Abstract:

Lyotropic liq. crystals of glycerol monooleate (GMO) and water binary mixts. have been extensively studied and their resemblance to human membranes has intrigued many scientists. Biol. systems as well as food mixts. 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 (C2-C18) at various water/GMO compns., on phase transitions from lamellar or cubic to reverse hexagonal (L$\alpha$-HII and Q-HII). The ability of the triglycerides to promote the formation of an HII mesophase is chain length-dependent. It was found that TAG mols. with very short acyl chains (triacetin) can hydrate the head groups of the lipid and do not affect the crit. 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, temp. and electrolytes effects were examd. In the presence of short and medium chain triglycerides, increasing temp. caused a transition from lamellar or hexagonal to L2 phase (highest CPP value). However, in the presence of long chain TAGs, increasing temp. to ca. 40 °C caused a formation of HII mesophase. In addn., it was found that in tricaprylin/GMO/water systems, the increase in temp. caused a decrease in the lattice parameter. The effect of NaCl on the HII mesophase revealed interesting results. At low concn. of tricaprylin (5 wt%), the addn. of only 0.1 wt% of NaCl was sufficient to cause the formation of well-defined HII mesophase, while further addn. of electrolyte increased the hexagonal lattice parameters. At higher TAGs concns. (10 wt%), addn. of electrolyte resulted in the formation of HII with modifications of the lattice parameter. All the examd. effects were more pronounced with increasing water content. [on SciFinder(R)]