Water exhibits many unusual properties that are essential for the existence of life. Water completely changes its character from ambient to supercritical conditions in a way that makes it possible to sustain life at extreme conditions, leading to conjectures that life may have originated in deep-sea vents. Molecular simulation can be very useful in exploring biological and chemical systems, particularly at extreme conditions for which experiments are either difficult or impossible; however this scenario entails an accurate molecular model for water applicable over a wide range of state conditions. Here, we present a Gaussian charge polarizable model (GCPM) based on the model developed earlier by Chialvo and Cummings [Fluid Phase Equilib. 150, 73 (1998)] which is, to our knowledge, the first that satisfies the water monomer and dimer properties, and simultaneously yields very accurate predictions of dielectric, structural, vapor-liquid equilibria, and transport properties, over the entire fluid range. This model would be appropriate for simulating biological and chemical systems at both ambient and extreme conditions. The particularity of the GCPM model is the use of Gaussian distributions instead of points to represent the partial charges on the water molecules. These charge distributions combined with a dipole polarizability and a Buckingham exp-6 potential are found to play a crucial role for the successful and simultaneous predictions of a variety of water properties. This work not only aims at presenting an accurate model for water, but also at proposing strategies to develop classical accurate models for the predictions of structural, dynamic, and thermodynamic properties.

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A thermodynamic approach is proposed to determine the dissociation conditions of salt hydrates and semiclathrate hydrates. The thermodynamic properties of the liquid phase are described with the SAFT-VRE equation of state, and the solid-liquid equilibria are solved by applying the Gibbs energy minimization criterion under stoichiometric constraints. The methodology is applied to water + halide salt systems, and an excellent description of the solid-liquid coexistence curves is obtained. The approach is extended to the water + tetra-n-butylammonium bromide (TBAB) binary mixture, and an accurate representation of the solid-liquid coexistence curves and dissociation enthalpies is obtained. The van der Waals-Platteeuw (vdW-P) theory combined with the new model for salt hydrates is used to determine the dissociation temperatures of semiclathrate hydrates of TBAB + carbon dioxide. A good description of the dissociation pressures of CO(2) semiclathrate hydrates is obtained over wide temperature, pressure, and TBAB composition ranges (AAD = 10.5%). For high TBAB weight fractions the new model predicts a change of hydrate structure from type A to type B as the partial pressure of CO(2) is increased. The model can also capture a change of behavior with respect to TBAB concentration, which has been observed experimentally: an increase of the TBAB weight fraction leads to a stabilization of the gas semiclathrate hydrate at low initial TBAB concentrations below the stoichiometric composition but leads to a destabilization of the hydrate at TBAB concentrations above the stoichiometric composition.

The statistical associating fluid theory for potentials of variable range SAFT-VR [Gil-Villegas, A.; et al. J. Chem. Phys. 1997, 106, 4168] is used to examine the fluid-phase behavior of mixtures of n-alkanes, alk-1-enes (R-olefins), and nitrogen with polyethylene. The molecules are modeled as flexible chains of tangent spherical segments, with segment-segment dispersive interactions treated via square-well potentials. The parameters of the polyethylene polymer are determined from those of the n-alkanes by using simple extrapolations with molecular weight. As a test of the extrapolated parameters for polyethylene, the absorption (vapor-liquid equilibria) and cloud curves (liquid-liquid equilibria) of n-pentane-polyethylene systems are predicted without adjustable binary parameters, and the effect of the polymer parameters on the phase behavior of the mixture is discussed. The liquid-liquid immiscibility curve is found to be very sensitive to the intermolecular potential parameters used to describe the polymer. The change in the lower critical solution temperature (LCST) for mixtures of n-alkanes of increasing chain length in polyethylene is also predicted. Good agreement with experimental data is also obtained for the absorption of small gases in amorphous polyethylene. The predictions of the SAFT-VR approach confirm experimental findings that the solubility of nitrogen increases with temperature while the solubility of heavier molecules such as ethene and but-1-ene decreases. Strong synergies can be observed when several gases absorb in the same polyethylene polymer. The co-absorption effects are explained in terms of the interactions between gas and polyethylene molecules. When the temperature of interest is below the melting temperature of polyethylene, the polymer exists in a semicrystalline state. In this case, the crystallinity of polyethylene has to be taken into account in order to predict the solubility of the various gases. We present an accurate theory to predict the crystallinity of polyethylene as a function of temperature. The approach, which is based on Flory's theory of copolymer crystallinity [Flory, P. J. Trans. Faraday Soc. 1955, 51, 848], is accurate for a large variety of polyethylene samples, and requires only the experimental crystallinity or polymer density at 25 °C as an input parameter. On combining the Flory and SAFT-VR approaches, we can predict the solubility of various gases in semicrystalline polyethylene samples, by assuming that the molecules of gas only absorb in the amorphous regions of the polymer.