Thermodynamic and structural studies of Triton X-100 micelles in ethylene glycol-water mixed solvents

Micellar properties of p-tert-octyl-phenoxy (9.5) polyethylene ether (Trito n X-100) in aqueous mixtures of ethylene glycol (EG) were determined using such techniques as surface tension, static and dynamic light scattering, an d fluorescence spectroscopy. Thermodynamics of micellization was obtained f rom the temperature dependence of critical micelle concentration values. Th e differences in the Gibbs energies of micellization of Triton X-100 betwee n water and binary solvent systems were calculated to evaluate the influenc e of cosolvent on the micellization process. From this study, it can be con cluded that the structure-breaking ability of EG and its interaction with t he oxyethylene groups of the surfactant are dominating factors in the micel lization process. Thermodynamics of adsorption of the solution-air interfac e was also evaluated. It was found that the surface activity of the surfact ant decreases slightly with increasing concentration of EG at a given tempe rature. By a combination of static and dynamic light scattering measurement s, a reduction of the micelle size was observed, mainly due to a decrease o f the micellar aggregation number, whereas the micellar solvation was not s ubstantially modified in magnitude with EG addition. However, the change of the surface area per headgroup of the surfactant suggested an alteration i n the nature of its solvation layer, produced probably by a certain partici pation of cosolvent in the micellar solvation layer. This point was corrobo rated from the fluorescence polarization studies of several luminescent pro bes, including coumarin 6, merocyanine 540, and rhodamine B. These experime nts revealed a slight increase of the micellar microviscosity. Finally, the proposed mechanism was also supported by the increase observed in the clou d point of Triton X-100, induced by the EG addition.