Salt effects on the dynamics of incorporation of organic coions into micelles

Time-resolved fluorescence decay measurements with pyrene as a probe have b een employed to study the dynamics of entry and exit of several homologous N-alkylpyridinium ions (R-Py+) from cationic hexadecyltrimethylammonium chl oride (CTACl) and acetate (CTAOAc) micelles over a wide range of concentrat ions of detergent and added common counterion salt (NaCl or NaOAc, respecti vely). The rate constants for entry of the N-alkylpyridinium ions into the micelle are insensitive to the alkyl chain length but increase with increas ing concentration of detergent or added salt. In contrast, the rate constan ts for micellar exit of the N-alkylpyridinium ions are quite sensitive to t he length of the alkyl chain attached to the pyridinium ion but relatively insensitive to salt or detergent concentration. Analysis of these results i n terms of a simple pseudophase model suggests that the transition state fo r incorporation of amphiphilic colons such as the N-alkylpyridinium ions in to ionic micelles is located just outside the Stern layer. These results pr ovide a unified picture of the relationship between dynamics and selectivit y of ion binding to ionic micelles.