1359
Langmuir 1994,10, 1359-1365
Thermodynamics of Micellar Solubilization for 1-Pentan01 in Weakly Interacting Binary Cationic Surfactant Mixtures at 25 "C A. Makayssi, R. Bury, and C . Treiner. Laboratoire d'Electrochimie, Universit&Pierre et Marie Curie, UA CNRS 430, 4, Place Jussieu, Paris 75005, France Received October 21, 1992. In Final Form: June 4,1993"
The partition coefficient P of 1-pentanol between micelles and water as well as the standard enthalpy of transfer AHt from water to micelles have been determined in aqueous mixtures of cationic surfactants using a calorimetricmethod of investigation. Various binary systems were used benzydimethyltetradecylammonium trimethyltetradecylammonium chloride (system I) in the presence of 0.05 mol/L NaCl(I1) and 0.1 moll L of NaCl(II1);trimethyltetradecylammonium + benzyldimethylhexadecylammoniumchloride (IV);benzyldimethylhexadecylammonium+ hexadecylpyridinium chloride (V). The atypical synergistic solubilization effect observed previously by system I, with two extrema displayed by the variation of the alcohol standard thermodynamic functions with micellar composition, disappears in the presence of salt. The results obtained conform to a regular type of solution behavior for both P and A& System IV also displays a regular solution profile. In the case of system V a distorsion appears which might be the consequenceof large structural fluctuationsat the mixed micellar surface. It is suggested that the nonregular behavior observed could be related to structural effects induced by the alcohol in the presence of benzalkonium salts due to the aromatic ring acting as a short second hydrocarbon chain on the alkylammonium head group.
+
Introduction One of the fundamental properties of aqueous surfactant solutions in the ability to form micelles and solubilize hydrophobic molecules or ions above a critical concentration (cmc). If the case of single surfactant solutions has been thoroughly studied, it is not so with mixed surfactants. It has been known for some time that the mixing of surfactants is generallly favorable to micellar stability and various models have been proposed to interpret this phenomenon.1-6 The question of the consequence of surfactant mixing on micellar solubilization is not as well documented. Recent publications have presented an attempt to provide a simple basis for the interpretation of existing The regular solution formalism was used to show that in most available cases, the change of micellar solubilization of polar hydrophobic molecules in the case of binary surfactant solutions is essentially governed by the strength of the interaction between the two unlike surfactants. This quantity may be deduced from the cmc change with micellar composition of the mixed surfactants.3 The general conclusion is that, as mixed micelles are formed because of favorable interactions between the two surfactant components, their mixing should be unfavorable to micellar solubilization (negative synergistic effect). (The case of nonpolar molecules which may display the opposite behavior in mixed anionic + ~
~~
~~
* To whom all correspondence should be addressed. *Abstract oublished in Advance ACS Abstracts,. May- 15, 1994. (lj Lange, H. Kolloid 2.2.Polym. 1953, 131,96. (2) Clint J. J. Chem. Soc., Faraday Trans 1 1975, 71,327. (3) (a) Rubingh D. N. In Surfactant in Solutions; Mittal, K. L., Ed.; Plenum Press: New York, 1979;Vol. 1,p 337. (b) Holland,P. M.; Rubingh, D. N. J. Phys. Chem. 1983,87, 1984. (4)Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984,262,948. (5) Nagarajan, R. Langmuir 1985, 1, 331. (6) Treiner, C.; Bocquet, J. F.; Pommier, C. J. Phys. Chem. 1986,90, 352. (7) Treiner, C.; Khodja, A. A.; Fromon, M. Langmuir 1987,3,729. (8) Treiner, C.; Nortz, M.; Vaution,C.; Puisieux, F. J.ColloidInterface Sci. 1988, 125, 261. (9) Treiner, c.; Nortz, M.; Vaution, C. Langmuir 1990, 6, 1211.
0743-7463f 9412410-1359$04.50/0
cationic mixtures will not be addressed in this study.l0J1) Most available data support the above conclusion. Exceptions among polar molecules concern a dye, orange OOT, in zwitterionic anionic mixtures,12 oleyl alcohol in the same type of mixed surfactant and benzodiazepam (an anxiolytic molecule)14or vitamin K1I5 in bile salt lecithin mixed micelles. In such cases, the surfactant mixing may induce the formation of either large micelles or vesicles, depending on micellar composition, and this structural factor could complicate the analysis of solubilization data with the simple regular solution model employed. We have recently shown that a positive synergistic effect could be observed for 1-pentanoP and benzyl using binary mixtures of two cationic surfactants: benzyldimethyltetradecylammonium chloride (C14BzCl) and trimethyltetradecylammoniumchloride (C14C1) (thereafter called system I). In the case of acetophenone in the same mixtures, a normal (negative) synergistic effect was obtained.18 It appeared therefore, that alcohols may induce a specificmicellar solubilization behavior in system I. The investigation method was calorimetry, which enables both the partition coefficient of the solute between micelles and water and the enthalpy of transfer between the pseudomicellar phase and water to be obtained. The solubilization of the two alcohols were accompanied by large enthalpy of transfer changes contrary to the case of the nonalcohol molecule which demonstrated that im-
+
+
(IO)(a) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn,J. F.
