Solubilization of Alkylpyridinium Ions in Cationic Micelles: Effect of the Electrostatic Repulsion

Abstract

Binding constants of n-alkylpyridinium, CnPd+ (n = 10, 11, 12, 13, 14, and 16), to CTAX micelles, where X = Br− and Cl−, 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, µt, were obtained from the distribution coefficients defined by the pseudophase model. From a plot of µt against the number of carbon atoms, a value of −2.5 kJ/mol was obtained for the incremental free energy per methylene group and 8.7, 11, and 13 kJ/mol for the electrostatic repulsion contribution sensed by CnPd+ in CTAOTOS, CTAB, and CTAC, respectively. CnPdX, with n = 10, are solubilized mainly in the aqueous intermicellar phase. Based on these results, a site of solubilization of the pyridinium head group in the interface is proposed. © 1993, American Chemical Society. All rights reserved.