Aqueous Phase Behavior of Diglycerol Fatty Acid Esters

Abstract

We have studied the phase behavior of homologous series of diglycerol fatty acid esters (Qn‐D, for n=10, 12, 14, and 16, where n represents the carbon number in the alkyl chain length of amphiphile) in aqueous solution as a function of temperature and surfactant concentration. The different equilibrium phases present over a wide range of composition and temperature studied were characterized by means of visual observation under normal and polarized light, and x‐ray scattering techniques at small (SAXS) and wide angle (WAXS) regions. In diglycerol monocaprate (Q10‐D) and diglycerol monolaurate (Q12‐D)/H2O systems, lamellar liquid crystal (Lα) phase is present in the surfactant rich region and it swallows an appreciable amount of water. The amount of water swallowed by the Lα phase was determined by plotting the interlayer spacing, d, as a function of reciprocal of the surfactant weight fraction Ws . In the dilute regions, dispersion of Lα phase in water is observed over a wide range of temperature. At higher temperatures, the Lα phase melts to isotropic two‐liquid phases in water rich region whereas to isotropic reverse micellar solution (Om) in surfactant rich region. The Lα‐Om transition temperature is increased on increasing the hydrocarbon chain length of the surfactant from Q10‐D to Q12‐D. There is surfactant solid phase in equilibrium with water up to 25°C in diglycerol monomyristate (Q14‐D)/H2O system and the solid phase could solubilize 25 wt% water. The melting temperature of solid phase is practically constant in a wide range of compositions. Both the solid present region and the extent of water solubilization are increased in diglycerol monopalmitate (Q16‐D)/H2O system. At lower surfactant concentrations, excess water appears and dispersion of solid in water is formed. The structure of the solid is identified by WAXS measurement and it is confirmed to α‐solid. Normal vesicular aggregates are formed in Lα+W regions in the Q14‐D/H2O system at 25°C.