Effect of sodium butanoate and sodium hexanoate on micelle formation of model cationic–nonionic and anionic–nonionic surfactant mixtures

Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 114–120

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Effect of sodium butanoate and sodium hexanoate on micelle formation of model cationic–nonionic and anionic–nonionic surfactant mixtures Musarat Jan, Aijaz Ahmad Dar, Ghulam Mohammad Rather ∗ Department of Chemistry, University of Kashmir, Srinagar 190006, J&K, India

a r t i c l e

i n f o

Article history: Received 22 May 2008 Received in revised form 27 September 2008 Accepted 24 October 2008 Available online 6 November 2008 Keywords: TX-100 DPC AOT Binary mixed micelles Sodium butanoate Sodium hexanoate Synergism Interaction parameter Surface tension

a b s t r a c t The formation of mixed micelles in cationic–nonionic (Dodecylpyridinium chloride (DPC)–Triton X-100) and anionic–nonionic (Aerosol OT (AOT)–Triton X-100) aqueous surfactant solutions has been studied at 25 ◦ C both in the presence and absence of sodium butanoate (NaBu) and sodium hexanoate (NaHx). The critical micelle concentration (cmc) and free energy of micellization have been estimated through surface tension measurements. Composition, activity coefficients and mutual interaction parameters of mixed micelles were evaluated in terms of different theoretical models like that of Clint, Rubingh, Rodenas, Maeda and Blankschtein, that explain binary system well. The analysis reveals that, in spite of synergism, very small mole fraction of the ionic surfactant is present in the mixed micelles. Blankschtein model predicts a continuous decrease in synergism for the DPC + TX-100 system both in NaBu and NaHx due to their salt effect. On the contrary, Rubingh’s treatment reveals an increase in synergism in the presence of NaHx above 30 mM as against a continuous decrease in NaBu in the whole concentration range. For the AOT + TX-100 system, where Blankschtein model is not applicable, Rubingh’s treatment results in a continuous decrease in synergism in the presence of NaBu as well as NaHx. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Many surfactant mixtures, especially ionic–nonionic, exhibit surface properties significantly better than either of the components alone. Such synergistic effects greatly improve many technological applications in areas such as emulsion formulation, emulsion polymerization, surface tension reduction, personal care and cosmetic products, pharmaceuticals and petroleum recovery. Mixtures of nonionic surfactants show ideality in respect of their mixing [1–3]. On the other hand, the mixtures of ionic–nonionic, anionic–cationic, and hydrocarbon–fluorocarbon surfactants exhibit highly non-ideal behavior [1–6], the anionic–nonionic mixtures showing significant synergism both in surface activity and micelle formation [7]. A very recent review on binary surfactant systems has been published by Sugihara et al. [8]. Different theoretical treatments like those of Lange [9,10], Clint [11], Rubingh and Mittal [12], Motomura et al. [13], Rodenas et al. [14] and Maeda [15] have been widely used. A number of studies [16-19] have been devoted to predict the properties of and interac-

∗ Corresponding author. Tel.: +91 194 2424900; fax: +91 194 2421357. E-mail address: [email protected] (G.M. Rather). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.10.036

tions in binary surfactant systems using these models. According to the Blankschtein molecular thermodynamic model [20–25], electrostatic interactions among the ionic and polar head groups cause non-ideality of surfactant mixtures. Presence of inorganic salts brings about a change towards ideality by effective screening of electrostatic interactions [26]. Sugihara and coworkers [27,28] have discussed mixed micelle formation between dodecylammonium surfactants with hydrocarbon or fluorocarbon counter anions, which differ in the number of carbon atoms. Similar other studies [16,29–31] explain the effect of counter ion hydrophobicity on aqueous stability and mixed micelle formation. Micellization of sodium dodecylsulfate with a series of nonionic n-alkylmalano-bis-N-methylglucoamides has been studied [32] both in the absence and presence of gelatin. Very recently, Aijaz et al. [33] have discussed mixed micellization of dodecyltrimethyl ammonium bromide and tetraethyleneglycol mono-n-decyl ether in absence and presence of sodium propionate. In this work, mixed micellization of two binary surfactant systems, namely (1) nonionic surfactant, Triton X-100 (TX-100) with cationic surfactant, dodecylpyridinium chloride (DPC), and (2) nonionic, TX-100 with anionic Aerosol OT (AOT), have been studied at 25 ◦ C in absence and presence of sodium butanoate (NaBu) and sodium hexanoate (NaHx). Selection of surfactant systems was based on two main reasons. Firstly, the hydropho-

