Hydrogen bonding and clusters in supercritical methanol–water mixture by neutron diffraction with H/D substitution combined with empirical potential structure refinement modelling

Abstract

ABSTRACTNeutron diffraction measurements of H/D isotopic substitution have been performed for seven H/D substituted methanol-water mixtures of 0.3 mol fraction of methanol (xM) under the supercritical (618 K, 100 MPa) and ambient (298 K, 0.1 MPa) conditions. The seven structure factors obtained were subjected to an empirical potential structure refinement (EPSR) modelling to derive all site-site pair correlation functions, coordination number distributions, spatial density functions, and cluster distributions. Water has a four coordinated structure in the first coordination shell under both ambient and supercritical conditions; however, the spatial density distribution of water molecules in the second coordination shell is delocalised under the supercritical condition. The mean coordination number of all atomic pairs with hydrophilic interactions decreases in the supercritical state. On the other hand, the mean coordination number of interactions between the hydrophobic part of methanol and water molecule is less sensitive to temperature. In the supercritical condition, water clusters with a wide size distribution are generated in a methanol-water mixture as well as in pure water. Since the critical temperature of a methanol-water mixture is lower than that of pure water, it can be concluded that the addition of methanol can generate fragment water clusters at a lower temperature.