Phase behaviour of monoglyceride/water systems

Abstract

Studies on the gel and lamellar phases formed by C_16–18 saturated monoglyceride, C_16–18SMG (Hymono 8903, a commercial distilled monoglyceride) in contact with water are described. Optical microscopy was used to study the mesomorphic behaviour as a function of temperature. Differential scanning calorimetry was utilised to determine the transition enthalpies between the mesophases. Surfactant mobility was studied by measuring proton NMR T₂ relaxation times for samples comprising C_16–18SMG/²H₂O. Molecular ordering was also studied by measuring ²H and ¹⁷O quadrupole splittings (by NMR) for systems comprising C_16–18SMG/²H₂O and C_16–18SMG/H₂ ¹⁷O (10% H₂ ¹⁷O in H₂ ¹⁶O). Over the temperature range studied (0–90 °C), gel, lamellar and cubic mesophases were found. For the gel phase, separate ²H NMR spectra, representing the ²H₂O solvent and the monoglyceride O²H groups, were observed. Two signals were also observed for the lamellar phase. From the relative intensities, one signal was assigned to the O²H group on the terminal carbon atom of the monoglyceride molecule, whilst the other signal arose from the ²H₂O solvent and the other monoglyceride O²H group. Analysis of these results demonstrate that there are about three bound water molecules per surfactant molecule for the lamellar phase at maximum hydration. Unusually, the degree of ordering of the water in the gel phase is much smaller than that of the water in the lamellar phase.