Reactivity and Models for Anion Distribution: Specific Iodide Binding to Sulfobetaine Micelles

The reaction of I- with methyl naphthalene-2-sulfonate (MeONs) is accelerated by the micellized sulfobetaind surfactants N-decyl, N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate. Concentrations of micellar-bound I- were determined by using ion-selective electrodes (ISE), and capillary electrophoresis. At low concentrations, I- incorporation fits Langmuir isotherms and is related to changes in micellar surface potentials. Rate effects of dilute KI are fitted quantitatively by a pseudophase model that describes I- binding in terms of a sorption isotherm, but at higher [KI], where the simple model predicts saturation, rates increase due to electrolyte invasion. This model considers transfer equilibria of both reactants between water and micelles and second-order rate constants in each pseudophase. Estimated second-order rate constants for reaction of MeONs with I- in the micellar pseudophase are 3.2- to 3.5-fold higher than the second-order rate constant, k(2w), in water, depending on surfactant structure and assumptions in the treatment.