Single-Site Local-Density Potentials for the Mesoscopic Representation of Water Based on the SAFT-VR Mie Equation of State.

Abstract

In this article, we present three mesoscopic models for water. All three models make use of local density-dependent interaction potentials, as employed within the Pagonabarraga-Frenkel framework [Pagonabarraga, I.; Frenkel, D. J. Chem. Phys. 2001, 115, 5015-5026]. The forms of these three interaction potentials are based on the free energy function of the SAFT-VR Mie equation of state (EoS) [Lafitte, T. J. Chem. Phys. 2013, 139, 154504]. Two of these models represent the water-water interaction as a spherically symmetric Mie interaction with temperature-dependent parameters, while the third model works with a temperature-independent Mie potential, but then explicitly models the effect of hydrogen bonding. All three models provide good predictions of the vapor-liquid equilibrium of water over a wide temperature range. They also give accurate predictions of the isothermal compressibility for both sub- and supercritical conditions. To model the interfacial tension of the vapor-liquid interface with our mesoscale simulations, we added a square-gradient term to our potential energy function. We show that the addition of this term has a minimal effect on the bulk properties of water. However, by parametrizing the coefficient of this term as a function of temperature, all three models again provide excellent predictions of water's interfacial tension over a wide temperature range. Of the three models, our preference is for the model that includes an association term, as this model can operate successfully over a wider range of conditions.