Cationic surfactant/bile salt interaction studied by fluorescence quenching

Fluorescence quenching measurements were performed on aqueous solutions of the cationic surfactant cetyltrimethylammonium halide (CTAX) and two bile salts, sodium cholate (NaC) and sodium deoxycholate (NaDC), to study the state of aggregation in the mixtures. Pyrene was used as a photoluminiscence probe in the study, and dimethylbenzophenone (DMBP) as the quencher. Analysis of time-resolved decay data with and without quencher using a simple kinetic model gave information of the different aggregation characteristics in the above two cases. Mixed micelles of CTAX/NaC were small and spherical at all compositions, while those of CTAX/NaDC tended to grow from spherical micelles to larger rod-like mixed aggregates at equimolar and close-to-equimolar concentrations. In the latter case more complex kinetics ensues and the fluorescence decays were treated using a generatized model for diffusion-controlled quenching along one dimension for infinitely long rod-like micelles. The mutual diffusion coefficient for the probe-quencherpair was determined.