Water behavior in nonionic surfactant systems I: Subzero temperature behavior of water in nonionic microemulsions studied by DSC

Subzero temperature differential scanning calorimetry (DSC) has been applied to a model nonionic microemulsion system, water/pentanol/dodecane/C₁₂(EO)₈, in order to determine the relative concentrations of bound and free water. On the basis of data thus acquired, conclusions are drawn relating to the low-temperature behavior of the systems studied, the surfactant hydration and the role of pentanol. It is shown that the surfactant becomes saturated with water at a ratio of three molecules of 'interphasal' water per ethylene oxide group. Free water can only be detected by DSC at higher water/surfactant ratios. Apparently nonfreezable water is not formed in the model system, at least not before the inversion from W/O to O/W microemulsion has occurred. The thickness of the bound water layer in the nonionic model system has been evaluated by several methods to be ca. 5 Å; i.e., two monolayers of interphasal water are closely associated with the surfactant. It is further demonstrated that although pentanol enhances water solubilization, and is present at the interface, its interaction with water or surfactant is not revealed by DSC.