Structure of water confined in bicontinuous cubic lipid-water mesophases

The first wide-angle neutron diffraction studies of the structure of water confined in bicontinuous cubic lipid-water mesophases are presented. By applying H-D substitution to the water component, the partial pair correlation functions g(HH)(r), g(XH)(r) and g(XX)(r) could be calculated from the measured total structure factors, where H and X denote the substituted and non-substituted atoms of the samples, respectively. Experiments were performed for two systems differing in lipid configuration, water fraction and temperature: monoolein-water (35% w/w, 35 degrees C) and monoelaidin-water (40% w/w, 50 degrees C). The conditions of hydration and temperature were chosen in order to obtain the cubic Q(II)(G) phase in both systems and to tune the water channel thickness. There is no evidence to suggest that the nature of the inter-bilayer water is substantially altered by interfacial interactions or to support the concept of structured water with correlations extending up to several tens of angstroms. Small but pronounced modifications of the HH and XH pair correlation functions are observed for both lipid systems as compared with bulk water. These modifications may be interpreted as a slight increase in the intermolecular distances in water due to confinement.