Bromide/Chtoride Counterion Exchange at the Surfaces of Dioctadecytdlmethylammonium Vesicles

Abstract

The exchange between bromide and chloride counterions at the surfaces of dioctadecyldimethylammonium vesicles has been examined through fluorescence quenching experiments. The quenching of the fluorescence of vesicle-incorporated naphthalene derivatives by bromide counterions has been studied under different experimental conditions. The data are analyzed by following the pseudophase ion-exchange model and assuming that the observed quenching is a direct function of the local quencher concentration at the vesicle surfaces. The fluorescence quenching behavior is found to be accounted for by the ion-exchange formalism when experiments are performed on vesicles having the same ionic composition for the contacting solution at both the outer and inner aqueous pseudophases. From this type of experiment, a bromide/chloride counterion exchange constant equal to 4 7 1 is obtained. Experiments performed by addition of a vesicle solution containing a single counterion (chloride) to an isotonic solution containing a mixture of bromide and chloride allow an evaluation of the ability of the vesicles to exchange the counterions at the outer surface while maintaining a single counterion at the inner surface. An analysis of the quenching data obtained by extrapolation to time zero indicates that the ability of the outer surface to exchange counterions under these conditions is lower than that predicted by the exchange model. The luminescence observed decreases with time in a manner that is indicative of fast diffusion of bromide across the vesicle bilayers. These results are interpreted in terms of the strain imposed on the bilayer by the asymmetrical counterion distribution, strain that can be relaxed by an increased rate of surfactant flip-flop leading to a thermodynamically stable state.