Rotational Relaxation of Nondipolar Probes in Triton X-100 Micelles in the Presence of Added Salt: Correlation of Lateral Diffusion Coefficient with “Dry” Micelle Radius

Abstract

In continuation of our efforts to understand the dynamics of solubilized species in large nonionic micelles, rotational relaxation of two structurally similar nondipolar probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been studied in Triton X-100 (TX-100) micelles in the presence of added salt. According to very recent light scattering studies, the hydrodynamic radius of TX-100 micelles increases by a factor of 2.2 upon the addition of 2.0 M NaCl due to increase in both the aggregation number and mechanically entrapped water. The micelles also get dehydrated in the presence of electrolyte due to decrease in the thermodynamically bound water, and consequently the “dry” micelle radius also increases, but only by a factor of 1.5. Time-resolved anisotropy of both the probes, DMDPP and DPP, in TX-100 micelles decays as a sum of two exponentials with two time constants, one corresponding to a fast reorientation time and the other to a slow one, over the entire range of salt concentration. The results are analyzed in terms of a two-step model consisting of fast-restricted rotation of the probe and slow lateral diffusion of the probe in the micelle that are coupled to the rotation of the micelle as a whole. However, as the concentration of NaCl is increased from 0.0 to 2.0 M, there is only a marginal increase (37% for DMDPP and 19% for DPP) in the slow component and almost no change in the fast component for both the probes. The lateral diffusion coefficients of both the probes obtained from their respective slow components are correlated with the dry micelle radius.