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bic chain lengths of the surfactants and hence their cmc values being quite different will result in non-ideal mixing. Further NaBu and NaHx by furnishing C3 H7 COO− and C5 H11 COO− which are counter ions for the DPC + TX-100 system but co-ions for AOT + TX100 system may affect micellization of these systems in different but interesting ways. The studies will provide information about the applicability of molecular thermodynamic models to these binary surfactant systems. The analysis of experimental surface tension data has been made in light of theoretical models of Clint, Rubingh, Maeda and Blankschtein to reveal their comparative performance. 2. Experimental 2.1. Materials and methods Triton X-100 (1,1,3,3-tetramethylbutylphenyl-polyoxyethylene), a polydisperse preparation with an average of 9.5 oxyethylene units per molecule, was a Sigma (99%) product and was used as such. 1Dodecylpyridinium chloride monohydrate (Merck, 99%) was used after two recrystallizations from methanol by acetone followed by drying for 24 h under vacuum. Sodium octylsulfosuccinate (AOT, Sigma) was used as received. Sodium butanoate (NaBu) and sodium hexanoate (NaHx) were prepared by neutralizing the respective aqueous acids with concentrated sodium hydroxide solution and then precipitating the dissolved salt with redistilled acetone. The crystals so obtained were recrystallized thrice from water by precipitation with acetone. The salts were dried and stored over fresh fused CaCl2 under vacuum. All the solutions were prepared in double distilled water. 2.2. Surface tension measurement Surface tension () of aqueous solutions was measured with a Kruss 9 (Germany) tensiometer, equipped with thermostatable vessel holder, by the platinum ring detachment method. The measured values were corrected according to the procedure of Harkin and Jordan [34]. A concentrated stock solution of surfactants was added in installments, using a Hamilton microsyringe, to 30 cm3 of pure water in a vessel placed in the vessel holder of tensiometer. Measurements were made after thorough mixing and temperature equilibration at 25 ± 0.1 ◦ C by circulating water from HAAKE GH thermostat. For measurements at a desired salt concentration, an equimolar stock solution of DPC and TX-100 was prepared in aqueous solution of desired salt concentration, and was then added in small aliquots to 30 cm3 of aqueous salt solution at the same concentration. The readings were taken in triplicate to check the reproducibility. A similar procedure was adopted for the AOT + TX100 mixed system. 3. Results and discussions Fig. 1(a and b) shows the variation of surface tension () with the logarithm of surfactant concentration (Ct ) for different constant composition mixtures of: (a) DPC (component 1) and TX-100 (component 2); (b) AOT (component 1) and TX-100 (component 2) respectively. The concentrations corresponding to breaks in the curves represent the critical micelle concentrations (cmc) in all the systems. The cmc values at 25 ◦ C for pure surfactants viz., DPC (15.75 mM), TX-100 (0.299 mM) and AOT (3.11 mM) are in good agreement with the literature values [11,35–39]. The very low cmc value of TX-100 reflects its much better propensity for micelle formation. A comparison of surface tensions of pure components at cmc also indicates higher surface activity of TX-100 compared to

Fig. 1. Plots of the surface tension, , of aqueous mixtures of: (a) DPC + TX-100, (b) AOT + TX-100, versus the total surfactant concentration, for different mole fractions ˛1 of ionic component at 25 ◦ C.

DPC and AOT. Similar plots were utilized to determine the cmc values for pure as well as 1:1 mixed systems in the presence of various NaBu and NaHx (10–50 mM) concentrations. The values at 25 ◦ C for the two binary mixtures at different stoichiometric compositions are given in Table 1a while those for the 1:1 mixtures at various NaBu and NaHx concentrations including the component cmcs are presented in Table 1b.

