SIMULATIONS OF THE STRUCTURE AND THERMODYNAMIC PROPERTIES OF WATER AT HIGH-PRESSURES AND TEMPERATURES

Precise pressure-volume-temperature (PVT) data for water exist to pressures of only 8.9 kbar and temperatures of 900-degrees-C. Since aqueous fluids are important in many processes at much higher pressures and temperatures, there is a need to develop reliable extrapolations of the low pressures experimental data. In this paper we present the results of molecular dynamics simulations of water properties over a wide P-T range. It has been shown that at a density of 1.0 g/cm3m, the TIP4P intermolecular potential (Jorgensen et al., 1983) very accurately reproduces the available thermodynamic and structural properties of water at pressures of 1 bar to 10 kbar (Brodholt and Wood, 1990). This is extended here by testing TIP4P at temperatures to 2500 K and 350 kbar. The PVT predictions are compared to the experimental data of Burnham et al. (1969) at low pressures, and with the data of Kormer (1968) and Bridgman (1942) at higher- pressures. We conclude that there is good reason to be confident of the predictive powers of the TIP4P intermolecular potential in the high-pressure and high-temperature regime. Heat capacities and structural properties have also been calculated and compared to available data. High-pressure simulations have also been made with two other intermolecular potentials which accurately represent water properties at low pressures, SPC/E (Berendsen et al., 1987) and WK (Watanabe and Klein, 1989). SPC/E predicts PVT properties that are similar to those of TIP4P, while WK is accurate only at densities of about 1.0 g/cm3. At high pressures the spherical potential of Belonoshko and Saxena (1991) predicts volumes that are within 0.5 m of those predicted by TIP4P. A modified Redlich Kwong (MRK) type equation of state based on the molecular dynamics simulations is presented which can be used at pressures in excess of 10 kbar. This is used to predict the location of the brucite dehydration reaction which has recently been determined to 120 kbar (Johnson et al., 1991).