Models for Specific Counterion Effects on the Incorporation of Charged Amphiphilic Substrates into Like-Charged Ionic Micelles

Article abstract of DOI:10.1021/j100245a020

Data for the quenching of the fluorescence of two anthracene-derived amphiphilic fluorescence probes (ethyldimethyl- (I) and trimethyl<3-(9-anthracenyl)propyl>ammonium bromide (II) in aqueous micellar solutions of the quencher detergents N-hexadecylpyridinium chloride (HPCl), bromide (HPBr), and nitrate (HPNO3) are analyzed in terms of a photophysical model which assumes static quenching of the micelle-incorporated probe and dynamic quenching of the residual fluorescence of the unincorporated probe by free detergent monomer and free counterions.This analysis provides values for the ground-state probe incorporation coefficient K_S which are compatible with independent estimates based on the quenching of the emission of the micelle-excluded Ru(bpy)3²⁺ ion by I under the same conditions.The results of the present study clearly show that K_S for incorporation of a charged amphiphilic substrate into the micellar phase of a like-charged detergent is quite sensitive to the concentration of detergent and added salt present.In addition to this general salt dependence, which can be ascribed to the effects of intermicellar ionic strength and intermicellar interactions on the micellar surface potential, the magnitude of K_S also exhibits a significant specific salt (counterion) dependence.Consideration of the consequences of selectivity in counterion exchange at the micelle surface on the effective micellar surface potential provides an analytical expression which rationalizes the obseved specific counterion effects on K_S for this type of substrate.The implications for the quantitative analysis of ground-state reactions and equilibria in ionic micellar solution are pointed out and methods for estimating K_S under diverse experimental conditions suggested.