Table 1a Experimental cmc values, exp C12 , at 25 ◦ C, of binary DPC + TX-100 and AOT + TX-100 systems at different stoichiometric compositions ˛1 . DPC + TX-100

AOT + TX-100

˛1

exp

0 0.1 0.25 0.5 0.75 0.9 1

0.299 0.32 0.37 0.468 0.76 1.35 15.747

C12 (mM)

˛1

exp

0 0.1 0.25 0.5 0.75 0.9 1

0.299 0.301 0.323 0.425 0.62 0.941 3.113

C12 (mM)

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M. Jan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 114–120

Table 1b C1 , C2 and exp C12 , at 25 ◦ C of binary 1:1 DPC + TX-100 and AOT + TX-100 mixed systems in the presence of various sodium butanoate and sodium hexanoate concentrations. [NaBu] (mM)

C1 (mM) DPC

C1 (mM) AOT

C2 (mM) TX-100

exp

C12 (mM) DPC + TX-100 + NaBu

exp

0 10.0 20.0 30.0 40.0 50.0

15.747 13.653 9.02 6.53 5.02 3.87

3.113 1.571 1.416 1.103 0.608 0.543

0.299 0.30 0.302 0.305 0.308 0.311

0.468 0.475 0.454 0.443 0.427 0.416

0.425 0.311 0.304 0.284 0.232 0.225

[NaHx] (mM)

C1 (mM) DPC

C1 (mM) AOT

C2 (mM) TX-100

exp

C12 (mM) DPC + TX-100 + NaHx

exp

0 10.0 20.0 30.0 40.0 50.0

15.747 7.52 4.81 3.23 2.30 1.678

3.113 2.511 2.011 1.752 1.554 1.213

0.299 0.306 0.311 0.317 0.321 0.328

0.468 0.474 0.466 0.459 0.428 0.389

0.425 0.334 0.315 0.307 0.298 0.292

3.1. Mixed micelle formation Assuming ideal behavior of surfactants in mixed micelles, the ideal cmc value, ideal C12 , and mole fraction, ideal X1 , of component 1 in the micelles are obtained from the following relations due to Clint [40] 1 Ideal C 12 Ideal

˛1 (1 − ˛1 ) + C1 C2

(1)

˛1 C12 C1 + ˛1 (C1 − C2 )

(2)

=

X1 =

where C1 and C2 denote cmc values of pure surfactants 1 and 2 respectively. ˛1 is the net mole fraction of component 1 in the binary system on surfactant only basis. The values thus obtained for the DPC + TX-100 system are presented in Table 2. For non-ideal mixing, the mixed cmc, C12 , and the micellar composition, X1 , as a function of ˛1, can be revealed in light of Rubingh’s [41,42] equation, which is based on the Regular solution approach X12 ln(C12 ˛1 /C1 X1 ) (1 − X1 )2 ln{C12 (1 − ˛1 )/C2 (1 − X1 )}

=1

(3)

The interaction parameter ˇ(R) , of mixed micelle formation is given by ˇ(R) =

ln(C12 ˛1 /C1 X1 )

(4)

(1 − X1 )2

and is related to the activity coefficients of the surfactants within the micelles as f1 = exp{ˇ(R) (1 − X1 )2 }

(5)

f2 = exp{ˇ(R) X12 )

(6)

The parameter ˇ(R) (assumed to be constant) is a measure of interaction between the components in mixed micelles and is obtained

C12 (mM) AOT + TX-100 + NaBu

C12 (mM) AOT + TX-100 + NaHx

from experimental cmc data. For most mixed micellar systems ˇ(R) is negative, implying that C12 is lower than the averaged value of the cmc of each surfactant. A negative value of ˇ(R) implies an attractive interaction; the more negative its value, the greater the interaction. The negative values of ˇ(R) have commonly been attributed to the interaction between the head groups leading to electrostatic stabilization. The values of X1(R) , f1(R) , f2(R) and ˇ(R) obtained from Rubingh’s equation for DPC + TX-100 mixed system are presented in Table 2. The results make it clear that ˇ(R) values are approximately constant and negative throughout the concentration range with an average value of −3.11, suggesting strong synergism in the system. It is also evident that Rubingh’s treatment predicts larger mole fraction of nonionic surfactant in the mixed micelles over the entire composition range, thus reflecting its stronger tendency to micellize than the cationic surfactant. It is very interesting to note that the sums of activity coefficients, f1(R) and f2(R) are round about unity. Rodenas et al. [14], using a simple theoretical treatment based on Lange’s model related activity coefficients f1 and f2 of surfactants in mixed micelles through Gibbs–Duhem equation. However, for cationic/nonionic binary system they used the relation: X1(Rd) =

