AOT REVERSED MICELLES INVESTIGATED BY FLUORESCENCE ANISOTROPY OF CRESYL VIOLET

AOT reversed micelles in n-heptane and n-dodecane as the continuous ph ase at various values of W = [H2O]/[AOT] were investigated through the decay of the total fluorescence, f(t), and of the fluorescence anisot ropy, r(t), of cresyl violet. The total fluorescence decay was monoexp onential in all cases investigated. The fluorescence lifetime decrease d with increasing W, with the extent of decrease diminishing at higher W values. No significant dependence on the continuous phase was obser ved. A biexponential expression for the fluorescence anisotropy yielde d adequate fits for all cases. For W < 10, one correlation time reflec ts the internal rotation of the probe in the micelle and the second th e 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 th e dye within its local hindering environment, r(b)(t) accounts for the motion of the micellar segment containing the dye, and exp((-)/phi(sp h) is the additional depolarization factor associated with the overall rotation of the micelle. Both r(p)(t) and r(b)(t) were expressed as t he sum of an exponential decay and a constant. The constant in the exp ression of r(p)(t) gives an indication of the width of the distributio n of the dye within the micellar segment; i.e., the constant is correl ated with the second orientational order parameter, S-p. Similarly, th e constant in the expression for r(b)(t) measures the second-order par ameter, 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 p hase. In contrast, S-b was higher in n-dodecane (0.7) than in n-heptan e (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 s uggested in the literature, and we refer to it as micellar ''breathing ''.