Mixed micellization of tetradecyltrimethylammonium bromide and Triton X-100 in water–ethanol mixtures, using potentiometric and surface tension techniques

Journal of Molecular Liquids 135 (2007) 128 – 134 www.elsevier.com/locate/molliq

Mixed micellization of tetradecyltrimethylammonium bromide and Triton X-100 in water–ethanol mixtures, using potentiometric and surface tension techniques Amir Abbas Rafati ⁎, Hamed Maleki Faculty of Chemistry, Bu-Ali Sina University, P.O. Box 65174, Hamedan, Iran Received 20 November 2005; accepted 15 November 2006 Available online 7 March 2007

Abstract The mixed micelles of tetradecyltrimethylammonium bromide (TTAB) and Triton X-100 (TX-100) in mixed solvent have been investigated using potentiometric and surface tension techniques. In order to estimate the interaction between the surfactants and ethanol, the cmc data were treated by using the conventional regular solution theory for mixed micelles. It was found that the interaction parameter values (β) varied with variation of solvent composition. In all cases the results showed deviation from ideal behavior. The stability of the mixed micelles was also discussed based on Maeda's approach. The obtained data show that mixed micellization became unfavorable with increasing alcohol concentration. Also, the maximum synergism in ethanol/water mixtures has been determined according to Rosen's new approach. The observed maximum synergisms were obtained from x1 = 0.25 to x1 = 0.35 at different concentrations of ethanol. © 2007 Elsevier B.V. All rights reserved. Keywords: Surfactant; Mixed micelle; Synergism; Surface tension; Potentiometry; Regular solution theory (RST)

1. Introduction Surfactant solutions are used widely in numerous technical applications such as in detergents, cosmetics, pharmaceuticals, enhanced oil recovery or surfactant-based separation processes (flotation). Surfactants are often mixed in water or hydroorganic media. This is so because (i) technical-grade surfactants are mixtures themselves, hence their purification process may be difficult or excessively expensive and (ii) the mixed system often behaves better than a single surfactant [1–4]. Surfactant mixtures often exhibit features deviating significantly from individual surfactants (i.e., they exhibit substantial synergism) [5,6]. To maximize beneficial effects of mixtures over individual surfactants, it is helpful to understand the interactions among surfactants in the mixtures. The micelle formation in an aqueous solution is known to be affected by organic additives, and there have been many in⁎ Corresponding author. Fax: +98 811 8272404. E-mail address: [email protected] (A.A. Rafati). 0167-7322/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2006.11.011

vestigations concerning the effect of organic additives on the cmc of individual surfactants [7–11]. Recently, increasing attention has been devoted to the study of the incorporation or solubilization of neutral molecules into micelle in aqueous solution. Some of the most studied solubilizates are alcohols, because of the important role they have in the preparation of microemulsions [12–18]. It is generally accepted that the alcohol binds to the micelle in the surface region, leading to three principal effects: (a) intercalation of alcohol molecules between the surfactant ionic head groups [19–23]; (b) decrease in dielectric constant at the micellar interface [24]; and (c) change in the molecular order of the interface region of the micelle [12]. For the literature cited here, the physicochemical studies on alcohols in aqueous mixture of surfactants are of chief interest, and it seems that the effect of alcohol addition in micellar solution of various mixed surfactant solutions has not been studied. In continuation of our work on micelle formation in non-aqueous solvents [25–29], a systematic attempt has been made to study the effect of alcohols on the micelle of various surfactants in mixed states.