J. Colloid Interface Sci. 1989,130, 254. (b) Scheuing, D. R.; Weers, J.
G. Langmuir 1990, 6,665. (11) Zhao, G. X.; Li, X. G. J. Colloid Interface Sci. 1991, 144, 185. (12) Iwasaki, T.; Ogawa, M.; Esumi, K.; Meguro, K.Langmuir 1991, 7, 30. (13) Abe, M.; Kubota, T.; Uchiyama, H.; Ogino,K. ColloidPolym. Sci. 1989,267, 365. (14) Rosoff, M.; Serajuddin, A. T. M. Int. J.Pharm. 1980,6, 137. (15) Nagata, M.; Yotsuyanagi, T.; Ikeda, K. J. Pharm. Pharmacol. 1988, 40, 85. (16) Bury, R.; Souhalia, E.; Treiner, C. J. Phys. Chem. 1991,95,3824.
(17) Bury, R.: Treiner, C.; Chevalet, J.; Makayassi, A. Anal. Chim. Acta 1991,-251, 69. (18) Makayssi, A.; Treiner, C. Colloid Polym. Sci. 1992, 270, 1124.
0 1994 American Chemical Society
Makayssi et al.
1360 Langmuir, Vol. 10, No. 5, 1994
portant structural changes occurred in the mixed micelles upon alcohol penetration. Thus, it was considered that this atypical behavior was worth further investigating by using 1-pentanol in different series of cationic mixtures which would differ from system I b y their head group or b y their alkyl chain length. Also the effect of salt was considered. The same calorimetric method was used in order to be able to compare the results with the same basic assumption.
Material and Methods C14BzC1(Aldrich) was a 99% pure compound. Its cmc, as deduced from conductivity experiments, was equal to 0.0019 mol/L a t 25 "C: C&l (TCI, Japan), cmc = 0.0053 mol/L; benzyldimethylhexadecylammonium chloride (C16BzC1) (Fluka purum, 97% pure), cmc = 0.00041 mol/L; hexadecylpyridinium chloride (ClSpyCl)(Sigma),cmc = 0.0010mol/L. Sodium chloride (Merck) was a very pure sample. Surface tension measurements for CI&l aqueous solutions showed a slight minimum close to the cmc. 1-Pentanol (Sigma) was a pure compound as deduced from gas chromatographic measurements. The alcohol concentration used was equal to 0.02 mol/L in all cases. This was considered as corresponding to a standard condition (see below). The standard enthalpy of solution of 1-pentanol in water at 25 "C was AH, = -7.78 i 0.04 kJ/mol in agreement with literature values.19 In the presence of 0.05 and 0.1 mol/L of NaC1, AH, was respectively equal to -7.89 f 0.06 and -7.79 i 0.05 kJ/mol. The pseudoadiabatic calorimeter used has been described in detail elsewhere." The surfactant concentration is varied from 0.01 to 0.1 mol/L (see below). The experiments were performed in a Dewar immersed in a bath-temperature regulated a t 25 h 0.02 "C. The enthalpy data are analyzed using relation 1based on the pseudophase model and on the experimental condition that the solute be dilute with respect to the surfactant in the micelleP
where AH",is the solute standard enthalpy of transfer from water to micelles, P is the partition coefficient between micelles and water, defined on the mole fraction basis, and Ct is the total surfactant concentration. AH",,, and AH",are respectively the measured alcohol enthalpy of solution in the surfactant solution and in pure water, b is an empirical constant which incorporates various physical effects which could not be properly introduced in the complex systems investigated. Such effects have been taken into account in more sophisticated treatments of enthalpy of solubilization In these derivations the solute is transferred from the monomer solution, below the cmc to the micelles, instead of the pure water reference state used in eq 1, and the enthalpy of micellization as well as the salting effect of the monomers on the solute activity are introduced. For example, in the treatment by Nguyen et al.,l9 the equation used with the present notation is:
where K, is the partition coefficient in the molal scale. The corrected (corr) index refers to the introduction of various physical parameters which take into account the contribution of the surfactant enthalpy of micellization and the effect of the solute on the surfactant cmc in the derivation leading to eq 2. It may be pointed out that De L i d s calculation21leads essentially to the same equation. The b term of eq 1is related to the parameters of eq 2 by (19) Nguyen, D.;Venable, R. L.; Bertand, G. L. Colloid Surf. 1992,65, 231. (20) De Lisi, R.;Genova, C.; Turco Liveri, V. J.Colloid Interface Sci.
1983,95,428. (21) De Lisi, R.;Milioto, M.; Castagnolo, M.; Inglese, A. J. Solution Chem. 1987,16, 373.
