PRESSURE AND TEMPERATURE EFFECTS ON CONFORMATIONAL AND HYDRATIONAL PROPERTIES OF LAMELLAR AND BICONTINUOUS CUBIC PHASES OF THE FULLY HYDRATED MONOACYLGLYCERIDE MONOELAIDIN - A FOURIER-TRANSFORM INFRARED-SPECTROSCOPY STUDY USING THE DIAMOND-ANVIL TECHNIQUE

We report on the effects of pressure and temperature on the phase beha vior and structural properties of aqueous dispersions of monoelaidin ( ME) using Fourier transform-infrared spectroscopy (FT-IR) in combinati on with the diamond anvil technique. The IR spectral parameters, such as frequencies, intensities, band shapes, and band splittings, were us ed to detect structural and dynamic changes upon change of pressure an d temperature. Analysis of these spectral parameters yields informatio n on conformer population, reorientational fluctuations, interchain in teraction, hydrogen bonding, and phase transformations. The monoacygly ceride ME was chosen for investigation because it exhibits various mes omorphic liquid-crystalline phases of different dimensionality, includ ing lamellar, a cubic-primitive (Q(II)(D)), and a body-centered cubic phase (Q(II)(P)). The latter two are inverse bicontinuous cubic phases . We have established the excess water p, T-phase diagram of ME over t he 1-23-kbar range at similar to 15-95 degrees C. In the high-pressure region, enhanced interchain interaction leads to a correlation field splitting of the CH2 bending and rocking modes, which is expected when entering the lamellar crystalline L-c phase that has the smallest par tial molar volume. As revealed by the phase diagram, the energetic deg eneracy of the cubic phases is broken. With increasing temperature or decreasing pressure, the cubic phases Q(II)(P) and Q(II)(D) are formed . Interpretation of the CH2 stretching and wagging modes for evaluatio n of conformational states in the fluidlike disordered (L-alpha, Q(II) (P), and Q(II)(D)) phases reveals different populations of gauche conf ormers and kinks in these fluidlike phases. From the analysis of the c arbonyl stretching mode vibrations we have been also able to detect, s mall but marked differences in the level of hydration of different bic ontinuous cubic phases. Compared with the Q(II)(D) phase of ME, the li pid chains of the body-centered cubic phase Q(II)(P) seem to contain a slightly higher population of gauche sequences and a slightly lower l evel of hydration of the carbonyl group. The results are compared with recent energetic models for intercubic phase transitions.