Light scattering and fluorescence studies of non-ionic surfactant binary mixtures formed by MEGA-10 and C 12E 8

Abstract

Binary mixtures of the sugar-based surfactant N-decanoyl- N-methylglucamide (MEGA-10) with the non-ionic octaethylene glycol monododecyl ether (C 12E 8) were investigated at different bulk mole fractions covering the whole composition range. The pyrene 1:3 ratio method was used to determine the critical micelle concentration. It was found that the mixed system essentially behaves ideally. Static and dynamic light scattering were utilized to obtain the apparent molecular weight and hydrodynamic radius, respectively, of mixed micelles as a function of the system composition and temperature. It was found that the hydrodynamic radius of micelles in mixtures rich in C 12E 8 increases with temperature as does this component in the mixed system. However, it was observed that micelles do not grow in size with temperature for mixed systems poor in the ethoxylated component. This effect is concomitant with the clouding phenomena, which only appears in the mixed systems with high mole fraction of C 12E 8 in the bulk. This behavior was attributed to the different hydration status of sugar moieties in relation to the polyoxyethylene head groups. On the other hand, fluorescence studies provided microenvironment information of mixed micelles. Steady-state fluorescence anisotropy in connection with time-resolved fluorescence studies of the neutral probe coumarin 6 (C6) solubilized in micelles allowed for the determination of the apparent rotational correlation time ( τ c) of the probe as a function of the composition of the mixed system. Our results indicated that τ c decreases with the C 12E 8 content, suggesting a faster rotational reorientation of the probe due to a less tight microenvironment.