An interstitial model for fluid water

Abstract

As in the usual interstitial model, disorder is introduced by the displacement of molecules from a bonded ice-like framework onto interstitial sites. However, in this model the interstitial sites are distributed in such a way that its properties can be derived from those of an Ising model on the framework sites. Frameworks considered are the three-dimensional diamond lattice and the two-dimensional honeycomb and square lattices. Accurate liquid/vapour phase diagrams are obtained, density maxima and compressibility minima being found on the liquid branch for appropriate ratios of the non-bonding interaction energy e to the bonding energy w. Results are compared with those of the zeroth-order and first-order approximations and isobars are calculated using the latter. For the range of e/w giving best qualitative agreement between the three-dimensional model and experiment, the first-order and accurate results are very close in the part of the state surface where anomalous water-like properties occur.