Pressure and Temperature Effects on Conformational and Hydrational Properties of Lamellar and Bicontinuous Cubic Phases of the Fully Hydrated Monoacylglyceride MonoelaidinA Fourier Transform-Infrared Spectroscopy Study Using the Diamond Anvil Technique

Abstract

We report on the effects of pressure and temperature on the phase behavior and structural properties of aqueous dispersions of monoelaidin (ME) using Fourier transform−infrared spectroscopy (FT-IR) in combination with the diamond anvil technique. The IR spectral parameters, such as frequencies, intensities, band shapes, and band splittings, were used to detect structural and dynamic changes upon change of pressure and temperature. Analysis of these spectral parameters yields information on conformer population, reorientational fluctuations, interchain interaction, hydrogen bonding, and phase transformations. The monoacyglyceride ME was chosen for investigation because it exhibits various mesomorphic liquid−crystalline phases of different dimensionality, including lamellar, a cubic-primitive (QIID), and a body-centered cubic phase (QIIP). The latter two are inverse bicontinuous cubic phases. We have established the excess water p,T-phase diagram of ME over the 1−23-kbar range at ∼15−95 °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 Lc phase that has the smallest partial molar volume. As revealed by the phase diagram, the energetic degeneracy of the cubic phases is broken. With increasing temperature or decreasing pressure, the cubic phases QIIP and QIID are formed. Interpretation of the CH2 stretching and wagging modes for evaluation of conformational states in the fluidlike disordered (Lα, QIIP, and QIID) phases reveals different populations of gauche conformers and kinks in these fluidlike phases. From the analysis of the carbonyl stretching mode vibrations we have been also able to detect small but marked differences in the level of hydration of different bicontinuous cubic phases. Compared with the QIID phase of ME, the lipid chains of the body-centered cubic phase QIIP seem to contain a slightly higher population of gauche sequences and a slightly lower level of hydration of the carbonyl group. The results are compared with recent energetic models for intercubic phase transitions.