. Colloid Interface Sci. 99 (1984) 427–434.
Synergistic interactions in the mixed micelles of cationic gemini with zwitterionic 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) surfactants has been investigated by measuring the surface tension of aqueous solution as a function of total concentration at various pH values from acidic to basic, under conditions of 298.15 K and atmospheric pressure. The results were analyzed by applying regular solution theory (RST), and Motomura’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 of the 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 solution have been extensively investigated as a result of their technological relevance in interfacial systems allowing for a better control of stability, flotation and rheology of dispersed system in different technological processes [1–3]. Many works have been developed in order to predict the ideal behavior of surfactants in aqueous solution regarding equilibrium surface tension and adsorption at fluid interface [4–10]. Micellar systems consisting of mixtures of surfactants of variable structure are of a high theoretical and industrial interest. The physicochemical properties of an aqueous solution constituted by two or more surfactants often present a very different character in comparison to those formed by single surfactants [11]. Mixtures of different surfactant types often exhibit cooperative interactions or synergism in their effects on the properties of a system [12]. This synergism can be attributed to nonideal mixing effects in the aggregates, which results in critical micellization concentrations (cmc) and interfacial tensions that are substantially * Corresponding author. Fax: +1 902 867 2414.
E-mail address: [email protected] (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 properties of the unmixed surfactants alone. Synergism is often modeled in the literature by employing the regular solution theory (RST) [13] with a negative interaction parameter. RST has proven successful in accounting for the nonideal behavior of a number of binary surfactant systems but it does not adequately describe the behavior of monomers in a surfactant solution, activity coefficient, and excess free energy in mixed surfactant systems [14]. Thermodynamic treatments have been developed to calculate micelle properties such as micellar composition [15–17]. Molecular thermodynamics allows the calculation of the total free energy of micellization and the prediction of a broad spectrum of solution properties [18]. Motomura’s theory is one such theory, which allows for the calculation of the free energy of micelle formation as a measure of molecular interactions. Surfactants consisting of two hydrophilic and hydrophobic groups have attracted considerable research interest over the last decade. These are called gemini or dimeric surfactants [19–22] because they have a structure that can be viewed as two monomeric surfactant units connected at or near the head groups by spacer. This kind of architecture provides solution properties that are dependent upon the nature and size of the head groups. A number of studies have reported their versa-
K. Singh, D.G. Marangoni / Journal of Colloid and Interface Science 315 (2007) 620–626
tile synthesis as well as their wide range of physicochemical properties [23–31]. Gemini surfactants are superior to the corresponding conventional monomeric surfactants in number of aspects. They have much lower critical micelle concentrations (cmc) values, better lime-soap dispersing, and better wetting properties. In view of their high cost, gemini surfactants are likely to be used in combination with conventional surfactants. Recently, several research papers have appeared in the literature regarding their mixing behavior with conventional surfactants (mainly ionic and nonionic). These mixtures are considered to have practical applications in many surfactant formulations. On the other hand, there have been very few studies on mixtures of conventional zwitterionic surfactants and gemini surfactants. Zwitterionic surfactants have a dipolar head bearing both positive and negative charges and have many interesting solution properties [32–36]. In general, zwitterionic surfactants are mild to skin and eyes, have low toxicity, and display excellent water solubility, high foam stability, and excellent surface tension reducing properties. To date, zwitterionic surfactants have been generally much less studied than other classes of surfactants. Their behavior is often desirable in relation to ionic and nonionic surfactants but the electrically neutral nature of zwitterionic surfactants clearly distinguishes them from ionic surfactants. As solutions in pure water, their behavior has been found to reassemble that of “ionic surfactant with excess salt.” The biodegradability of such compounds and their reduced skin and eye irritation are also of great importance. In combination with other surfactants, these surfactants find their applications in, laundry detergents, shampoos, and other cosmetics products [37,38]. Due to the wide spread use of cationic/zwitterionic mixed surfactants systems both commercially and industrially, it would be very beneficial to develop a predictive theory for these kinds of mixtures [39]. The objective of this study was to investigate the mixed gemini/zwitterionic systems, and to observe the presence of cooperative interactions at various pH values ranging from acidic to basic.