−(1 − ˛1 )˛1 d ln C12 + ˛1 d˛1

(7)

which is similar to that of Motomura’s equation [13] for 1:1 cationic–nonionic surfactant pairs. Using Eq. (7), micellar mole fraction, X1(Rd) , of DPC was calculated from d ln C12 /d˛1 while activity coefficients were calculated using the equations: f1(Rd) =

˛1 C12 X1(Rd) C1

(8)

f2(Rd) =

(1 − ˛1 )C12 (1 − X1(Rd) )C2

(9)

Table 2 Micellar composition, X1 , interaction parameter, ˇ, and activity coefficients, f1 and f2 , of binary mixtures of DPC and TX-100 at 25 ◦ C at different stoichiometric coefficients, ˛1 , using Rubingh, Rodenas and Blankschtein’s approaches. ˛1

X1(R) /X1(Rd)

ideal

0.10 0.25 0.50 0.75 0.90

0.033/0.026 0.062/0.069 0.154/0.140 0.237/0.211 0.337/0.555

0.002 0.006 0.019 0.054 0.146

X1

ˇ(R)

f1(R) /f1(Rd)

f2(R) /f2(Rd)

pred

−3.00 −2.68 −3.27 −3.23 −3.36

0.06/0.07 0.09/0.08 0.10/0.11 0.15/0.17 0.23/0.14

1.00/0.99 1.00/1.00 0.92/0.91 0.83/0.80 0.68/1.01

0.317/0.331 0.356/0.396 0.469/0.587 0.759/1.131 1.388/2.554

C12(B) /ideal C12 (mM)

pred

X1(B) /opt X1(B)

0.040/0.081 0.085/0.060 0.153/0.091 0.238/0.094 0.334/0.174

Pred

Pred

f1(B) /opt f1(B)

0.050/0.096 0.066/0.087 0.098/0.102 0.152/0.103 0.237/0.152

Blankschtein’s predictive and optimal parameters, viz., f1 and f2 , have been evaluated using predictive ˇ value (pred ˇ(B) = −3.24) and its optimal value, shown are the ideal, ideal C12 , and predicted, pred C12(B) , cmc values. (R) = Rubingh’s approach, (B) = Blankschtein’s approach.

f2(B) /opt f2(B)

0.995/0.982 0.978/0.989 0.927/0.977 0.833/0.976 0.697/0.919 opt

ˇ(B) = −2.77. Also

M. Jan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 114–120

Fig. 2. Variation of cmc, C12 (mM), of binary mixture of DPC and TX-100 at 25 ◦ C with mole fraction of, ˛1 , of ionic surfactant. Dashed line (): ideal C12 with ˇ = 0; full line (): pred C12 with pred ˇ(B) = −3.24; dotted line: best fit to the experimental data () with opt ˇ(B) = −2.77.

The values of X1(Rd) , f1(Rd) and f1(Rd) , included in Table 2, show that both the Rubingh and Rodenas treatments lead to similar conclusion that nonionic surfactant predominates the mixed micelles. The activity coefficients from Rodenas treatment are identical to those from that of Rubingh except at very high bulk mole fraction of ionic component. The results are in conformity with our recent results [16]. Blankschtein and Shiloach [25] proposed a molecular thermodynamic model which is used to predict cmc as well as the values of specific interaction parameter, Pred ˇ(B) , for non-ideal binary surfactant mixtures where at least one of the surfactant mixtures is ionic. This model requires, as input, only the cmc values of pure surfactants, the overall surfactant composition, solution conditions like temperature, salt concentration and chemical structure of the hydrophobic and hydrophilic moieties of the surfactant species. The theory gives reasonably satisfactory results for nonionic–ionic mixtures containing single tailed surfactants with linear hydrocarbon or phenyl-hydrocarbon chains. The molecular thermodynamic theory assumes that electrostatic interaction among ionic head groups contributes to the non-ideality of surfactant mixing, and hence Pred ˇ (B) corresponds to the excess electrostatic free energy associated with mixed micelle formation. Besides, the predicted value of ˇ can be compared with the experimentally deduced optimal value, opt ˇ (B), that best fits the experimental cmc, C12 , versus ˛1 data in a least squares fit sense. The predicted value of micellar mole fraction of surfactant 1, Pred X1(B) , the activity coefficients Pred f1(B) and Pred f Pred C 2(B) and the predicted cmc values, 12(B) , corresponding to Pred ˇ (B) = −3.24 are given as a function of ˛1 in Table 2 which also includes the optimal values of the above parameters, viz., opt X1(B) , opt f opt f 1(B) and 2(B) corresponding to the optimal interaction paramopt ˇ(B) = −2.77. eter Fig. 2 represents the variation of C12 of the DPC + TX-100 system with overall composition ˛1 under various treatments. It is evident