A.A. Rafati, H. Maleki / Journal of Molecular Liquids 135 (2007) 128–134

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The analysis of micellar systems made of mixed surfactants requires experimental techniques that provide unambiguous determination of the distribution of different species in solution and their partition between solution and aggregates. For electrically charged amphiphiles, potentiometric methods can be considered, using selective electrodes. Recent extensive development of these electrodes permits their use in the study of different equilibrium reactions involving ionic surfactants [30–33]. For the simplest case of a binary surfactant system without any significant net interaction between surfactant types, the mixture cmc (cmcMix) obeys the ideal solution theory [34]. When two surfactants forming a mixed micelle have different head groups, the cmc is not predicted by the ideal mixing theory. An extension of the Lang–Shinoda approach [35–37] has been developed to include mixtures of nonionic and ionic surfactants. A simpler approach has been given by Rubingh [38] and it applies the regular solution theory (RST) to the mixed micelle, thus allowing for the interaction between the surfactant molecules within the mixed micelle. In analyzing the mixtures of surfactants, RST is the most widely used model. In the present work, the various aspects of micellization of TTAB and TX-100 mixture in the presence of ethanol have been investigated, using potentiometric and surface tension techniques. We use an ion-selective electrode sensitive to cationic surfactant (i.e. TTAB) and other surfactant (i.e. TX100) is neutral with a low cmc. Moreover, though the experimental results are very precise, the interpretation of the electromotive force (emf) curves lacks precision and thus surface tension is required to analyze the data.

(Methrohm 60501-100), as reference electrode. The cell configuration used for the potential measurements was

2. Experimental

where [S ]f and [C ]s are concentration of free surfactant and coion (here, sodium ion from backing electrolyte), respectively. Since the surfactant is dissociated completely into ions below the cmc, the plot of emf against log[S ]t ([S ]t is total concentration of surfactant) obeys Nernstian behavior. Hence, the slope of the line is referred to 2:303 RT F and the intercept, ° +/Na + .This calibration line can be used for the determination Esur of monomer surfactant ion concentration above the cmc by adjusting data on the calibration line. A least mean squares method was used for determination of the slope, 2:303 RT F , and ° +/Na +, for each set of data [26,28,30,39,40]. intercept, Esur

2.1. Chemicals Tetradecyltrimethylammonium bromide (TTAB) was obtained from Sigma (approx. 99%) and used without further purification. TX-100 (N 99%) and ethanol (N99%) were obtained from Merck and used as received. All solutions were prepared with doubly distilled water. 2.2. Methods All experiments were carried out at a temperature of 30 °C and this was maintained with ± 0.1 °C by circulating thermostated water (Pharmacia Biotech). The surface tension of surfactant solutions was determined using an automatic tensiometer, KSV Instruments, model Sigma 70 (Helsinki, Finland) employing the ring method. The ion-selective electrode sensitive to TTA+ was used for the measurement of the free concentration of surfactant ion, [S]f, in equilibrium with micelles at different conditions. The membrane ion-selective electrode (MISE) used in the present work was originally constructed to investigate the behavior of mixed surfactants in water/ethanol media. The procedures used to construct this MISE are well documented [26–29]. The emf measurements of the surfactant selective electrode were measured relative to a commercial sodium ion electrode

Surfactant Electrode ðMISEÞ j test solution j reference electrode

The following equations can be written for different electrode potentials, according to Nernst's equation Esurþ ¼ Esur þ þ

RT ln asurþ F

ð1Þ

ENaþ ¼ ENa þ þ

RT ln aNaþ F

ð2Þ

where T, R, F, asur+, and aNa+ represent the absolute temperature, gas constant, Faraday's constant, activity of surfactant ion, and ° + and ENa ° + activity of sodium ion, respectively. Esur+, ENa+, Esur indicate the sodium, surfactant and corresponding standard electrode potentials, respectively. We have assumed that at low ionic strength the mean activity coefficient of different ions irrespective of charge and shape leads to unity. At constant sodium ion concentration, which applies to this experiment, we can use the following equation for the determination of free surfactant concentration emf ¼ Esur þ =Naþ þ 2:303

RT log½S f F

ð3Þ

where Esur þ =Naþ ¼ ENaþ −ENaþ −2:303

RT log½C s F

ð4Þ

3. Results and discussion In the absence of any significant net interaction between surfactant types, the ideal solution theory predicts the cmc of mixture as follows 1 y1 1−y1 ¼ þ cmcMix cmc1 cmc2

