Transitions induced by solubilized fat into reverse hexagonal mesophases

Lyotropic liquid crystals of glycerol monooleate (GMO) and water binary mixtures have been extensively studied and their resemblance to human membranes has intrigued many scientists. Biological systems as well as food mixtures are composed of lipids and fat components including triacylglycerols (TAGs, triglycerides) that can affect the nature of the assembly of the mesophase. The present study examines the effect of TAGs of different chain lengths (C ₂-C₁₈) at various water/GMO compositions, on phase transitions from lamellar or cubic to reverse hexagonal (L_α- H_II and Q-H_II). The ability of the triglycerides to promote the formation of an H_II mesophase is chain length-dependent. It was found that TAG molecules with very short acyl chains (triacetin) can hydrate the head groups of the lipid and do not affect the critical packing parameter (CPP) of the amphiphile; therefore, they do not affect the self-assembly of the GMO in water, and the mesophase remains lamellar or cubic. However, TAGs with medium chain fatty acids will solvate the tails of the lipid, and will affect the CPP of the GMO, and transform the lamellar or cubic phases into hexagonal mesophase. TAGs with long chain fatty acids are very bulky, not very miscible with the GMO, and therefore, kinetically are very slow to solvate the lipid tails of the amphiphile and are difficult to accommodate into the lipophilic parts of the GMO. Their effect on the transitions from a lamellar or cubic phase to hexagonal is detected only after months of equilibration. In order to enhance the effect of the TAG on the phase transitions in the GMO/triglyceride/water systems, temperature and electrolytes effects were examined. In the presence of short and medium chain triglycerides, increasing temperature caused a transition from lamellar or hexagonal to L₂ phase (highest CPP value). However, in the presence of long chain TAGs, increasing temperature to ca. 40°C caused a formation of H_II mesophase. In addition, it was found that in tricaprylin/GMO/water systems, the increase in temperature caused a decrease in the lattice parameter. The effect of NaCl on the H_II mesophase revealed interesting results. At low concentration of tricaprylin (5 wt%), the addition of only 0.1 wt% of NaCl was sufficient to cause the formation of well-defined H_II mesophase, while further addition of electrolyte increased the hexagonal lattice parameters. At higher TAGs concentrations (10 wt%), addition of electrolyte resulted in the formation of H_II with modifications of the lattice parameter. All the examined effects were more pronounced with increasing water content.