A quantitative treatment of micellar effects upon deprotonation equilibria

The apparent base dissociation constants, K_B, for deprotonation of benzimidazole in dilute NaOH go through minima with increasing concentration of cetyltrimethylammonium ion surfactants (CTAX, X = Cl, Br, NO₃) at concentrations above the critical micelle concentration (cmc). However, K_B decreases smoothly if the concentration of surfactant counterions is maintained at 0.1 M. In both cases the micellar effect on K_B follows the sequence CTACl > CTABr > CTANO₃. These observations are consistent with a pseudophase ion-exchange model in which the micelle binds OH^- and both forms of the indicator, increasing deprotonation and decreasing K_B, but at the same time the counterion of the surfactant displaces OH^- from the micellar surface so that K_B goes through a minimum with increasing [CTAX]. This model leads to a very simple quantitative treatment which fits the data for [CTAX] > 0.01 M. A similar pattern of behavior has been found for deprotonation of phenols and oximes in CTABr in the presence of borate buffer. Intrinsic basicity constants in the micellar pseudophase can be estimated which are larger than those in water by approximately one order of magnitude.