ð5Þ

where y1 is the solution mole fraction of the surfactant 1 (solution composition) with critical micelle concentration cmc1, and cmc2 denotes the cmc for the second surfactant alone. The ideal mixing theory has been successful in explaining the properties of mixtures composed of surfactants with similar chemical structures; however, it fails for mixtures containing chemically

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the larger the absolute value of β the stronger the interaction, whether attractive or repulsive. For the two-surfactant system, the micellar composition of each component is given by xi ¼

Fig. 1. Plot of the surface tension vs. the total surfactant concentration of the system TTAB/TX-100 at different mole fractions of TTAB in solution.

dissimilar surfactants. In the present work the individual surfactant types interact strongly and more elaborate analysis is required to gain molecular insight. In this case Eq. (5) can be recasted to reflect the activity coefficients, γi, of the individual surfactants within the mixed micelle 1 y1 1−y1 ¼ þ cmcMix g1 cmc1 g2 cmc2

ð6Þ

On the other hand, nonideality can be analyzed by using a regular solution theory (RST) which introduces an interaction parameter to characterize the interactions between the two surfactant species in the mixed micelle. This interaction parameter is related to the activity coefficients of the surfactants within the micelle by lng1 ¼ ð1−x1 Þ2 b

ð7Þ

lng2 ¼ x21 b

ð8Þ

ðci −mi Þ ðci −mi Þ þ ðcj −mj Þ

ð11Þ

where ci and cj are total concentrations of i and j species respectively, and mi and mj are corresponding monomeric concentrations. In a particular experiment the total concentrations ci and cj will be known and since i is an ionic monomer, mI can be determined by using the surfactant selective electrode in cells without a liquid junction in the usual work. Experimentally, it is difficult to determine the concentration of the monomer of a nonionic surfactant in equilibrium with the mixed micelles in such systems. Because of this, the nonionic components were chosen to have a low cmc such that the monomer concentration above the mixed cmc was low enough to be negligible. In addition, a relatively high total concentration, cj, of the nonionic component can be used such that (cj − mj) ≈ cj. In this study the monomer concentration of nonionic surfactant (mj) is equal to the nonionic surfactant concentration at which cmc is obtained by surface tension. Then for such systems Eq. (11) may be approximated by xi ¼

ðci −mi Þ ðci −mi Þ þ cj

ð12Þ

Eq. (9) must be solved iteratively for xi, whereupon substitution into Eq. (10) results in immediate solution for β. Initially, the Rubingh method was used for calculating xi as monomer concentration in mixed micelles. The values of xi do not converge iteratively for xi where Eq. (9) has been used. Alternatively, a surfactant selective electrode fabricated in the laboratory was applied for determination of xi and finally evaluating of β [6,38]. Fig. 1 shows a typical plot of surface tension against the logarithm of total concentration of surfactant at different mole fractions of TTAB/TX-100 mixture in 1% (v/v) ethanol/water.

where x1 is the mole fraction of the surfactant 1 in the mixed micelle, which can be calculated solving iteratively the equation   Mix x1 ln yx1 1cmc cmc1  ¼1 ð9Þ ð1−y1 ÞcmcMix 2 ð1−x1 Þ ln ð1−x1 Þcmc2 Subsequently, the interaction parameter, β, can be evaluated from the equation   Mix ln yx1 1cmc cmc1 b¼ ð10Þ ð1−x1 Þ2 Attractive interactions between two surfactant types result in a negative β value, positive β values imply a net repulsion, and

Fig. 2. Plot of emf cell against logarithm of total concentration of surfactants for TTAB/TX-100 mixtures at different mole fractions of TTAB in solution.