Table I. Enthalpy of Solution of 1-Pentanol ( m = 0.02 mol/L) in Mixtures of [C&l + C ~ ~ B Z C+I ]0.05 NaCl: System I1 A H o , - AH", (kJ/mol) 0 1.04 1.34 1.70 1.92 2.12 2.22 2.28 2.40 0.10 1.27 1.56 1.86 2.01 2.28 0.30 1.36 1.47 1.72 1.92 2.21 2.75 2.81
X
Ct - cmc AH',, - AH", Ct - cmc (mol/L) X (kJ/mol) (mol/L) 0.017 0.50 1.16 0.0185 0.027 1.64 0.0285 0.037 1.97 0.0385 0.047 2.18 0.0485 0.057 2.35 0.0585 0.067 2.74 0.0684 0.077 2.89 0.0725 3.12 0.088 0.0885 0.0188 0.80 0.96 0.0142 0.0288 1.20 0.0192 0.0388 1.65 0.0292 0.0588 2.06 0.0392 0.0688 2.64 0.0592 0.0185 2.91 0.0692 0.0235 3.07 0.0792 0.0285 3.24 0.0892 0.0335 1.00 0.42 0.0093 0.0485 0.0177 1.10 0.0639 1.99 0.0291 0.0785 2.41 0.0401 2.62 0.0471 2.86 0.0591 3.27 0.0711 3.73 0.0791 3.84 0.0911 4.15 0.1191
a Not analyzed because the enthalpy versus concentration curve presented a strong hump around 0.05 surfactant concentration.
b = AH",,, - AH", + '/2[cmc(2.303k, + (1 + r)PJI,] (3) y is the degree of counterion dissociation of the micelles, AH, is the enthalpy of micellization, and k, is the familiar Setchenow constant. Such informations could not be easily obtained for the binary surfactant systems investigated, hence the introduction of the parameter b in eq 1. The left-hand side of eq 1 is plotted as a function of the dominator of the right hand-side of this equation for increasing P values (with b = 0) until the best straight line is obtained. The values of the nonzero intercept b are presented on Table V. Nguyen's experiments are so performed that the enthalpy of solution of the solute at each surfactant concentration is measured a t various solute concentrations and the results are extrapolated to zero to ensure conditions as close as possible to standard ones. In the procedure adopted by us, a single (small) concentration of alcohol is added to the solution vessel. In its simplest form eq 2 reduces to eq 1. Nguyen et al. have published standard enthalpy of solution data for 1-pentanol, benzyl alcohol, and 3-methyl-1-butanol in C12Na solutions a t 25 "C with surfactant concentration up to 0.27 mol/L. They obtain by using eq 2, respectively, the following partition coefficients: 21.6,13.0, and 14.6. Using their data with eq 1, one gets, in the same molal scale, the following numbers: 25.9, 13.0, and 14.9. Taking into account the standard deviations of the enthalpy runs, the agreement between the two sets of data is satisfactory. Thus, the simpler equation (1)was used throughout this paper. Tables I to IV present the raw experimental data, and Table V presents the results of the analysis. It must be stressed that, on physical grounds, these procedures are valid under the condition that the micellar aggregates do not change significantly in shape or size in the surfactant concentration range studied. Thus, the range of Ct values should be kept to a minimum compatible with an unambiguous determination of the two unknown parameters of the model used. On analytical grounds, it can easily be shown that the best conditions for the use of eq 1depend upon the range of P values considered. For a value of P = 1000, the concentration range adopted (Ct cmc < 0.1 mol/L) is the smallest compatible with a determination of both thermodynamic parameters involved with the desired precision. Under these conditions, the range of P values compatible with the use of eq 1 extends from about 100 to 4000 (mole fraction scale).
