FLUORESCENT PROBE SOLUBILIZATION IN THE HEADGROUP AND CORE REGIONS OF MICELLES : FLUORESCENCE LIFETIME AND ORIENTATIONAL RELAXATION MEASUREMENTS

Experimental results demonstrate that the fluorescent probes 2-(N-hexadecylamino)-naphthalene-6-sulfonate (HANS) and 2-(N-decylamino)-naphthalene-6-sulfonate (DANS) are solubilized in two distinct regions, that is, the headgroup and core, within micelles of cetyltrimethylammoniumbromide (CTAB), tetradecyltrimethylammoniumbromide (TTAB), dodecyltrimethylammoniumbromide (DTAB), cetyltrimethylammoniumchloride (CTAC), and tetradecyltrimethylammoniumchloride (TTAC). The fluorescence lifetime decays for both chromophores are biexponential in all the different micelles. The population associated with the shorter lifetime (U1 = 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 (U2 = 19-20 ns) because there is no significant quenching by bromide counterions. Evidence of water penetration places them fairly close to the core-Stern layer interface. Time-dependent fluorescence anisotropy is analyzed in terms of these two populations. The measurements show that the orientational relaxation of the probes in the hydrocarbon core region 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 the core and headgroup regions of the micelles can be determined. In this paper experiments on and analysis of the fluorescence lifetimes and the orientational relaxation of two fluorescent probes, 2-(N-hexadecylamino)-naphthalene-6-sulfonate (HANS) and 2-(N-decylamino)-naphthalene-6-sulfonate (DANS), in five micelles, cetyltrimethylammoniumbromide (CTAB), tetradecyltrimethylammoniumbromide (TTAB), dodecyltrimethylammoniumbromide (DTAB), cetyltrimethylammoniumchloride (CTAC), and tetradecyltrimethylammoniumchloride (TTAC), are reported. Careful analysis of the data and comparison of the micelle data with data taken in a variety of homogeneous solvents of varying concentrations of micelle counterions reveals a remarkable feature of these systems. While the majority of the HANS and DANS chromophores are located in the headgroup region (Stern layer) of the micelles, a detectable amount of the probes dissolves in the hydrocarbon core. The location of the probes in the two regions is evidenced by differences in lifetimes and orientational relaxation times. The fraction of the probe chromophores that reside in each region can be determined by analyzing the time-dependent data. The use of fluorescent probes has become a widespread approach for investigating properties of micelles, vesicles, and membranes. 1-8 Various types of fluorescence experiments have been employed to study these systems, but steady-state fluorescence spectroscopy remains the most widely used technique. For a method based on the use of fluorescent probes to be applied successfully, the fluorescent molecule must be tailored to probe the desired environment within the microheterogeneous medium under investigation. To ascertain that the probe is located in the target environment, various probe properties, like the positions and shapes of the absorption and fluorescence bands, can be compared to those in bulk-phase systems that mimic the conditions in the microenvironments. The choice of these comparative systems is not always straightforward, since the conditions in bulk-phase analogues of the environments in the microheterogeneous system can actually be quite different from the latter system. 9 This is true in particular for the hydrocarbon core of a micelle. It is often mimicked with hydrocarbon solvents, thus neglecting the notable water content of the regions close to the interface layer, where water penetration can be significant. Because the fluorescence properties of many probes are quite sensitive to the presence of water, especially if there is hydrogen bonding, it becomes necessary to understand the behavior of the probe in considerable detail before an unequivocal interpretation of the experimental results