Phase Behavior of Pentaglycerol Monostearic and Monooleic Acid Esters in Water

Abstract

The phase behavior in water of pentaglycerol monostearate (C18G5) and pentaglycerol monooleate (C18:1G5) surfactants has been studied as a function of temperature and surfactant weight fraction, W s . The equilibrium phases present at each composition and temperature studied were characterized by means of visual observation under normal and polarized light, differential scanning calorimetry (DSC), and X‐ray scattering, both at small (SAXS) and at wide angle (WAXS). In the temperature range 0–46°C, C18G5 presents a thermotropic α‐gel structure. However, at higher temperatures, the α‐gel phase melts and a lamellar liquid crystalline (Lα) phase is formed. The amount of water that can be solubilized by α‐gel and Lα was determined by plotting the interlayer distance, d, as a function of the reciprocal of W s . Water is soluble in the α‐gel phase up to 21 w/w% water concentration and in the Lα phase up to 30 w/w% water concentration. At higher water concentrations, excess water appears and a dispersion of α‐gel (α‐gel+W) and lamellar liquid crystal (Lα+W) in water is formed, respectively. In contrast, C18:1G5 is liquid in the whole range of temperatures studied (0–100°C). While at low temperatures, C18:1G5 presents a Lα structure, at about 63°C Lα melts and an isotropic liquid reverse micellar solution (Om) phase is formed. The amount of water that can be solubilized by both Om and Lα increases with temperature.