117

from the figure that Pred C12(B) is in good agreement with best fit experimental C12 values corresponding to opt ˇ(B) = −2.77. The predicted and fitted curves are almost indistinguishable in this case suggesting strong synergism in the mixed system. The ideal cmc values are however, significantly different from the experimental values especially above ˛1 = 0.50. The activity coefficients from Rubingh, Rodenas and Blankschtein treatment are in good agreement with each other. Only at ˛1 = 0.90, f(Rd) values are significantly different from f(B) and f(R) values. The overall analysis therefore, reveals that the binary mixed system (DPC + TX-100) is well represented by Regular solution approach. The results of the analysis for mixed AOT + TX-100 surfactant system, according to various models are presented in Table 3. The table also includes optimal micellar parameters corresponding to opt ˇ = −1.91. The results indicate that ˇ (R) is negative throughout the concentration range with an average value of −2.18, suggesting strong synergism in the system. Rubingh’s treatment again predicts a larger fraction of nonionic surfactant in the mixed micelle a result also confirmed by Haque et al. for sodium oleate/alcohol ethoxylate C12.5 (EO)6.5 system using tensiometry [16]. Rodenas treatment leads to similar conclusion. The activity coefficients from this treatment are comparable to those of Rubingh’s treatment except at ˛1 = 0.90. Fig. 3(a and b) represents the variation of micellar composition (X1 ) of the DPC + TX-100 and AOT + TX-100 mixed systems with the overall composition, ˛1 , under various treatments. It is seen that the ideal X1 values are significantly lower except at ˛1 = 0.90. The analysis thus reveals that the AOT + TX-100 as well as the DPC + TX100 mixed systems are well represented by Regular solution theory. Thermodynamic stability of mixed micelles can be represented in terms of Gmic , the Gibbs energy of mixed micelle formation which can be computed by using opt ˇ(B) and opt X1(B) and is defined by Maeda [15] as: Gmic = RT (B0 + B1 X1 + B2 X12 )

(10)

where B0 = ln C2 B2 = −ˇ B1 + B2 = ln

(11a) (11b)

C  1

(11c)

C2

Fig. 4(a and b) shows the plot of Gmic as a function of bulk mole fraction, ˛1, of ionic surfactant for the DPC + TX-100 and AOT + TX100 mixed systems respectively at 25 ◦ C according to Eqs. (10) and (11) using opt ˇ(R) and opt X1(R) ; avg ˇ(R) and X1(R) . The figures reveal that the stability of mixed micelles decreases with the incorporation of ionic monomer in them. Free energy of micellization obtained from the above treatment is compared with that obtained from the thermodynamic relation o GM = RT ln(cmc)

(12)

where cmc is experimentally determined critical micelle conceno versus ˛ are also included in Fig. 4(a and tration. The plots of GM 1

Table 3 Micellar composition, X1 , interaction parameter, ˇ, and activity coefficients, f1 and f2 , of binary mixtures of AOT and TX-100 at 25 ◦ C at different stoichiometric coefficients, ˛1 , from Rubingh and Rodenas models. Also included are optimal values corresponding to opt ˇ(B) = −1.91. ˛1

X1(R) /X1(Rd) /opt X1(B)

ideal

0.10 0.25 0.50 0.75 0.90

0.079/0.076/0.070 0.147/0.101/0.118 0.220/0.119/0.170 0.345/0.374/0.283 0.482/0.269/0.357

0.010 0.031 0.087 0.224 0.464

X1

␤(R)

f1(R) /f1(Rd) /opt f1(B)

f2(R) /f2(Rd) /opt f2(B)

ideal

−2.49 −2.38 −1.92 −1.95 −2.14

0.121/0.126/0.192 0.176/0.256/0.226 0.310/0.571/0.268 0.432/0.445/0.375 0.564/1.010/0.454

