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The effect of the micellar composition of mixed micelles of n -dodecyl-β- d -maltoside (β-C 12 G 2 ) 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-C 12 G 2 by the small and charged ones of SDS. [ABSTRACT FROM AUTHOR]

Abstract: Mixed micelles of n-octyl-β-d-thioglucoside (OTG) and octaethylene–glycol monododecyl ether (C12E8), two non-ionic surfactants belonging to the alkyl glucosides and polyoxyethylene alkyl ether families, respectively, were investigated by using light scattering and fluorescence probe techniques. From the determination of the critical micelle concentration (cmc), by the well-established pyrene 1:3 ratio method, it was found that the mixed system behaves ideally, the micellization process being clearly controlled by the ethoxylated surfactant. The micellar hydrodynamic radius as a function of temperature, composition and concentration was obtained by dynamic light scattering measurements. It was observed that the micellar size increases with temperature, this growth being more pronounced as the relative proportion of the ethoxylated surfactant was increased. The behavior of the micellar size with the total surfactant concentration was also found to be dependent on temperature and composition. The clouding temperature, characteristic of the ethoxylated surfactants, was increased with the addition of the sugar surfactant. Lastly, possible structural changes in the micellar palisade layer were examined by steady-state fluorescence anisotropy in conjunction with time-resolved fluorescence studies with the hydrophobic probe coumarin 6 (C6). The obtained results indicate that the participation of the ethoxylated surfactant induces a slightly more polar palisade layer, whereas the probe carries out a faster rotational reorientation as a result of a less compact environment. All these observations were attributed to the different structure of the head groups of both surfactants and, as a consequence, to their different hydration. [Copyright &y& Elsevier]