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was ±0.05 mN/m. The pH of the solutions was controlled with sodium hydroxide (NaOH) and hydrochloric acid (HCl), and pH measurements were carried out with Digi-Sense digital pH meter from Cole-Parmer Instrumental Co., USA triply deionized 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 the mole fraction (αi ) of the respective surfactant, defined by αi =
[Si ] , [Si ] + [Sj ]
(1)
where [Si ] and [Sj ] are the molar concentrations of the surfactants i and j in the mixed solutions. The cmc was taken as a point of intersection by fitting the pre-micellar and postmicellar data in linear equations as shown in Fig. 1. The errors in 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 shown as a function of the logarithm of total surfactant concentration, in Fig. 1. In order to determine the cmc, two linear fits were used to each of the isotherm. The first line was fitted to the interval of concentration characterized by a linear decrease in surface tension and the second to the region of concentration with nearly constant surface tension. The cmc values for the pure surfactants and the various mixtures have been evaluated by linear fitting of pre-micellar and post-micellar data (Fig. 1). The experimental cmc values evaluated from surface tension measurements at various pH conditions and the ideal
2. Experimental 2.1. Materials and method The cationic gemini surfactants, dialkylenebis(alkyldimethylammonium bromide) (12-6-12 and 12-4-12) was synthesized according to the method reported elsewhere [19]. These surfactants were purified by repeated crystallizations from ethanol/acetone mixtures and finally from pure ethanol. The purity of these surfactants was judged from a plot of surface tension vs logarithm of surfactant concentration. Because there was no appreciable minimum visible in the surface tension 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 received. The surface tension measurements were carried out with a Krüss (K-8) tensiometer using a platinum ring at constant temperature (298 ± 0.1 K). The measurement accuracy
Fig. 1. Variation of the surface tension vs logarithm of concentration of pure 12-4-12, pure EBB, and their mixtures in water at 25 ◦ C. The solid line represents 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 various binary surfactant mixtures (Figs. 2a–2f). α1 (1 − α1 ) 1 = + , ∗ cmc cmc1 cmc2
(2)
where α1 is the mole fraction of surfactant 1 (i.e. EBB) in the total mixed solute, and cmc1 and cmc2 are the critical micelle concentrations of pure components, respectively. The deviation of cmc from cmc* indicates nonideality in the mixed micelles. A nonideal mixing behavior is expected for the present binary mixtures 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).
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show that the cmc values of each mixture vary nonlinearly with respect to the change in bulk mole fraction, and that the cmc values are lower than the cmc* for all the binary mixtures. It is to be mentioned here that a lower value of cmc than the corresponding cmc* value indicates mixed micellization due to some sort of attractive interactions operating between the unlike components. Due to the presence of dimeric gemini head groups in combination with those of bulky zwitterionic head groups in the Stern layer of the mixed micelles, a nonideal behavior is expected in the mixed state. A quantitative interpretation of the results can be carried out by considering the regular solution approximation. This theory allows for the calculations of the micellar mole fraction as well as the interaction parameter by using equations, respectively, X12 ln(cmc α1 /cmc1 X1 ) = 1, (1 − X1 )2 ln(cmc(1 − α1 )/cmc2 (1 − X1 )) ln(cmc α1 /cmc1 X1 ) β= , (1 − X1 )2
(3) (4)
where X1 is the micelle mole fraction of surfactant 1 (i.e. EBB) in the mixed micelles and β is the interaction parameter which indicates the magnitude of interactions operating between the unlike components in the mixed micelle state. The micelle mole fraction in the ideal state (Xideal ) can also be evaluated by applying Motomura’s theory [41], which is based upon excess thermodynamic quantities, equation Xideal = (α1 cmc2 ) α1 cmc2 + (1 − α1 )cmc1 . (5) Deviation in X1 from the corresponding Xideal values would indicate the presence of nonideality in the mixed micelles. This variation between both X1 and Xideal values has been shown graphically in Figs. 3a–3f. In Fig. 3, X1 values are mostly lower than the Xideal values, with exception of 12-6-12 + EBB systems at pH 7 (Fig. 3b). Here the X1 values are higher than Xideal values in the starting mole