0.984/0.978/0.991 0.949/0.901/0.974 0.911/0.806/0.946 0.857/0.924/0.858 0.608/0.431/0.784

0.329 0.386 0.545 0.928 1.604

C12 (mM)

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M. Jan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 114–120

Fig. 3. Variation of micellar mole fraction X1 with bulk mole fraction ˛1 of: (a) DPC + TX-100 and (b) AOT + TX-100 binary systems according to different treatments.

b). The agreement is fair at low ˛1 values with increasing deviation as the proportion of ionic surfactant increases. It is well known [43,25] that the incorporation of a cationic or anionic surfactant into nonionic micelles introduces electric charge on to the micellar surface and, hence repulsion between the micelles. The charge on the micellar surface also leads to destabilization of the mixed micelles. Rubingh’s treatment (Eqs. (3–6)) has been used to determine the important parameters X1 , ˇ(R) , f1 and f2 of mixed micelles of

Fig. 4. Gibbs energy of mixed micelle formation as a function of the bulk mole fraction, ˛1 , of ionic component for the binary mixtures of: (a) DPC + TX-100; (b) AOT + TX-100, at 25 ◦ C according to Eqs. (10) and (11) using micellar mole fraction calculated with: RT ln(cmc) (), Blankschtein’s () and Rubingh’s () treatments.

DPC + TX-100 in the presence of sodium butanoate and sodium hexanoate at 25 ◦ C. The computed values are listed in Table 4. The table also lists values of ideal cmc from Clint Eq. (1). The predicted cmc, pred C12(B) , the interaction parameter, pred ˇB , the micellar mole fraction, pred X1(B) and the activity coefficients, pred f1(B) and pred f2(B) from the Blankschtein predictive model [25] are included for com-

Table 4 Micellar composition, X1 , interaction parameter, ˇ, and activity coefficients, f1 and f2 , of binary mixtures of DPC and TX-100 at 25 ◦ C at different sodium butanoate and sodium hexanoate concentrations in light of Blankschtein predictive (subscript B) and Rubingh’s (subscript R) models. [Salt] (mM)

ideal

C12 /pred C12(B) (mM)

ˇ(R) /pred ˇ(B)

X1(R) /pred X1(B)

f1(R) /pred f1(B)

f2(R) /pred f2(B)

