EQUILIBRIUM PROPERTIES OF WATER AND ITS LIQUID-VAPOR INTERFACE

A previously formulated self-consistent field lattice theory for molecules with orientation-dependent interactions is applied to a lattice-gas model for water. The theory is a generalization of the quasi-chemical treatment and hence accounts for local correlations. The calculated equations of state and liquid-vapor phase diagrams agree at least qualitatively with the experimental behavior of water. The well-known anomalous behavior of water as reflected in the maximum of the isobaric density is reproduced by the theory. Within this model, it is possible to analyze the relation between this behavior and the hydrogen-bonded structure of water. Interfacial properties of water are also investigated. Although the calculated interfacial tension is lower than that experimentally found, also for this property, the trends are reproduced. The theoretical liquid-vapor interfacial tension shows a minimum in its temperature coefficient, as is found experimentally. At low temperatures, the calculated interfacial thickness and the density decay length of the fluid are an order of magnitude larger than for simple molecules, and a minimum as a function of temperature is predicted. To assert the effects of the orientation-dependent nature of the intermolecular interactions of water, results for a model with energetically isotropic molecules are also given and compared to those for the water model.