Dynamic partitioning of an aromatic probe between the headgroup and core regions of cationic micelles

Fluorescence lifetime measurements of 2-ethylnaphthalene (2EN) probes in the cationic micelles tetradecyl-trimethylammonium chloride (TTAC), tetradecyl-trimethylammonium bromide (TTAB), dodecyl-trimethylammonium bromide (DTAB), and dodecyl-trimethylammonium chloride (DTAC) demonstrate the close proximity of the 2EN to the Br- counterions. The fluorescence lifetimes in TTAB and DTAB are much shorter than in TTAC and DTAC because of fluorescence quenching caused by the external heavy atom (Br-) effect. These results indicate that 2EN resides to a substantial degree in the headgroup region of the micelles. However, detailed analysis of the fluorescence decays shows that the decays are nonexponential in TTAB and DTAB. For micelles with chloride counterions, the fluorescence decays are single exponentials. Fluorescence measurements on nonmicelle solutions with bromide ions are also single exponentials. The results indicate that 2EN is exchanged between the headgroup and the core regions on a short time scale. Analysis of the data with a kinetic model provides information on the faction of the 2EN in the headgroup regions and on the exchange rates. The results show that exchange between the headgroup and the core occurs on a 5 to 10 ns time scale, depending on the micelle. At any given time, similar to 80% and similar to 50% of the 2EN are found in the headgroup regions of the tetradecyl and dodecyl micelles, respectively.