fractions and then come close to Xideal , and there after X1 values are lower than Xideal . This result is quite consistent with the previous findings of Singh and Bakshi for gemini/zwitterionic (10-2-10 + DPS) combinations under the same set of conditions [11]. The variation of X1 and Xideal in the 12-4-12 + EBB systems is more or less consistent with the 12-6-12 + EBB systems (Figs. 3d–3f). A lower X1 than that of corresponding Xideal value indicates that the mixed micelles are rich in gemini component, while X1 value close to Xideal value indicates ideal mixing. A close inspection of Fig. 3 indicates that a change in the pH of the system from neutral to acidic or basic allows the relatively lesser amount of zwitterionic component in the mixed micelles. This effect is very pronounced when the pH value increases from 7 to 10 (Figs. 3a and 3b). A similar behavior is observed for 12-4-12 + EBB mixed surfactant systems (Figs. 3d, 3e, and 3f). These results are further evaluated on the basis of the variation of β values for all the present mixtures (Eq. (4)). In regular solution theory, the component molecules are assumed to be of comparable volume, completely interchangeable, and the interaction energy is expressed as a sum of pair-wise, nearest
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neighbor interactions. The β12 parameter is still useful even in the presence of other existing theories in indicating the nature of the interaction. A better interpretation of the interaction parameter as well as the shapes of the resulting aggregates can be achieved by considering the molecular thermodynamic approach [16,18,42] when the cmc values of unlike components are close enough to each other. However, this is not the case for most of the present mixtures. β values calculated for all the mixtures from Eq. (4), has been plotted against the bulk mole fraction in Fig. 4a (12-6-12 + EBB, shown in supplementary Fig. S1), furthermore, average β(βavg ) values are also plotted against the pH of solution in Fig. 4b. Fig. 4b demonstrates that the βavg values for all the mixtures are negative. A negative βavg value can be attributed to mixed micelle formation due to synergistic interactions. Therefore, all the present mixtures undergo mixed micelle formation due to attractive interactions operating between the gemini and zwitterionic components. However, a close inspection of Figs. 4a and 4b demonstrates that the magnitude of synergism increases with an increase in bulk mole fraction of zwitterionic component for all pH values, and maximum synergism exist under the basic conditions. Furthermore, the lower βavg value for the 12-4-12 + EBB systems than the 12-6-12 + EBB (except at pH 10), indicates that synergism in the mixed micelles of gemini/zwitterionic surfactant mixtures decrease with an increase in the length of the spacer. 3.2. Evaluation of nonideality from Motomura’s theory Quantitatively, deviation from ideal mixing in the micelles is demonstrated by excess Gibbs free energy of micelle formation (g M,E ), which was calculated by applying Eqs. (6)–(8), resulting from the Motomura’s theory [43] g M,E = RT X1M ln f1M + X2M ln f2M , (6) where f1M and f2M are the activity of surfactant 1 (i.e. EBB) and surfactant 2 (i.e. gemini) in micelle evaluated by applying equations CX1 = C10 f1M X1M
(for EBB),
(7a)
CX2 = C20 f2M X2M
(for gemini).
(7b)
In this equation C is the cmc of the mixture, C10 and C20 are the cmcs of pure EBB and gemini surfactant, respectively, X1M and X2M are the micellar mole fractions in term of EBB and gemini surfactants, respectively. X2M for the gemini surfactant is evaluated through equation X1 X 2 ∂C M . X2 = X 2 − (8) C ∂X2 T ,P The g M,E values calculated for EBB/gemini systems and 12-412 + EBB systems at various pH values have been plotted in Fig. 4c (12-6-12 + EBB, shown in supplementary Fig. S1). All g M,E values are negative, which means that mixed micelle formation by these two surfactants is a favorable process. Gharibi et al. [14] have observed a similar type of nonlinear variation in
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Fig. 3. Micellar mole fractions, X1 , Xideal determined from RST (points) and Motomura’s theory (lines), respectively, versus mole fraction of zwitterionic, α EBB , 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.
g M,E for the mixed surfactant systems of CTAB and Triton X100. The variation of g M,E is very close to the variation of the β values. Furthermore, the average excess Gibbs free energy M,E of micelle formation (gavg ) have been evaluated and compared with the variation in βavg (Figs. 4b and 4d) and a very similar trend has been observed.
