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Langmuir 1993,9, 2066-2070
Solubilization of Alkylpyridinium Ions in Cationic Micelles: Effect of the Electrostatic Repulsion Consuelo Gamboa and Andres F. Olea' Departamento de Qutmica, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Casilla 325, Santiago, Chile Received November 4, 1992. I n Final Form: May 7, 1993 Binding constants of n-alkylpyridinium, C,Pd+ (n = 10, 11,12,13,14, and 161,to CTAX micelles, where X = B r and C1-, and to CTAOTOS were determined by ultrafiltration and a steady-state fluorescence method. These micelles have different dissociation degrees and, consequently, different micellar surface potentials. Free energies of transfer, A,were obtained from the distribution coefficientsdefined by the pseudophase model. From a plot of A against the number of c*n atoms, a value of -2.5 kJ/mol was obtained for the incremental free energy per methylene group and 8.7,11, and 13kJ/mol for the electrostatic repulsion contribution sensed by C,Pd+ in CTAOTOS, CTAB, and CTAC, respectively. C,PdX, with n = 10,are solubilized mainly in the aqueous intermicellarphase. Based on these results, a site of solubilization of the pyridinium head group in the interface is proposed.
Introduction One of the most important features of micelles is their capacity to solubilize hydrophobic molecules in aqueous solution.13 Incorporation of substrates into micelles has been analyzed through different theoretical approaches$ and the pseudophase model is the one most commonly used. In this model, solubilization is treated as a distribution of a substrate between two pseudophases: the micellar phase and the aqueous phase.- The thermodynamic parameters obtained from this kind of study are the partition coefficient and the free energy of transfer, A,which is assumed to be composed of additive contributions from different groupsg = ~ l +~n , ~ r ,+ pel (1) where p ~ denotes r the contribution of the parent aromatic group, pc is the incremental free energy per methylene group, n, is the number of these groups attached to the parent group, and pel is the electrostatic contribution due to the Coulombic interaction between a formal charge on the substrate and the charged micellar surface. Some attempt have been made to calculate the last term in the micellization process,8 but as Tanford indicated, this is not a simple task, primarily because of the presence of counter ion^.^ On the other hand, measurements of pel are rather ~ c a r c e . ~ & ~ 3 In this paper we report the results obtained from a solubilization study of long chain alkylpyridinium ions, C,Pd+, with chain lengths ranging from n = 10 to 16, in pt
(1) Shinoda,K.,Ed.Soluent Properties ofSurfactant Solutions;Marcel Dekker: New York, 1967. (2) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular System; Academic Press: New York, 1975. (3) Mukerjee, P. In Solutions Chemistry of Surfactants;Mittal, K. L., Ed.; Plenum: New York 1979; Vol. 1, p 157. (4) Sepulveda, L.; Lissi, E.; Quina, F. Adu. Colloid Interface Sei. 1986, 25, 1. (5) Almgren, M.; Grieeer, F.; Thomas, J. K. J. Am. Chem. SOC.1979, 101, 279. (6) Tachiya, M. J . Chem. Phys. 1982, 76, 340. (7) Moroi, Y.; Noma, H.; Matuura, R. J. Phys. Chem. 1983,87,872. (8) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934. (9) Tanford, C. The Hydrophobic Effect,2nd ed.; John Wiley: New York, 1988. (10) Bunton, C.; Sepulveda, L. J. Phys. Chem. 1979,83,680. (11) Hirme, C.; Sepulveda, L. J. Phys. Chem. 1981,85, 3689. (12) Bunton, C.; Romsted, L.; Thamavit, C. J. Am. Chem. SOC.1980, 102,3900. (13) Gamboa, C.; Sepulveda, L.; Rim, H. J. Phys. Chem. 1989, 93, 5540.
cetyltrimethylammonium, CTA+, micelles with chloride, bromide, and p-toluenesulfonate (OTOS-) counterions.