Langmuir, Vol. 10, No. 5, 1994 1361
Micellar Solubilization Table 11. Enthalpy of Solution of 1-Pentanol (m = 0.02 mol/L) in Mixtures of [C&l + C14BzCll + 0.10 NaCI: System I11
AH",,,- AH," Ct - cmc
AH",, - AH," Ct - cmc
X
(kJ/mol)
(mol/L)
X
(kJ/mol)
(mol/L)
0.0
0.46 0.97 1.35 1.91 1.95 2.12 2.23 0.82 1.22 1.50 1.75 1.95 2.20 2.29 2.49 2.52 0.96 1.56 1.83 2.02 2.27 2.46 2.65 2.93
0.0082 0.0182 0.0282 0.0480 0.0582 0.0682 0.0882 0.0093 0.0191 0.0293 0.0393 0.0493 0.0543 0.0593 0.0693 0.0893 0.009 0.019 0.029 0.039 0.049 0.059 0.0695 0.089
0.60
0.98 1.53 1.82 2.11 2.26 2.56 2.71 0.58 1.07 1.43 1.72 2.15 2.46 2.55 0.96 1.57 1.87 2.75 2.96 3.05
0.0093 0.0193 0.0293 0.0393 0.0493 0.0593 0.0693 0.009 0.019 0.029 0.039 0.059 0.079 0.087 0.0094 0.0194 0.0294 0.0694 0.0794 0.0894
0.10
0.30
0.8W
1.0
Table IV. Enthalpy of Solution of 1-Pentanol (m= 0.02 mol/L) in Mixtures of ClePyCl + ClaBzCl: System V
X
AH",,. (kJ/mol)
Ct-cmc (mol/L)
1.01 1.74 2.27 3.36 3.45 3.52 1.05 1.71 2.20 2.62 3.65 3.76 3.82 1.08 1.66 2.21 2.66 2.84 3.26 3.45 3.53 1.25 1.85 2.38 2.80 3.03 3.46 3.73 3.95
0.009 0.019 0.029 0.069 0.079 0.089 0.0092 0.0202 0.0302 0.0390 0.0802 0.0902 0.0992 0.0093 0.0203 0.0293 0.0403 0.0493 0.0593 0.0703 0.0793 0.0094 0.0194 0.0294 0.0403 0.0494 0.0694 0.0794 0.0894
0.0
0.10
0.20
0.40
Not analyzed because of a strong hump in the enthalpy versus concentration curve (see text). Table 111. Enthalpy of Solution of 1-Pentanol (m = 0.02 mol/L) in Mixtures of [C&1 + ClaBzCl]: System IV
X
AH",,, - AH", Ct - cmc (kJ/mol) (mol/L)
0.0
0.05
0.10
0.50 0.79 1.38 1.50 1.69 1.88 2.00 2.23 2.32 0.79 1.23 1.78 2.00 2.13 2.31 2.39 2.46 1.01 1.36 1.76 2.01 2.10 2.35 2.67
0.0076 0.0136 0.028 0.0318 0.0374 0.0447 0.0547 0.0747 0.0834 0.0078 0.0179 0.0379 0.0479 0.0579 0.0679 0.0879 0.0979 0.0185 0.0285 0.0385 0.0485 0.0585 0.0685 0.0985
AH",,,- AH", X
(kJ/mol)
Ct - cmc (mol/L)
0.30
0.71 1.03 1.53 1.95 2.26 2.50 2.72 2.78 0.82 1.37 1.54 1.93 2.37 2.87 2.99 3.17 0.84 1.50 2.14 2.52 2.66 3.30 3.40
0.0091 0.0141 0.0291 0.0391 0.0491 0.0691 0.0791 0.0891 0.0093 0.0143 0.0193 0.0293 0.0393 0.0493 0.0593 0.0693 0.0095 0.0195 0.0294 0.0395 0.0445 0.0695 0.0745
0.65
0.75
Light-scattering and fluorescence measurements on micellar solutions up to 0.1 mol/L and in absence of added salt indicate a negligible micellar change for either single cationic or anionic surfactant solution^.^^^^^ However, in the case of surfactant mixtures such as, for example, C&l+ CleCl,24an initial decrease of micellar aggregation number N is observed a t small total surfactant concentration followed by a leveling off, the onset of which depends upon the micellar composition. Such a change may be interpreted by the variation of micellar composition with surfactant concentration. No structural change need be invoked. (22)Mazer, N.A.;Benedek, G. B.; Carey, M. C. J.Phys. Chem. 1976,
80, 1075.
(23)Candau, S.J.; Hirsch, E.; Zana, R. J.Phys. Chem. 1984,45,1263. (24)Malliaris, A.;Binana-Limbele,W.; Zana, R. J. Colloid Znterface Sci. 1986,110, 114.
X
AH",, - AH", (kJ/mol)
Ct - cmc (mol/L)
1.16 1.68 2.12 2.46 2.86 3.02 3.42 3.53 3.80 1.17 1.33 2.05 2.75 3.14 3.31 3.71 1.09 1.64 2.40 2.75 3.33 3.52 3.81
0.0145 0.0195 0.0295 0.0395 0.0495 0.0595 0.0695 0.0795 0.0895 0.0095 0.0149 0.0203 0.0410 0.050 0.060 0.070 0.0096 0.0196 0.0296 0.0396 0.0506 0.0603 0.0796
0.60
0.95
0.98
Table V. Thermodynamic Parameters for the Micellar Solubilization of 1-Pentanol in Cationic Surfactant Mixtures systemnumber I1
I11
IV
V
X
P
Mot (kJ/mol)
b
uAH,,,..~
0.0 0.1 0.3 0.5 0.8 1.0
1630 1450 810 460 750 1180" 2040 900 950 980 1190 1550 1540 1300 1070 1050 1240 1520b 1750 1030 940 810 920 900 1170 1520b
3.62 3.27 4.81 6.62 6.02 5.65 3.4 3.92 4.0 4.12 4.21 3.55 3.09 4.24 4.33 5.56 5.73 6.36b 4.98 5.66 5.83 6.10 6.18 6.3 6.34 6.36b
-0.19 0.18 0.30 0.35 -0.02
0.04 0.03 0.08 0.06 0.06
-0.35 0.28 0.48 0.43 0.28 -0.15 0.23 -0.28 0.07 0.05 -0.18
0.06 0.08 0.06 0.06 0.05 0.03 0.03 0.01 0.06 0.10 0.04
0.10 0.19 0.25 0.52 0.05 0.29 -0.08
0.04 0.05 0.07 0.04 0.09 0.13 0.10
0.0 0.1 0.3 0.6 1.0 0.0 0.05 0.1 0.3 0.65 0.75 1.0 0.0 0.1 0.2 0.4 0.6 0.95 0.98 1.0
AHt obtained by extrapolation andpcalculated by eq 1. b Values obtained by extrapolation of the data to X = 1from system V: same value as for system IV (pure ClsBzCl). UAH,,, is the standard deviation obtained from the data analysis with eq 1. The situation may be different in the presence of salt. But then the cmc is considerably decreased and the minimum surfactant concentration used in the calorimetric experiments is then sufficiently remote from the cmc so that the micellar composition may be safely assumed as equal to the stoichiometric one and the aggregation number of the mixed micelles independent of surfactant concentration.