NaBu 0 10.0 20.0 30.0 40.0 50.0

0.587/0.470 0.587/0.477 0.584/0.457 0.583/0.444 0.579/0.432 0.576/0.421

−3.27/−3.24 −3.00/−2.98 −2.89/−2.83 −2.70/−2.64 −2.63/−2.55 −2.50/−2.44

0.154/0.153 0.151/0.149 0.177/0.175 0.195/0.194 0.215/0.211 0.233/0.230

0.096/0.098 0.115/0.117 0.142/0.145 0.174/0.175 0.198/0.204 0.230/0.230

0.925/0.927 0.933/0.936 0.913/0.917 0.902/0.904 0.885/0.893 0.873/0.878

NaHx 0 10.0 20.0 30.0 40.0 50.0

0.587/0.470 0.588/0.424 0.584/0.405 0.577/0.385 0.563/0.364 0.549/0.346

−3.27/−3.24 −2.44/−3.19 −2.10/−2.94 −1.78/−2.74 −1.79/−2.59 −1.90/−2.46

0.154/0.153 0.169/0.207 0.191/0.235 0.214/0.264 0.253/0.291 0.297/0.321

0.096/0.098 0.186/0.136 0.253/0.179 0.333/0.226 0.368/0.272 0.391/0.321

0.925/0.927 0.932/0.873 0.926/0.850 0.922/0.826 0.892/0.803 0.845/0.776

M. Jan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 114–120

parison. Here NaBu and NaHx are assumed to behave as inorganic salts. It is clear that both ˇ(R) and pred ˇ(B) show continuous decrease in magnitude with increasing concentration of NaBu. Blankschtein model predicts that an increase in the concentration of inorganic salt, that mainly contributes due to its electrical effect, leads to more effective screening of electrostatic interactions bringing the mixture closer to ideality, i.e., ˇ → 0. This indicates that the salt effect of Na+ and Bu− ions predominates over the solubilization effect of Bu− ions which would lead to partial neutralization of charge on the mixed micelles. Fig. 5(a) depicts the variation of experimental, ideal and predicted cmc values of 1:1 DPC + TX-100 mixed system with [NaBu]. It is clear that the variation is well represented by Blankschtein predictive model in the whole NaBu concentration range. This confirms that only electrical effects are significant. Contrary to a small but continuous decrease in the magnitude of pred ˇ(B) with [NaHx] (Table 4), ˇ(R) decreases more rapidly initially and increases slowly after passing through a minimum. The micelles of DPC + TX-100 system being positively charged, an increase in the concentration of NaHx leads to more effective screening of electrostatic repulsions and hence the mixture is brought closer to ideality. The same trend is observed in ˇ(R) up to 30 mM. Beyond this the concentration effect is reversed. This may be due to specific interaction of NaHx with the mixed micelles at higher concentration. Hydrophobic hexanoate ions, furnished by NaHx, may get solubilized into the mixed micelles, thereby neutralizing the micellar surface charge which results in decreasing the electrostatic repulsion between micelles. A comparison of ideal, experimental and predicted cmc values of 1:1 DPC + TX-100 mixed system shows that although both exp C12 and pred C12(B) decrease similarly with [NaHx], the plots do not overlap as in the presence of NaBu. The values at high [NaHx] are lower relative to those in NaBu indicating better solubilization of Hx− ions into the positively charged mixed micelles. This is reflected by the increase in the magnitude of ˇ(R) which otherwise should have become less negative had only the electric effects been significant. The different behaviour of DPC + TX-100 mixed system in the presence of NaBu and NaHx is because of the fact that the butyrate ion has smaller hydrophobic chain length, and hence its solubilization tendency must be smaller than that of Hx− ion. Rubingh treatment has also been used to find out the important parameters of mixed micellization viz., ˇ(R) , f1 and f2 for the AOT + TX-100 mixed system in the presence of the above salts. The results are listed in Table 5. The table also contains ideal cmc values from Clint Eq. (1). Initial addition of salts in both cases increases the magnitude of ˇ, i.e., synergism, more so by NaHx. However further addition of salts leads to gradual decrease in ˇ. AOT + TX-100 mixed micelles are negatively charged and the charge on the micellar sur-

119

Fig. 5. Variation of experimental, predicted and ideal cmc, C12 , of 1:1 mixed DPC + TX-100 system versus: (a) sodium butanoate concentration; (b) sodium hexanoate concentration, at 25 ◦ C.

face gives rise to repulsion between the micelles. On addition of NaBu or NaHx, the original charge distribution is swamped, leading to screening of repulsion. Since the concentration of additive we used lies between 10 and 50 mM, we do not observe any solubilization effect. Also the magnitude of avg ˇ(R) is more for AOT + TX-100 mixed system in the presence of NaHx than in the presence of NaBu which may be due to difference in the length of carboxylate co-ions furnished by them.

Table 5 Micellar composition, X1 , interaction parameter, ˇ, and activity coefficients, f1 and f2 , of binary mixtures of AOT and TX-100 at 25 ◦ C at different sodium butanoate and sodium hexanoate concentrations in light of Rubingh’s model. AOT + TX-100 + NaBu

AOT + TX-100 + NaHx

[NaBu] (mM)

ideal

0 10.0 20.0 30.0 40.0 50.0

0.545 0.504 0.498 0.474 0.408 0.396

C12 (mM)

X1(R)

ˇ(R)

f1(R)

f2(R)

[NaHx] (mM)

ideal

0.220 0.322 0.333 0.357 0.422 0.436

−1.92 −2.57 −2.54 −2.46 −2.38 −2.34

0.310 0.307 0.322 0.361 0.452 0.475

0.911 0.765 0.754 0.731 0.654 0.641

0 10.0 20.0 30.0 40.0 50.0

0.545 0.545 0.539 0.537 0.532 0.516

C12 (mM)

X1(R)

ˇ(R)

f1(R)

f2(R)