Experimental Section Materials. n-Alkylpyridinium bromides, C,Pd+Br (n = 10, 11,13,14),were prepared by refluxing pyridine (Aldrichfreshly distilled) and the respective n-alkyl bromide (Aldrich) for 24 h. The cooled, solid materials were washed with acetone and dried under vacuum. Dodecylpyridniumchloride (Aldrich)and cetylpyridnium chloride (Sigma)were used as received. The surfactants CTAB and CTAOTOS were obtained from Sigma and used without further purification. CTAC was obtained from CTAB solutions by passing it through a Dowex 1x8-200 ion exchange resin (Aldrich),drying,and crystallizing. The resin was previously treated with chloride acid; the purity of the CTAC was checked from surface tension measurements of aqueous solutions. The critical micellar concentration (cmc)obtained was 1.3 X 10-8M, in good accord with reported value.12 Pyrenesulfonate and pyrenetetrasulfonate were purchased from Molecular Probes. Measurements of Surface Potentials. Micellar surface potentials were estimated from the shift in the ground state pK, of Phenol Red solubilized in micelles, following the method of Fromherz and Fernandez.14 The basic and acid forms of Phenol Red show two different absorption bands centered at 572 and 420 nm, respectively. The pK,'s of the probe in water, in charged micelles, and in a neutral micelle were obtained from a plot of AIAo v8 pH, where A and A0 denote the absorbance at 572 nm for a given pH and for a pH where only the basic form is present. Phenol Red (0.1 g) was dissolved with 0.05 M NaOH (5.7 mL) and diluted to 200 mL. A Phenol Red stock solution (1.3 X 1od M) was prepared by diluting 1Wfold. Ultrafiltration indicated the probe is completely incorporated to the cationic micelles. The pK measurements were made in 0.01 M surfactantsolutions containing 2.6 X lV7M of the dye. An aliquot of 3 mL was added to a UV cuvette and both pH and absorption spectra were recorded. The pH was measured with a thin Schott Gerate combinedelectrode,connected to an Orion Research Model7Ol-A digital ionalyzer. Absorption spectra were registered in a Shimadzu UV 160 spectrophotometer. This method has been recently reviewed by Drummond et al.l6J6 who pointed out that the measured potentials depend on many different factors. The most criticized assumption of this treatment is the use of the pK value determined in nonionic micelles as the pK0, instead of that obtained from the study of organic solvent-water mixtures. (14) Fromherz, P.; Femandez, M. J. Phys. Chem. 1977,81, 1755. (15) Drummond, C. J.; Grieser, F.; Healy, T. W. J. Phys. Chem. 1988, 92, 2604. (16) Drummond, C. J.; Grieser, F.; Healy, T. W. Faraday Diecuss. Chem. SOC.1986,81,95. Dnunmond, C . J.; Grieser, F.; Healy, T. W. J. Chem. SOC.,Faraday Trans. 1989,85,521-577.
0743-746319312409-2066$04.00/0 0 1993 American Chemical Society
Solubilization of C z d + in Cationic Micelles
Langmuir, Vol. 9, No. 8, 1993 2067
However, Drumond et al.lS have demonstrated that for cationic micelles this assuption is valid. For the systems studied here, pKo must be the same because just the counterion has been changed. Measurements of Distribution Coefficients. In the pseudophase model the solubilization process is represented by the equilibrium:
1.5
I T
for which the equilibrium constant Ks is given by where [Swl and [SM] denote molar concentrations of substrate in aqueous and micellar pseudphase, respectively, and [DM]is the concentration of micellized surfactant. The distribution coefficient defined on a mole fraction basis, K,,is given by
-
L
[CTAB],.
(4)
5
6
IO2
f
where XUand XWare the substrate mole fractions in the micellar and aqueous phases. These quantities are related to ISM] and [SW]through the following equations
LO '-
B
(5)
-'1-
xw= [Sw] +[SWI cmc + 55.5 K, is related to KSby
30
20
'
At low substrate occupation numbers ([Sd [DMI