Makayssi et al.
1362 Langmuir, Val. 10, No. 5, 1994
1 '
04
0
20
40
-
60
80
100
C Cmc ( mmolll ) Figure 1. Variation of the enthalpy of solution of 1-pentanol ( m = 0.02 mol/L) as a function of micelle concentration: (a) pure C14Ck (b) C&l + C16BZC1 ( X = 0.75); (C) c1&1+ c & c l ( X = 0.20). In some cases, a slight hump was observed on the experimental enthalpy versus surfactant concentration plot in a narrow surfactant concentration range around Ct = 0.05 mol/L. The magnitude of that hump depended upon the micellar composition. In such cases, either the hump was ignored if it was barely larger than the order of magnitude of the experimental error on AHm, or the run was altogether rejected if the magnitude of the hump was too large. Figure 1 illustrates the various profiles encountered. The relative error on each value of AHm, being of the order of f 0.8 % , this means, practically, that deviations from the smooth curves of up to 2% were considered as acceptable (Figure lb); curve c in Figure 1 was not analyzed. The phenomenon must be induced by some structural micellar change. It must be stressed that the hump was found as often and was of the same magnitude in absence or in the presence of added sodium chloride. The solute mole fraction in the micelles may be estimated from the relationz1
xalc
=
Km 1 + K(C, - cmc)
Km
1+ K(Ct - cmc)
various mixed micellar systems meaningful. Finally, it is interesting to note that the effect of solute concentration on P values in micellar systems is still a matter of controversy and seems to depend upon the experimental method used.27 It has been recently shown%*=that conductivity versus concentration curves for cationic surfactants with long hydrocarbon chain and their mixtures present two breaks. It was the case for most of the systems which were used for the present study. The first one corresponds to the cmc; the second, which occurs at a surfactant concentration of the order of magnitude of 3-5 times the cmc, indicates the occurrence of surfactant structural changes. With the low cmc of the cationic surfactants used, the second break was always observed below the lowest surfactant concentration which could be used in the calorimetric experiments. Whatever the structural reason for this behavior, it could be assumed that, in the absence of added alcohol, the micellar solution remained essentially unchanged above the second break. In the presence of salt, the cmc's were considerably decreased and considered as equal to zero when applying eq 1. Finally it was assumed that the micelle composition is equal to the stoichiometric composition. This assumption is valid the higher the total surfactant concentration, the closer the cmc of the two surfactants to each other, and the closer the mixed micelles to thermodynamic ideality. For the systems under investigation, these conditions applied above a total surfactant concentration of 0.02 mol/L, a limit compatible with our analytical procedure. The enthalpy of solution of 1-pentanol as a function of surfactant concentration was studied in several mixtures for the following systems in order to test various surfactant solutions structural parameters: (1)testing the effect of salt, CI4BzCl+ C1&1 in the presence of 0.05 and 0.1 mol/L of NaCl (systems I1 and 111);(2) changing the hydrocarbon chain length, ClsBzCl + C1&1 (system IV); (3) changing the head group, C1&zC1+ Cl6PyCl (system V).
