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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: The temperature and NaCl concentration dependence of the size of mixed micelles of n-octyl-β-d-thioglucoside (OTG) and Triton X-100 (TX100), two non-ionic surfactants belonging to the alkyl glucosides and polyoxyethylene alkyl ether families, respectively, has been investigated using the dynamic light scattering technique. Micellar size increases with temperature, with this micellar growth occurring faster when the temperature approaches the cloud point temperature. The extent of this growth is less pronounced at higher relative proportions of OTG in solution. In addition, the cloud point temperature was investigated for aqueous and NaCl solutions of OTG and Triton X-100 mixtures. The experimental results can be explained assuming the clouding process to be a liquid–liquid phase separation which takes place as a result of reduced hydration of the head group in the surfactant molecules caused by the temperature rise. The thermodynamic parameters of the clouding phenomenon have been calculated using the phase-separation model and considering the cloud point to be the threshold temperature for solubility. This study found that the enthalpy–entropy compensation relationship holds for this process. [Copyright &y& Elsevier]

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]

Abstract: Binary mixtures of the sugar-based surfactant N-decanoyl-N-methylglucamide (MEGA-10) with the non-ionic octaethylene glycol monododecyl ether (C12E8) 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 C12E8 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 C12E8 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 C12E8 content, suggesting a faster rotational reorientation of the probe due to a less tight microenvironment. [Copyright &y& Elsevier]

Abstract: The clouding behavior of two non-ionic mixed surfactant systems (n-dodecyl-β-d-maltoside/n-dodecyl-hexaethylene-glycol and n-decyl-β-d-maltoside/n-dodecyl-hexaethylene-glycol) was investigated over the entire composition range in pure water and in 1M NaCl. The evolution of micellar size as a function of temperature was monitored by dynamic light scattering and a unimodal distribution pattern of aggregates consistent with a single scattering species was found. Our results confirm that an increase of temperature induces micellar growth and that clustering of micelles occurs as the temperature approaches the cloud point. It was observed that, with increasing temperature, the higher the n-dodecyl-hexaethylene-glycol content the greater the micellar growth. The presence of sugar surfactant monomers in the mixed micelle enhances the cloud point of the system. This phase behavior is sensitive to the presence of NaCl in the micellar solution which induces cloud point depression. For a certain system composition, the cloud point decreases as the alkyl chain length of the sugar-based surfactant increases. Cloud-point thermodynamics were evaluated assuming that clouding indicates the solubility limit when phase separation occurs. The micelle–solvent interactions were characterized from the compensation temperatures, which were determined from the enthalpy–entropy plots. [Copyright &y& Elsevier]