Hydration and thermodynamic equilibrium of non-ionic surfactant in solution

In this article we present a combined analysis of volumetric [1] and molecular dynamics (MD) [2] determinations of the interfacial properties of non-ionic surfactant aggregates belonging to the family of the oligooxyethylene glycol (CiEj). The volumetric experiments were made on surfactant having i=12 and j ranging from 5 to 8 EO unit and concern the density, and the sound velocity measurements on aqueous solutions of the surfactants. For concentrations up to about 40 wt.% the density of the solutions cross that of pure solvent at given temperatures. The presence of crossing temperatures Tρ allows us to describe the surfactant solutions in terms of an ideal mixture of pure solvent and an effective solute. On the ground of a simple hydration model for the micellar aggregate, we obtained the dependence of the degree of hydration and of the osmotic compressibility on the solute molar volumes and on the temperature. On the other hand, the compressibility embodies a term associated with a local osmotic exchange between the aggregates and the external solvent. To obtain a microscopic resolution of the interfacial properties, MD simulations were performed on a spherical C12E6 micelle. The dependence of the aggregate hydration on the temperature obtained by MD agrees with the results from density experiment. However, the MD clearly indicates that dehydration mainly occurs at the exposed hydrophobic area. Besides the MD results have shown that the micellar interface is characterized by significative interaction among the hydrophilic termination. These interactions leave a large hydrophobic area exposed to the solvent. Despite the complexity of the problem, in the context of our hydration model and within 40 wt.%, the properties of these solutions can be defined by their specific hydration numbers.