Theoretical Background It has been shown previously that the simple equations (5 and 6),which were derived for nonpolar solutes in mixed solvents, may be helpful to represent free energy and enthalpy data for dilute polar molecules in mixed micellar systems6 In P, = (1 - x ) In P,
+ x In P2 + Bx(1- x )
(5)
(4)
+1
withP = 55.5Kvwhere p i s the surfactant partial molar volume, K the partition coefficient in the molar scale, and m the concentration of alcohol. Above a total micellized concentration Ct equal to 0.05 mol/kg with a concentration of alcohol of 0.02 mol/L, eq 4 as applied to the case of 1-pentancl in single C&1 solutions shows that zde= 0.23. (P= 1500 and V = 0.321 L/mol, a value obtained using results for C14Br25 and the difference of the partial molar volume of the anions.) Under such conditions, the alcohol may be considered as having the properties of adilute solution in the sense that the value of P is still independent of alcohol concentration.26 This hypothesis may be questioned at lower surfactant concentrations. As the solute mole fraction increases above say, x d c > 0.25, P could vary with surfactant concentration because of solute solute interactions. However the linearity of the enthalpy plot based on eq 1as characterized by the low values of the standard deviation crAH,, (Table V) may be taken as indication that if such an activity coefficient effect is present, its consequenceon the values of the two unknown parameters of eq 1 is not significant under the present concentration conditions. Furthermore, it may be pointed out that these same conditions were applied to all binary systems studied, thus making a comparison between the behavior of the solute in the
+
(25) De Lisi, R.;Milioto, S.; Triolo, R. J.Solution Chem. 1988,17,673. (26) (a) Treiner, C.; Khodja,A. A,; Fromon, M.; Chevalet, J. J. Solution Chem. 1989,18, 217. (b) Abuin, E.B.; Lissi, E. A. J. Colloid Interface Sci. 1983, 95,183.
B and 6Bt6T are empirical parameters which describe the departure from ideality of the two thermodynamic functions. Indexes 1 and 2 refer to each single micelle property. Within the framework of the regular solution model, these coefficients correspond to the interaction between the two unlike surfactant components of the mixed micelles. From eqs 5 and 6 the standard entropy function could be calculated. Likewise, the experimental entropy of transfer can be obtained from the classical thermodynamic relations relating the standard free energy and standard enthalpy to the entropy of transfer from water to micelles using the data of Table V. However the entropy changewith micellar composition for the various binary systems will not be discussed in t h e present investigation as these variations do not carry additional information which could be unambiguously interpreted in structural terms at the present stage. The regular solution model as applied to the variation of cmc with surfactant micellar composition makes use of a simple activity coefficient such as, for example, log f i = expEPx121, where P is an empirical coefficient which quantifies the departure from ideality of the binary micellar ~ y s t e m .If~the regular solution modelwas strictly (27) Gao, 2.;Wasylishen, R. E.; Kwak, J. C. T.J.Phys. Chem. 1989, 93,2190. (28) Treiner, C.; Makayssi, A. Langmuir 1992, 8, 794.
Micellar Solubilization
Langmuir, Vol. 10, No. 5, 1994 1363
in the case of a two-phase
In 0.0
0.2
0.4
0.6
0.8
1.o
X
Figure 2. Variation of the partition coefficient of 1-pentanolas a function of micellar composition: C14C1 + C14BzCl,0; same system in presence of 0.05 mol/L of NaCI, +.
applicable to mixed micelles, the @ coefficient would be equal to the coefficient B of eq 5. It is of course not the case, but it was shown for a barbituric acid in a series of various surfactant mixtures ranging from cationic nonionic to anionic + cationic systems, that @ is proportional to B.9 The more negative the B coefficient, the more negative the B coefficient and, hence, the larger the decrease of micellar solubilization upon mixed micelle formation. In the present cases, the following coefficients were obtained:28C14BzC1+ C1&1,@ = -0.50; C16BzC1+ C14C1, @ = -0.36; Cl6BzCl+ c16Pyc1, @ = -0.8. The magnitude of these parameters indicates, as expected for a pair of surfactants of like charge, that the interactions between the surfactants within micelles are of small magnitude; according to the model used, the solubilization behavior of the alcohol should be similar in the above binary systems. Note that effect of salt on the value of @ is usually very small.8
+
Results and Discussion Various structural parameters will be considered separately. 1. Systems I to 111: The effect of salt. Figure 2 presents the variation of P with micellar composition for 1-pentanol as a function of C14BzC1 mole fraction in absence and in presence of a single constant quantity of NaC1, for the sake of clarity, 0.05 mol/L. A few experiments were performed on system I in addition to those obtained for the previous in~estigati0n.l~ Note that in Figures 2-6, X corresponds to the mole fraction of the second component mentioned on each figure caption. A solubilization maximum had been found previously in the C&l-rich region: its magnitude is confirmed by the additional experiments, a result which is contrary to expectation as deduced from eq 5. This was the result which prompted us to start the present investigation. The main observation is that in the presence of salt, a regular solution behavior is observed (Figure 2). However, the addition of NaCl decreases P in the C&l-rich region while the reverse trend is observed in the Cl4BzCl-rich mixtures. In view of the abnormal behavior of system I noted without salt based upon eq 5, it would be useful to choose a fundamental albeit simplistic approach for the discussion of the effect of salt as well. Equation 7 may be used to describe the basic salt effect on solute solubilization