4. Discussion A collective interpretation of the present results, evaluated from regular solution theory, indicates nonideality for all the binary mixed systems and synergistic interactions are observed under 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) g M,E determined from M,E versus pH 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) gavg of solution for all combinations.
ever, that combinations of a zwitterionic surfactant based on a basic carboxylate group that may be protonated and an anionic surfactant may exhibit strong synergism [44,45]. The reason for this is that the zwitterionic surfactant will be protonated, i.e. transferred into a cationic surfactant, in the micellization process. One way to express this is that formation of mixed micelles between a cationic and an anionic surfactant is so energetically favored that it drives protonation of the carboxylate group although the bulk pH is much higher than the pKa for the unimer. In the present study an attempt has been made to study the effect of pH induced protonations/deprotonation of carboxylate/carboxylic group of EBB on the mixed micelle formation between EBB and cationic gemini surfactants, additionally, the effect of spacer has also been studied. The head group of zwitterionic surfactant studied here carries both a quaternary ammonium group and a carboxyl group as well. At low pH, i.e. in acidic conditions, EBB has only a positive charge localized on the quaternary ammonium group due to protonation of 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 surfactant because of the deprotonation of carboxylic group, and again synergism is imparted in the mixed micelles, but to a greater extent. The latter behavior has been observed for previously 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 surfactant monomers into the micelles of gemini surfactants which leads to the synergistic effect. Apart from this, increase in spacer length also affects synergistic interactions, most likely due to the increase in hydrophobicity at the level of head group and the modification in the distance between the head groups of amphiphiles in the micellar phase. Negative Gibbs free energy of micellization for all the mixtures indicates that mixed micelle formation is thermodynamically favorable. Furthermore, M,E a close variation of βavg , and gavg vs pH clearly indicates that strongly interacting surfactant has greater feasibility of mixing.
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5. Conclusions Synergistic mixing behavior of cationic gemini and carboxylate based zwitterionic surfactant can be modified under different conditions of pH, and further modification is also possible from the change in the spacer length of gemini component. Maximum synergistic effect has been observed under basic conditions, where EBB acts like a true zwitterionic surfactant. In all cases mixed micelles are rich in gemini component and incorporation of zwitterionic surfactant monomers into the micelles of gemini surfactant is most likely the cause of synergistic effect. A collective interpretation by regular solution approximation and Motomura’s theory indicates that strongly interacting surfactant molecule has greater feasibility of mixing. These observations may be of interest in practical applications of gemini and zwitterionic surfactant combinations. Acknowledgments The research grants from NSERC (Research Grant and Research Capacity Development), the Atlantic Innovation Fund, StFX University, and Newpark Resources, are thankfully acknowledged. Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2007.06.062. References [1] J.C.T. Kwak, Polymer–Surfactant Systems, Dekker, New York, 1998. [2] R.M. Hill, in: K. Ogino, M. Abe (Eds.), Mixed Surfactant Systems, in: Surfactant Sci. Ser., vol. 46, Dekker, New York, 1993, chap. 11. [3] S. Ghosh, S.P. Moulik, J. Colloid Interface Sci. 208 (1999) 357–366. [4] E.H. Lucassen-Reynders, J. Lucassen, D. Giles, J. Colloid Interface Sci. 82 (1981) 150–157. [5] M.J. Rosen, X.Y. Hua, J. Am. Chem. Soc. 59 (1982) 427–431. [6] M.J. Rosen, Surfactant and Interfacial Phenomena, second ed., Wiley, New York, 1989, chap. 11. [7] M.J. Rosen, Langmuir 7 (1991) 885–888. [8] D.N. Rubingh, T. Jones, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 176– 182. [9] I. Reif, P. Somasundaran, Langmuir 15 (1999) 3411–3417. [10] P. Wydro, M. Paluch, Colloids Surf. A Physicochem. Eng. Aspects 245 (2004) 75–79.
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