A.A. Rafati, H. Maleki / Journal of Molecular Liquids 135 (2007) 128–134 Table 1 cmc values obtained from surface tension (cmc1⁎) and emf measurements (cmc2⁎) for TTAB/TX-100 mixtures at different mole fractions of ionic surfactant in solution y1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

cmc1⁎ (mM)

cmc2⁎ (mM)

Ethanol percentage (%v/v)

Ethanol percentage (%v/v)

2%

5%

10%

20%

30%

2%

5%

10%

20%

30%

0.218 0.201 0.208 0.274 0.291 0.364 0.420 0.633 1.07 –

0.400 0.229 0.218 0.301 0.326 0.404 0.442 0.714 1.040 –

0.227 0.226 0.299 0.312 0.356 0.446 0.467 0.704 1.21 –

0.329 0.457 0.460 0.552 0.521 0.651 0.805 0.935 0.932 –

0.751 0.944 1.090 1.010 1.350 1.280 1.700 2.070 1.860 –

0.670 1.540 2.360 2.580 3.000 3.390 3.320 2.900 2.230 3.680

0.605 1.510 2.300 3.290 2.380 3.000 2.270 4.560 3.680 3.990

0.330 1.740 2.700 2.600 3.200 3.070 4.120 3.870 3.570 3.420

0.220 1.260 2.340 4.170 3.370 2.240 3.700 4.000 2.900 4.800

0.415 1.060 1.300 3.220 4.670 7.130 6.020 5.800 6.500 9.870

In the surface tension experiments, the first micelles formed cause a sharp break in the surface tension curve. It is believed that this break is owing to micellization of TX-100. Since the cmc value of TX-100 is smaller than TTAB then TX-100 micelles will be formed earlier. Fig. 2 shows the changes of emf against the logarithm of total concentration of surfactants for TTAB/TX-100 mixtures at some typical mole fractions. The first region shows the Nernstian slope indicating that there is no measurable interaction between ionic and nonionic surfactant in the form of free monomers. Since the surfactant is dissociated completely into ions below the cmc [25], the plot of emf against log ci shows Nernstian behaviour. In the second region a break was found in the linear line that is shown as first critical micelle concentration (cmc1⁎) where the interaction between ionic and nonionic surfactants begins leading to the formation of TX-100 micelles. cmc2⁎ is the beginning of the third region where the

monomer concentration has reached a maximum and may present ionic micelles saturated by non-ionic surfactant. The emf data were plotted against total surfactant concentration, [S]t. As the surfactant selective electrode responses only to the monomeric form of surfactant, the emf should decrease after micellization because the micellization process decreases the monomer concentration of the surfactant. The cmc values for mixed and pure solutions obtained from surface tension and emf measurements for TTAB/TX-100 systems in different water/ethanol compositions are tabulated in Table 1. The cmc values, which are measured by surface tension, represent the total surfactant concentration in solution.

Table 2 β, y1, x1, γ1 and γ2 values calculated based on RST for TTAB/TX-100 system at cmc⁎ (obtained from surface tension measurements) in different ethanol concentrations EtOH (% v/v)

y1

x1

β

γ1

γ2

2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.084 0.172 0.209 0.181 0.227 0.238 0.289 0.314 0.404 0.134 0.135 0.203 0.153 0.200 0.213 0.282 0.288 0.405 0.168 0.212 0.180 0.223 0.248 0.260 0.328 0.359 0.452 0.169 0.089 0.159 0.153 0.247 0.261 0.298 0.373 0.476 0.188 0.167 0.171 0.250 0.234 0.312 0.334 0.395 0.492

− 3.16 − 4.03 − 4.01 − 2.68 − 2.93 − 2.40 − 2.54 − 1.75 − 1.22 − 4.39 − 3.30 − 3.95 − 2.27 − 2.48 − 2.02 − 2.50 − 1.38 − 1.54 − 4.67 − 4.46 − 2.86 − 3.00 − 2.75 − 2.20 − 2.73 − 1.89 − 1.16 − 4.64 − 1.85 − 2.42 − 1.68 − 2.67 − 2.13 − 1.89 − 2.21 − 3.65 − 4.86 − 3.12 − 2.39 − 3.22 − 2.10 − 2.93 − 2.29 − 2.35 − 4.12

