Thermal Analysis of Water-in-Oil Microemulsions

Abstract

In previous papers[1–4], we reported the study of the dielectric and calorimetric properties of a water/oil (w/o) microemulsion at increasing water concentration (C, mass fraction), by focusing attention mainly on the roles of the water and the interphase of the system. In particular, by Differential Scanning Calorimetry (DSC), the measurement of the enthalpy change (ΔH) associated with well identified thermal events, showed that w/o microemulsions are characterized by the presence of a “free water” fraction (AHw≠0 at 273 K), (Fig. 1). Furthermore, it was shown that, for very low water concentrations, (0.024≤C ≤0.222), the enthalpy change due to the melting of the dodecane present, (ΔH_d at 263 K), appeared as “more endothermic” than expected; as if a Iarger amount of oil than that really contained in the sample had melted! (Fig. 2). Finally, a careful analysis carried out on several samples in the range (0.222 <C<0.4), allowed us to detect two concentrations where time dependent phenomena were found to occur upon ageing of the samples. (Fig. 2). As a result in these samples, the free water fraction evaluated from the experimentally measured ΔH_w tends to zero, paralleled by an increase of the heat of melting of the dodecane up to a value much higher than that corresponding to the dodecane content of the sample. Since the latter behaviour was observed every time the enthalpic contribution ΔH_w was either absent or disappearing, the larger amount of heat absorbe by the more endothermic dodecane transition was interpreted in terms of the presence of another endothermic contribution occurring in the same temperature interval and ascribed to the fusion of “interphasal water” at 263 K.