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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: 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]