0.071 0.063 0.081 0.165 0.174 0.240 0.277 0.439 0.648 0.037 0.085 0.809 0.197 0.204 0.285 0.275 0.497 0.579 0.040 0.062 0.146 0.163 0.210 0.300 0.291 0.459 0.705 0.041 0.215 0.181 0.300 0.220 0.312 0.394 0.419 0.367 0.040 0.115 0.193 0.164 0.292 0.250 0.361 0.424 0.345

0.978 0.887 0.840 0.916 0.860 0.873 0.809 0.842 0.820 0.924 0.942 0.850 0.948 0.906 0.912 0.820 0.892 0.777 0.877 0.819 0.912 0.861 0.845 0.862 0.745 0.784 0.788 0.876 0.986 0.941 0.961 0.850 0.865 0.845 0.735 0.437 0.842 0.917 0.932 0.818 0.892 0.753 0.775 0.693 0.370

5

10

20

30

Fig. 3. Experimental (•) and predicted (lines) critical micelle concentration of TTAB/TX-100 mixtures as a function of the mole fraction of TTAB. The solid line is the best fit to the data according to the regular solution approach. The concentration of EtOH is 2% v/v, typically.

131

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Table 3 β values determined from plot of lnγ1 against (1 − x1)2 and using RST in different concentration of EtOH EtOH (% v/v)

β

2 5 10 20 30

− 3.06 − 2.33 − 1.41 − 0.88 − 0.55

Fig. 3 shows the cmc⁎ values obtained from surface tension experiments for the TTAB/TX-100 system investigated as a function of the mole fraction of ionic surfactant in the solution (y1) at 2% (v/v) EtOH solution. It can be observed that mixed cmc⁎ experimental values in water/ethanol mixture are lower than those obtained by assuming ideal behavior (Eq. (5)). This behaviour was observed for all percentages of ethanol. It was found that in the presence of ethanol, the β values varied with the composition of surfactants (see Table 2). In order to obtain a characteristic β value for these systems, we have performed a regression analysis of the cmc⁎ using a least mean square method. The best fits of data in 2% (v/v) EtOH solution are shown in Fig. 3 (solid line). In this figure, it can be observed that mixed cmc experimental values are lower than those obtained by assuming an ideal behavior, i.e., Eq. (5). This negative deviation from ideality indicates attractive interaction between the two surfactants forming the mixed system. In a first attempt to understand synergism, we used the RST plot to understand our experimental results. The validity of the RST for the systems has been examined by the plot of lnγi vs. (1 − xi)2 according to Eqs. (7) and (8). In all cases the plot is a straight line with slope β over the entire range of composition. The slope values (β) are tabulated in Table 3. The main reason for synergism when an ionic surfactant is involved, comes from the reduction of electrostatic repulsion between charged heads when a nonionic surfactant is intercalated in the micelle. This enables a correct prediction of the sign and order of magnitude of β in the simplified molecular thermodynamic model [41]. However, in this model, specific

interactions due to the chemical structure of the surfactants used are not investigated, because they are very difficult to evaluate. The predictions are then the same for surfactants having similar geometrical characteristics but different structures, whereas the experimental results are different. As mentioned above, variation of β values with composition in water/ethanol mixtures were observed. Particularly, we found that β values become more negative as the mole fraction of the TTAB decreases. In general, this behavior could be rationalized taking into account the role of the repulsive interactions of the head groups of the cationic surfactant in the stability of the mixed micelles. Note that the intercalation of nonionic surfactants in the mixed micelle prevents these repulsive interactions

Table 4 Interaction parameters according to Maeda's approach for the different mixed solvent composition EtOH (% v/v)

