DYNAMICS OF COUNTERION EXCHANGE IN AQUEOUS MICELLAR SOLUTION - SALT EFFECTS ON THE COUNTERION EXIT RATE

Time-resolved fluorescence decay measurements with pyrene as probe have been employed to study the dynamics of exit of the N-ethylpyridinium ion and Cu2+ from micelles of sodium dodecyl sulfate (SDS) over a wide range of concentration of SDS and added common counterion salt (NaCl or Na2SO4). The rate constants k(q) for intramicellar quenching decrease slightly with increasing added salt due to salt-induced micellar growth. The rate constants k(-) for micellar exit are modestly dependent on SDS concentration and strongly dependent on the added salt concentration. A simple pseudophase model, which assumes that the counterion exit rate is governed by the micellar surface potential, nicely rationalizes both the detergent and added salt effects on k(-) and provides an estimate of the effective electrostatic work for counterion escape from the SDS micelle.