AOT REVERSED MICELLES INVESTIGATED BY FLUORESCENCE ANISOTROPY OF CRESYL VIOLET

Abstract

AOT reversed micelles in n-heptane and n-dodecane as the continuous phase at various values of W = [H2O]/[AOT] were investigated through the decay of the total fluorescence, f(t), and of the fluorescence anisotropy, r(t), of cresyl violet. The total fluorescence decay was monoexponential in all cases investigated. The fluorescence lifetime decreased with increasing W, with the extent of decrease diminishing at higher W values. No significant dependence on the continuous phase was observed. A biexponential expression for the fluorescence anisotropy yielded adequate fits for all cases. For W < 10, one correlation time reflects the internal rotation of the probe in the micelle and the second the overall rotation of the micelle. For W greater than or equal to 10, the fluorescence anisotropy can be expressed alternatively by r(t) = r(p)(t) r(b)(t) exp((-)t/phi(sph); r(p)(t) denotes the anisotropy of the dye within its local hindering environment, r(b)(t) accounts for the motion of the micellar segment containing the dye, and exp((-)/phi(sph) is the additional depolarization factor associated with the overall rotation of the micelle. Both r(p)(t) and r(b)(t) were expressed as the sum of an exponential decay and a constant. The constant in the expression of r(p)(t) gives an indication of the width of the distribution of the dye within the micellar segment; i.e., the constant is correlated with the second orientational order parameter, S-p. Similarly, the constant in the expression for r(b)(t) measures the second-order parameter, S-b, of the equilibrium distribution of the micellar segments with respect to the normal to the micellar surface. It was found that S-p was about 0.9. The value of S-p did not depend on the continuous phase. In contrast, S-b was higher in n-dodecane (0.7) than in n-heptane (0.4). The relaxation time, phi(b), in r(b)(t) measures the dynamics of the wobbling motion of the micellar segment and was found to be on the order of several nanoseconds. As expected, phi(b) is higher in n-dodecane than in n-heptane. The motion of micellar segments has been suggested in the literature, and we refer to it as micellar ''breathing''.