B0

B1

B2

2

− 8.43 − 8.43 − 8.43 − 8.43 − 8.43 − 8.43 − 8.43 − 8.43 − 8.43 − 8.40 − 8.40 − 8.40 − 8.40 − 8.40 − 8.40 − 8.40 − 8.40 − 8.40 − 8.18 − 8.18 − 8.18 − 8.18 − 8.18 − 8.18 − 8.18 − 8.18 − 8.18 − 7.81 − 7.81 − 7.81 − 7.81 − 7.81 − 7.81 − 7.81 − 7.81 − 7.81 − 6.92 − 6.92 − 6.92 − 6.92 − 6.92 − 6.92 − 6.92 − 6.92 − 6.92

− 0.34 − 1.21 − 1.19 0.14 − 0.11 0.43 0.28 1.07 1.60 − 1.51 − 0.42 − 1.08 0.61 0.39 0.85 0.38 1.49 1.33 − 2.16 − 1.96 − 0.36 − 0.50 − 0.25 0.30 − 0.23 0.61 1.34 − 2.18 0.62 0.05 0.79 − 0.20 0.34 0.58 0.26 − 1.18 − 2.56 − 0.82 − 0.09 − 0.92 0.21 − 0.63 0.01 − 0.04 − 1.81

3.16 4.03 4.01 2.68 2.93 2.40 2.54 1.75 1.22 4.39 3.30 3.95 2.27 2.48 2.02 2.50 1.38 1.54 4.67 4.46 2.86 3.00 2.75 2.20 2.73 1.89 1.16 4.64 1.85 2.42 1.68 2.67 2.13 1.89 2.21 3.65 4.86 3.12 2.39 3.22 2.10 2.93 2.29 2.35 4.12

5

10

20

30

Fig. 4. Typical plot of x1 vs. y1 for the determination of maximum synergism in 5% v/v EtOH. The open circle represents surface tension data whereas the closed circles are data obtained by potentiometric technique.

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from achieving the electrostatic stabilization of the micelle. It is interesting to point out that some authors [3,42] have justified similar variations of β with the composition in systems formed by cationic and nonionic surfactants consisting of polyoxyethylene (POE) group, based on the repulsive interactions between the cationic head groups and oxonium ions formed in the hydrophilic moiety of the nonionic surfactant. 3.1. Maximum synergism in water/ethanol mixtures Rosen [43–45] has proposed a new approach for the determination of maximum synergism in mixed micelle formation. According to Rosen's approach, the maximum synergism occurs at x1 = y1. The plot of evaluated x1 (using Eq. (9)) versus y1 shows a curve of which the intersection with the line x1 = y1 (dashed line) gives the maximum synergism (see Fig. 4). The observed maximum synergisms were obtained from x1 = 0.25 to x1 = 0.35 at different concentrations of ethanol. 3.2. Maeda approach Recently, Maeda [46] has proposed a new approach for mixed micelles involving ionic species. In the formulation of Maeda, which is based on the phase separation model, the thermodynamic stability is described by ΔG°m , which is given as a function of the mole fraction of the ionic component in the mixed micelle
ð13Þ DG-m ¼ RT B0 þ B1 x2 þ B2 x22 where B0 is an independent term related to the cmc of the nonionic component by B1 ¼ lnC1

ð14Þ

(C1 is the cmc of the nonionic surfactant).

Fig. 5. Stability of TTAB/TX-100 mixed micelles vs. the composition of the mixed micelle. Symbols are indicated in the plot.

