The ascorbyl palmitate-water system: Phase diagram and state of water

Abstract

This work addresses the ascorbyl palmitate (Asc16) aqueous systems. It has been studied through differential scanning calorimetry (DSC) and polarizing microscopy (PM) at different temperatures over a wide range of concentrations (weight fractions (w/w) between 0.05 and 1.0), allowing the determination of a complete phase diagram. When possible, the findings were corroborated with literature data. Systems below C ≈ 0.48 wt fraction show hydrated crystals in an isotropic liquid, which lead to a lamellar liquid crystal when heated to about 60 °C. Above this concentration, the phase transition occurs at about 80 °C, giving a cubic liquid crystal which in turn becomes lamellar liquid crystal at about 90 °C. The texture of this lamellar mesophase is different to that produced at lower concentrations. It has been determined, through DSC analysis of the melting peaks of water, computer simulation of the hydration of Asc16 molecules and a model of the hydrated aggregates (hydrated crystals and lamellar mesophase), that low concentration liquid crystals exist provided bulk water is present in the system. Two other kinds of water but bulk were detected: a first hydration shell, with water molecules strongly attached by hydrogen bonds to the oxygen and hydrogen atoms of the polar headgroup, which does not melt and is then undetectable by DSC (11.47 ± 0.95 water molecules per surfactant molecule) and a second hydration layer containing 59 ± 17 water molecules per surfactant one (apart from those in the first hydration layer). The number of water molecules in the second hydration layer decreases as the surfactant concentration increases, and vanishes at C = 0.62 ± 0.01 wt fraction (computer simulation and structure model data) or at C = 0.667 ± 0.007 (DSC). At very high concentrations the formation of mesophases is preceded by the melting of the chains, giving a waxy texture when viewed at the polarizing microscope, maintaining the structure of crystals because the anhydrous polar bilayer remains “solid”.