Probing Water State during Lipidic Mesophases Phase Transitions

Abstract

We investigate the static and dynamic states of water network during the phase transitions from double gyroid ( $Ia\bar{3}d$ ) to double diamond ( $Pn\bar{3}m$ ) bicontinuous cubic phases and from the latter to the reverse hexagonal (H_II ) phase in monolinolein based lipidic mesophases by combining FTIR and broadband dielectric spectroscopy (BDS). In both cubic(s) and H_II phase, two dynamically different fractions of water are detected and attributed to bound and interstitial free water. The dynamics of the two water fractions are all slower than bulk water due to the hydrogen-bonds between water molecules and the lipid's polar headgroups and to nanoconfinement. Both FTIR and BDS results suggest that a larger fraction of water is hydrogen-bonded to the headgroup of lipids in the H_II phase at higher temperature than in the cubic phase at lower temperature via H-bonds, which is different from the common expectation that the number of H-bonds should decrease with increase of temperature. These findings are rationalized by considering the topological ratio of interface/volume of the two mesophases.

Keywords: dielectric spectroscopy; interfaces; lipidic mesophase; phase transitions; water dynamics.