Fourier-transform infrared spectroscopy study of dioleoylphosphatidylcholine and monooleoylglycerol in lamellar and cubic liquid crystals.

The liquid-crystalline phases of the systems monooleoylglycerol (MO)/water, dioleoylphosphatidylcholine (DOPC)/water, and MO/DOPC/water have been studied by Fourier-transform infrared (FTIR) spectroscopy. In the latter ternary system, the sn-3 OH group of MO competes with water to interact with the polar head group of DOPC, and an intramolecular hydrogen bonding of MO is broken up. The hydration of the ester carbonyl groups in the lamellar phases of both the MO/water and DOPC/water systems increases with increasing water content. Similarly, the addition of small amounts either of MO to a DOPC/water system or of DOPC to an MO/water system also results in an increase in the hydration of the ester carbonyl groups. This leads to an unfavorable hydrocarbon-water contact which is counteracted by the formation of a cubic phase, except for the DOPC/water system, where the lamellar phase is stable also at the highest water concentrations. The phase behavior of the different systems can be described in terms of lipid monolayer curvature and molecular packing in the lipid aggregates. Finally, it is shown by the water association band in the FTIR spectrum that the water hydrogen bonding is considerably different in the liquid-crystalline phases than in bulk water.