0.220 0.294 0.316 0.330 0.343 0.363

−1.92 −2.99 −2.98 −2.96 −2.95 −2.72

0.310 0.226 0.248 0.265 0.279 0.311

0.911 0.772 0.742 0.724 0.706 0.699

120

M. Jan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 114–120

4. Conclusion The above studies reveal the following important facts: (1) The analysis shows significant deviation of the binary surfactant systems from ideal behavior. It also indicates only a small mole fraction of ionic surfactant in the mixed micelles. (2) Micellar composition and interaction parameter calculated from different models give similar results except at very high mole fraction of ionic surfactant. This may be due to the utilization of almost similar equations in these approaches, which differ only in the method of evaluation of ˇ except for Rodenas model that uses dependence of cmc data on ˛1 . (3) The results indicate a decrease in cmc on addition of NaBu as well as NaHx in pure as well as in 1:1 binary mixtures. (4) Blankschtein model predicts a continuous decrease in synergism due to salt effect of NaBu and NaHx. Rubingh treatment on the other hand, indicates a continuous decrease in synergism in case of DPD + TX-100 mixed system in the presence of NaBu but a slight increase in the presence of NaHx above 30 mM concentration. However, in the AOT + TX-100 mixed system, synergism decreases slowly both in NaBu as well as NaHx after an initial increase. Acknowledgement We are highly thankful to Prof. M.A. Qurishi, the HOD, for moral support. References [1] M.J. Rosen, Surfactants and Interfacial Phenomena, Willey, New York, 1989, Chapter 11. [2] M.J. Rosen, X.Y. Hua, Synergism in binary mixtures of surfactants II. Some experimental data, J. Am. Oil Chem. Soc. 59 (1982) 582–585. [3] R. Nagaranjan, Molecular theory for mixed micelles, Langmuir 1 (1985) 331–341, references cited therein. [4] T. Handa, P. Mukerjee, Surface tension of nonideal mixtures of fluorocarbons and hydrocarbons and their interfacial tension against water, J. Phys. Chem. 85 (1981) 3916–3920. [5] M.J. Rosen, Z.H. Zhu, T. Goa, Synergism in binary mixtures of surfactants: II. Mixtures containing mono- and disulfonated alkyl- and dialkylphenylethers, J. Colloid Interface Sci. 157 (1993) 254–259. [6] P.G. Cumins, J. Penfold, E. Staples, A study on structure of mixed cationic/nonionic micelles, Langmuir 8 (1992) 31–35. [7] T. Okano, Y. Abe, D.-K. Hotta, T.-Y. Nakano, G. Sugihara, S.-G. Oh, Binary mixed systems of anionic/nonionic surfactants: synergistically enhanced surface activity and relationship among compositions of monomers in bulk (X2 ), micelles (Y2 ) and adsorbed film at air/water interfaces (Z2 ), J. Jpn. Oil Chem. Soc. 49 (2000) 915–928. [8] G. Sugihara, S. Nagadome, S.-W. Oh, J.-S. Ko, A review of recent studies on aqueous binary mixed surfactant systems, J. Oleo Sci. 57 (2008) 61–92. [9] H. Lange, Kolloid Z. 131 (1953) 96. [10] H. Lange, K.H. Beck, Kolloid Z. Z. Polym. 251 (1973) 1327. [11] J.H. Clint, Micellization of mixed nonionic surface active agents, J. Chem. Soc. Faraday Trans. 1 71 (1975) 1327–1334. [12] D.N. Rubingh, K.L. Mittal (Eds.), Solution Chemistry of Surfactants, vol. 1, Plenum Press, New York, 1979, p. 337. [13] K. Motomura, M. Arantono, K. Ogino, M. Abe, in: K. Ogino, M. Abe (Eds.), Mixed Surfactant Systems, Dekker, New York, 1993, p. 99. [14] E. Rodenas, M. Valiente, M.S. Valluafruela, Different theoretical approaches for the study of mixed tetraethyleneglycol-mono-n-dodecyl ether/hexadecyltrimethylammonium bromide micelles, J. Phys. Chem. B 103 (1999) 4549–4554. [15] H. Maeda, A simple thermodynamic analysis of the stability of ionic/nonionic mixed micelles, J. Colloid Interface Sci. 172 (1995) 98–105. [16] Md. Haque, A.R. Das, A.K. Rakshit, S.P. Moulik, Properties of mixed micelles of binary surfactant combinations, Langmuir 12 (1998) 4084–4089.

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