A molecular-based theory for the thermodynamic properties of water

Following the rules set by the molecular theories of fluids, a perturbed form of the Helmholtz free energy for water has been developed. The reference term corresponds to short range water, and is approximated by the properties of a primitive model; the perturbation term is given by contributions of the dispersion forces and the dipole-dipole interaction. The method is first verified by applying it to TIP4P water and then used for real water without reference to any specific potential. The parameters of the model are determined in order to obtain the best representation of the vapour pressure and coexistence liquid densities from the triple point to 643.15K; no attempt is made to fit the critical region. Despite a number of approximations employed, the accuracy of the equation of state is comparable with that of the modified Redlich-Kwong-Soave equation and SAFT Yukawa-dipole-dipole equation, and considerably better than the accuracy of SAFT-HS and SAFT-VR equations. Because of its true molecular footing, the equation remains reliable also for various thermodynamic properties outside the coexistence region. It reproduces the anomaly in the isothermal compressibility, locating its minimum at T = 38 °C (versus the experimental value T = 46 °C) for P = 1 bar. It also predicts a density maximum, but outside the experimental temperature range (at temperatures below the triple-point temperature).