Low temperature study of w/o microemulsions by differential scanning calorimetry and dielectric measurements

Abstract

The temperature dependence of the dielectric constant and loss tangent in parallel with differential scanning calorimetry (DSC) were used to study the low-temperature properties of a w/o microemulsion in the concentration interval (C, mass fraction) 0.024 ≤ C < 0.4, within the temperature range (20°C ÷ -150°C). For 0.222 < C < 0.4 the dielectric constant exhibits, with decreasing temperature, a sharp peak centred around - 35 °C. It is shown that the latter cannot be justified exclusively in terms of a Maxwell-Wagner interfacial polarization. By means of DSC analysis the temperatures of the different thermal transitions occurring in the w/o microemulsion were obtained; a differentiation between samples that possess a « free water » fraction (0.222 < C < 0.4) and those that do not (0.024 ≤ C ≤ 0.222) was made; the enthalpic change associated with the melting of the dodecane-oil (ΔHd) and the water (ΔHw), contained in the samples was determined against increasing concentration. An enthalpic contribution around - 10 °C due to water adsorbed at the interphase region was detected superimposed on that of the dodecane (- 9.6 °C). Polydispersed samples were identified by means of the analysis of the frequency dependence of the dielectric loss of «liquid» microemulsions at T = 20 °C. Samples exhibiting a time dependent behaviour were found in which, upon ageing, the free water Δ Hw tends to zero accompanied by the increase of the dodecane Δ Hd contribution. The phenomenon was interpreted in terms of the formation of 4 H2O-molecule hydration structures on the hydrophilic groups of the surface active molecules. The hydration process develops at constant surface tension at the expenses of the free water fraction of the dispersed phase. The energy balance of the latter process was investigated and the « surface increment » enthalpy associated with the formation of the 4 H2O-molecule hydrated structures was estimated.