FLUORESCENT-PROBE SOLUBILIZATION IN THE HEADGROUP AND CORE REGIONS OFMICELLES - FLUORESCENCE LIFETIME AND ORIENTATIONAL RELAXATION MEASUREMENTS

Experimental results demonstrate that the fluorescent probes 2-(N-hexa decylamino)-naphthalene-6-sulfonate (HANS) and 2-(N-decylamino)-naphth alene-6-sulfonate (DANS) are solubilized in two distinct regions, that is, the headgroup and core, within micelles of cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammoniumbromide (TTAB), dodecyltrim ethylammoniumbromide (DTAB), cetyltrimethylammoniumchloride (CTAC), an d tetradecyltrimethylammoniumchloride (TTAC). The fluorescence lifetim e decays for both chromophores are biexponential in all the different micelles. The population associated with the shorter lifetime (tau(1) congruent to 4-5 ns) is located in the Stern layer, where reduction of the fluorescence lifetime occurs because of quenching induced by the bromide counterions, The second population of chromophores is located in the hydrocarbon core region of the micelle. In this environment the chromophores have a considerably longer lifetime (tau(2) congruent to 19-20 ns) because there is no significant quenching by bromide counte rions. Evidence of water penetration places them fairly close to the c ore-Stern layer interface. Time-dependent fluorescence anisotropy is a nalyzed in terms of these two populations. The measurements show that the orientational relaxation of the probes in the hydrocarbon core reg ion is considerably slower than orientational relaxation in the Stern layer. When the lifetime measurements and the orientational relaxation measurements are combined, the partitioning of the chromophores in th e core and headgroup regions of the micelles can be determined.