A COMPUTER-SIMULATION STUDY OF THE LIQUID-VAPOR COEXISTENCE CURVE OF WATER

The liquid-vapor coexistence curve of a model water (the extended simple point charge model, SPCE) is evaluated by molecular dynamics simulation in the (N,V,E) ensemble. It is shown that the simulated system (N = 256 water molecules) is too small to present a spinodal decomposition and, hence, can be described by a classical equation of state whose the critical parameters (T(c) = 651.7 K, rho(c) = 0.326 g/cm3, and P(c) = 189 bar) are found to be very close to that of real water (T(c) = 647.13 K, rho(c) = 0.322 g/cm3, and P(c) = 220.55 bar). The critical parameters for SPCE water in the thermodynamic limit are deduced from the simulation data employing Wegner type expansions for the order parameter and the coexistence curve diameter; here also the values of the critical parameters (T(c) = 640 K, rho(c) = 0.29 g/cm and P(c) = 160 bar) are close to that of real water. The temperature dependence of the dielectric constant for water and steam at orthobaric densities is next evaluated between ambient and T(c); the agreement with the experimental data is quite remarkable (e.g., epsilon(SPCE) = 81.0 at 300 K and epsilon(SPCE) = 6. at T(c) instead of 78.0 and 5.3, respectively, in real water). The modifications experienced by water's architecture with the temperature are deduced from the evaluation of the atom-atom correlation functions. It is shown that a structural change occurs in the temperature range 423-473 K. This important reorganization is characterized by a shift of the second shell of neighbors from 4.5 to 5.5 A and the loss of almost all angular correlations beyond the first solvation shell. Moreover, it is observed that the average number of hydrogen bonds per molecule n(HB) scales with the density all along the saturation curve. In the same way the values of n(HB) for orthobaric densities seems to follow a law analogous to the law of rectilinear diameter for orthobaric densities.