133

The parameters B1 and B2 are related to the cmc values of pure systems by   C2 ¼ B1 þ B2 ln ð15Þ C1 The last coefficient, B2, is equivalent to − β in the regular solution theory. When B2 is evaluated, one can obtain B1 from Eq. (15). Maeda's approach parameters obtained for our systems are tabulated in Table 4. Now, based on the description proposed by Maeda, we have estimated the stability of the mixed systems as a function of the micellar composition. Fig. 5 shows the dependence of ΔG°m on micellar composition, where ΔG°m is defined as the stability relative to that of TX-100 pure micelles. As can be seen in Fig. 5, the stability of the mixed system decreases when the participation of the co-surfactant increases. This behavior is reasonable as the incorporation of ionic monomers is associated with an increase of the charge density, causing the electrostatic destabilization of the system. Comparison between free energy of micellization in various percentages of ethanol shows that mixed micellization is unfavorable with increasing alcohol concentration according to increasing ΔG°m values (see Fig. 5). References [1] R.M. Hill, in: K. Ogino, M. Abe (Eds.), Mixed Surfactant Systems, Marcel Dekker, New York, 1993, p. 317. [2] B. Kronberg, Curr. Opin. Colloid Interface Sci. 2 (1997) 456. [3] T.R. Desai, S.G. Dixit, J. Colloid Interface Sci. 177 (1996) 471. [4] A. Shiloach, D. Blankschtein, Langmuir 14 (1998) 1618. [5] J.F. Scamehorn, Phenomena in Mixed Surfactant Systems, ACS Symposium Series, vol. 311, American Chemical Society, Washington, DC, 1986. [6] P.M. Holland, D.N. Rubingh, Mixed Surfactant Systems, ACS Symposium Series, vol. 501, American Chemical Society, Washington, DC, 1992. [7] P.H. Elworthy, A.T. Florence, C.B. McFarlane, Solubilization by Surface Active Agents and Its Application in Chemistry and the Biological Sciences, Chapman and Hall, London, 1968. [8] T. Nakagawa, in: M.J. Schick (Ed.), Nonionic Surfactants, Marcel Dekker, New York, 1967, p. 558. [9] K. Shinoda, Solvent Properties of Surfactant Solutions, Marcel Dekker, New York, 1967. [10] K.L. Mittal, P. Mukerjee, in: K.L. Mittal (Ed.), Micellization, Solubilization and Microemulsions, vol. 1, Plenum Press, New York, 1977, p. 1. [11] J.H. Fendler, E. Fendler, Catalysis in Micellar and Macromolecular Systems, Academic Press, New York, 1975. [12] F. Quirion, J.E. Desnoyers, J. Colloid Interface Sci. 112 (1986) 565. [13] L.V. Dearden, E.M. Woolley, J. Phys. Chem. 91 (1987) 2404. [14] R. De Lisi, A. Lizzio, S. Milioto, V. Turco Liveri, J. Solution Chem. 15 (1986) 623. [15] E. Caponetti, S. Causi, R. De Lisi, M.A. Floriano, S. Milioto, R. Triolo, J. Phys. Chem. 96 (1992) 4950. [16] G. Tardajos, E. Junquera, E. Aicart, J. Chem. Eng. Data 39 (1994) 349. [17] M. Kahlweit, R. Strey, D. Haase, J. Phys. Chem. 89 (1985) 163. [18] E. Vikingstad, H. HØiland, J. Colloid Interface Sci. 64 (1978) 522. [19] M. Manabe, A. Tokunaga, H. Kawamura, M. Shiomi, K. Hiramatsu, Colloid Polym. Sci. 280 (2002) 929. [20] M.A. Safarpour, A.A. Rafati, H. Gharibi, J. Chin. Chem. Soc. 46 (1999) 983. [21] M. Almgren, S. Swarup, J. Colloid Interface Sci. 91 (1983) 256. [22] M.S. Bakshi, J. Chem. Soc., Faraday Trans. 89 (1993) 4323. [23] R. Nagarajan, C.C. Wang, J. Colloid Interface Sci. 178 (1996) 471. [24] R. Zana, Adv. Colloid Interface Sci. 57 (1995) 1. [25] R. Palepu, H. Gharibi, D.M. Bloor, E. Wyn-Jones, Langmuir 9 (1993) 110.

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