P = Po exp(2.303kSm,) (7) where Po is the partition coefficient without salt, mE is the molar salt concentration, and ks is the salting constant. ks is positive for 1-pentanol in the presence of NaCl (ks = 0.22).30 Thus, P should slightly increase with the addition of salt because of the decrease solubility of the alcohol in the aqueous phase. The decreasing effect observed on Figure 2 on the C&I-rich micellar composition domain would therefore appear as abnormal and the solubilization behavior in C14BzC1-richend as normal. This interpretation ignores evidently the effect of salt on the micellar surface where the 1-pentanolmolecule is generally assumed to be located, at least at low alcohol concentration. Partition coefficients as deduced from solubility (saturation) measurements of 1-pentanol in sodium dodecyl sulfate solutions (C12Na) remain essentially constant upon addition of up to 0.2 mol/kg of NaC1.31 Using a gas chromatographic method at controlled (low) 1-pentanol activity in the same chemical system leads to the same conclusion.26 Finally in the case of a barbituric acid, butobarbitone, in mixtures of ClzNa with a nonionic surfactant,8 it was observed that P hardly changes upon addition of 0.2 mol/L of NaCl in the whole range of micellar composition in agreement with what could be expected from the negligible variation of @ in presence of salt in this binary system. (Note that if P remains constant, the solubility of 1-pentanol decreases by about 50% upon the addition of 0.025 mol/kg of NaCl to a Cl2Na solution of 0.04 mol/kg.32) Solubilization behavior upon addition of very high salt concentration^^^ should not be compared to the present results which were obtained at relatively low salt concentration. It seems therefore that based on eq 7 and examples such as that provided above, the dramatic decrease of P with added NaCl concentration observed with systems I1 and 111(not shown on Figure 2) should be looked upon as abnormal and that it is the behavior in the C14BzCl-rich region which is the expected one. It may be pointed out that in the absence of alcohol, system I presents no anomaly on the mixed cmc versus composition curve. Thus, the effects observed on the two parameters, P and AHt, are induced by the addition of alcohol, even at the relatively dilute concentrations employed. The enthalpic results on Figure 3 also show some interesting features. In absence of salt, the variation of AHt with micellar composition is small in the C&l-rich region and changes dramatically in the ClrBzCl-rich composition region. Benzyl alcohol presents essentially the same features in system 1,16 whereas acetophenone displays a close to ideal behavior with a value of the empirical coefficient of eq 6 close to zero.18 The addition of salt decreases the endothermic maximum until, at 0.1 mol/L of NaC1, a plateau value is attained in the whole range of micellar composition. The system then behaves as if it had become ideal. One may conclude that the origin of the atypicalPmaximum observed for the alcohols is predominantly entropic and the normal behavior is essentially enthalpic. Addition of salt gradually breaks (29) Fromon,M.;Chattopadhyay,A. K.;Treiner,C. J. ColloidInterface Sci. 1984, 102, 14. (30) Aveyard, R.; Heselden, R. J. Chem. Soc., Faraday Trans. 1 1975, 70, 312. (31) Hoiland, H.; Ljosland, E.; Backlund, S. J. Colloid Interface Sci. i w . ---, 707.4137. (32) Blokhus, A. M.; Hoiland, H.; Gilje, E.; Backlund, S. J. Colloid Interface Sci. 1988, 124, 125. (33) Ozeki, S.; Ikeda, S. J.Phys. Chem. 1985, 89, 5088. ----I
Makayssi et al.
1364 Langmuir, Vol. 10, No. 5, 1994
1
10,
4z
20.0
0.2
X
0.4
0.6
1.o
0.8
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Figure 3. Variation of AHt as a function of micellar composition: C&l+ CI4BzC1,0 ;same system in presence of 0.10 mol/L of NaCl, +.
k
I
1500
goo4 0.0
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I 1.0
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Figure 4. Variation of the partition coefficient of 1-pentanolas a function of micellar composition: CI4Cl+ C&Cl. up the micellar structure responsible for the abnormal behavior displayed in absence of electrolyte. 2. System IV: Hydrocarbon Chain Length Effect. The hydrocarbon chains of the two surfactants of system I are of equal length. System IV increases the dissymmetry between the alkyl and the aryl surfactants by adding two methylene groups on the benzalkonium derivative. Figure 4 shows the results obtained for the C1&1 + C16BzC1 mixtures (the experiments in pure C&zCl solutions were performed in presence of a small quantity of C16Pycl: see below). The P profile for 1-pentanol displays a typical regular solution behavior; no maximum is observed. The corresponding enthalpic curve (Figure 5) however shows some distinct characters in the Cl&l-rich region. The precision of the enthalpy results (and the smoothness of the Pversus composition curves) authorizes consideration of such enthalpy changes as the consequence of meaningful micellar modifications. The enthalpy (or entropy) curves often display specificities which are cancelled out when the free enthalpy function is considered because AHt and TGSt are of the same sign and order of magnitude. This is the case here. In the C16BzCl-richmicellar composition, a close to ideal behavior is observed. Hence the main difference between the results of Figures 3 and 5 does not occur in the C&l-rich micellar composition domain but in the benzalkonium-rich one (note the different scales in these figures), with the large endothermic maximum with C14BzC1 replaced by a linear variation with C16BzCI. It has been noted before that when the hydrocarbon
Figure 5. Variation of AHt as a function of micellar composition: CI4Cl+ Cl&izC1.
