Effect of the micellar composition on the rotational relaxation dynamics of Coumarin 153 in mixed micelles of n-dodecyl-β-d-maltoside and sodium dodecyl sulfate

Abstract

The effect of the micellar composition of mixed micelles of n-dodecyl-β-d-maltoside (β-C12G2) and sodium dodecyl sulfate (SDS) on the photophysics and rotational diffusion of Coumarin 153 has been investigated. State-steady and time-resolved fluorescence spectroscopic data, including time-resolved fluorescence anisotropy ones, were used to assess the effect of the presence of the ionic surfactant on the local environment of mixed micelles. The probe molecule was found to reside close to the micellar interface, where it senses a relatively polar environment. Hydrodynamic radii data of micellar aggregates, as determined from dynamic light-scattering studies, indicated that the micellar size is controlled by a combined effect of steric and electrostatic interactions. Moreover, the two-step model together with the wobbling-in-cone one were used to analyze the fluorescence anisotropy decay, which proved to be biexponential in nature. Our experimental results suggest that the translational diffusion of the probe within micelle is the main factor controlling the depolarization mechanism of Coumarin 153 in micellar media. We conclude that the presence of the ionic surfactant in the mixed micelle causes a reduction of the micellar microviscosity. This finding was attributed to a significant relaxation of the local structure of the micellar interface as a result of the substitution of the bulky, rigid, and strongly hydrated headgroups of β-C12G2 by the small and charged ones of SDS.