Protonation of anionic indicators in the presence of cationic micelles

The protonation equilibria of two sulfonephthalein indicators (BCG and BPB) and one sulfonated azo dye (T-azo-R) in the presence of a cationic surfactant (alkylbenzyldimethylammonium chloride) at a concentration higher than its critical micellar concentration (c.m.c.) have been investigated. In particular the effect of the ionic composition of the solution was studied, with the purpose of testing a model which regards the micelles as a true microphase behaving like a strong base anion-exchange resin. The potential difference at the micelle/aqueous solution interface accounts for the distribution equilibria of charged species, which are set up according to the Gibbs–Donnan equation. Thus the ion-exchange constants are expected to be equal to one, while the ion-exchange coefficients depend on the activity coefficients of the charged species partitioned between the micellar and solution phases. According to the model a simple relation is derived between the apparent protonation constants in the presence of micelles, and those in aqueous solution, which has been verified for the systems considered, for other previously studied organic reagents and in the case of interaction of T-azo-R with albumin in acidic solution.