chain length is larger than 12 methylene groups, further increase in chain length has a small influence on the partition coefficients of polar solute^.^ It is therefore interesting to recall that the maximum solubilization of the dye o-tolueneazo-8-naphthol (OOT) in mixtures of betaines with C12Na,12increases by a factor of 2 when the zwitterionic surfactant chain length is increased from 12 to 14. This effect, which is in the opposite direction to that observed with systems I and IV, still indicates the much greater influence of surfactant chain length on micellar solubilization for mixed micelles when compared to single micellar solutions. In the case studied by Iwasaki et al., the increase of solubilization follows the increase of micellar aggregation numbers. 3. System V: Head-Group Effect. The effect of changing the head group, keeping the same hydrocarbon chain length, was investigated on the hexadecyl derivatives with the pyridinium group replacing the trimethylammonium group. The experiments could not be performed in pure C16BzCl solutions because of the rather low solubility of that surfactant in water. Thus, a solution composition with 0.02 mol fraction of ClePyCl was studied with C16BzC1, which was enough to obtain a clear solution up to 0.1 total surfactant concentration. The values of P and AHt obtained at several micellar compositions were extrapolated to obtain the pure C&zC1 result. This result was also used for that surfactant in system IV. The decrease of P upon addition of either component to the single surfactant solution is faster than for the other systems (Figure 6) and could not be represented by a regular solution behavior (eq 5). Apparently, addition of a small quantity of the second surfactant component is highly unfavorable to the alcohol micellar solubilization until a plateau value is observed. The AHt versus composition curve should be considered with the same observations than for the preceeding system regarding the confidence attached to the A& data. The data could be fitted to eq 6 with a positive value for the excess enthalpy term. In conclusion, it appears that system I should be considered as an isolated case which displays an extremely atypical solubilization behavior in the case of alcohols. However, all the cationic surfactant pairs displayed specificities which are not amenable to the simple representation inferred by eq 5. As the experimental conditions were identical for all systems studied, the calorimetric technique cannot be blamed for the abnormal results obtained. The present study shows the evident limitation of the simple thermodynamic approach adopted
Micellar Solubilization
Langmuir, Vol. 10, No. 5, 1994 1365
1800
P
1400
1000
600 0.0
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Figure 6. Variation of the partition coefficient of 1-pentanol as a function of micellar composition: ClsPyCl + C16BzC1.
4.5 0.0
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Figure 7. Variation of AHH,as a function of micellar composition: C&c1 + Cl&zCl.
Any interpretation of such data can only be speculative. One may recall that double-tailed surfactants form vesicles. Most certainly, vesicles and micelles present different solubilization characteristics. The aromatic group attached on the methylene moiety of the substituted ammonium head group of the benzalkonium salts may be considered as a short, bulky second hydrocarbon chain. The estimation of the ratio of total hydrocarbon volume V over surface head-group a' and chain length I as a guideline for structural information on aggregation type (V/a01)34is not straightforward in the case of the benzalkonium series. However, Magid and ~ o - w o r k e rhave s~~ shown using light-scattering experiments that doubletailed surfactants of the type of sodium p(pentylhepty1)benzenesulfonate present two types of micellar size distributions corresponding to large micelles in equilibrium with small ones. Nagarajan and Ruckenstein36 have predicted that, for some of such surfactants, coexisting micelles and vesicles may be formed. Zana has shown that in the case of the dodecylalkyldimethylammonium bromide series, the surfactants start to behave as doubletailed compounds when one dodecyl chain is associated to a butyl chain.37 An increase in aggregation number is not sufficient to promote an increased solubilizationas is shown by the normal (regular) behavior of alcohols in mixed anionic + cationic surfactant mixture^.^ However, in all cases where an increase in micellar solubilization occurs above the ideal behavior, the size of the aggregates also increases. These remarks suggest two working hypotheses for future work in this area: (i) Benzalkonium salts promote, in the presence of a small quantity of alcohol and under specific conditions dictated by structural surfactant constrains and micellar composition, the formation of small vesicles in equilibrium with micelles. (ii) The effect of alcohol on the mixed micelle structure is larger the smaller the mutual interaction between the unlike cationic components. Confirmation of such hypotheses would need the determination of mixed micellar aggregation numbers in the presence of small alcohol quantities.
to mixed micelles with interactions between the unlike components governing the sign and the amplitude of the micellar solubilization changes. A plausible suggestion would be to assume that if the interactions between the (34)Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. SOC., two surfactant components are weak, then the addition of Faraday Trans 1 1976,72,1525. a solute which interacts more strongly with one of them (35) Magid, L. J.; Triolo, R.; Gulari, E.; Bedwell, B. In Surfactants in may alter more deeply the mixed micellar structure than Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984;Vol. 1, p 405. in the case of strong intramolecular interactions between (36) Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1979,71, the unlike surfactants. This effect, which may be at work 580. in the present cationic mixtures did not seem to occur (37) Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983,91, with most mixed surfactant systems previously a n a l ~ z e d . ~ ~276.
