No abstract

The test particle method is used to evaluate by molecular dynamics calculations the solubility of rare gases and of methane in water between the freezing point and the critical point. A quantitative agreement is obtained between solubility data and simulation results when the simulated water is modeled by the extended simple point charge model (SPCE). From a thermodynamical point of view, it is shown that the hierarchy of rare gases solubilities in water is governed by the solute–water interaction energy while an entropic term of cavity formation is found to be responsible for the peculiar temperature dependence of the solubility along the coexistence curve, and more precisely, of the solubility minimum exhibited by all the investigated solutes. Near the water critical point, the asymptotic behaviors of the Henry’s constant and of the vapor–liquid partition coefficient, respectively, as deduced from the simulation data follow with a good accuracy the critical laws recently proposed in the literature for these quantities. Moreover, the calculated partial molar volume of the solute shows a steep increase above 473 K and becomes proportional to the isothermal compressibility of the pure solvent in the vicinity of the critical point as it is observed experimentally. From a microscopic point of view, the evaluation of the solute–solvent pair distribution functions permits to establish a relationship between the increase of the solubility with the decrease of the temperature in cold water on the one hand, and the formation of cages of the clathrate-type around the solute on the other hand. Nevertheless, as soon as the boiling point of water is reached the computer simulation shows that the water molecules of the first hydration shell are no longer oriented tangentially to the solute and tend to reorientate towards the bulk. At higher temperatures a deficit of water molecules progressively appears around the solute, a deficit which is directly associated with an increase of the partial molar volume. Although this phenomenon could be related to what is observed in supercritical mixtures it is emphasized that no long range critical fluctuation is present in the simulated sample.

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 (Tc=651.7 K, ρc=0.326 g/cm3, and Pc=189 bar) are found to be very close to that of real water (Tc=647.13 K, ρc=0.322 g/cm3, and Pc=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 (Tc=640 K, ρc=0.29 g/cm3, and Pc=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 Tc; the agreement with the experimental data is quite remarkable (e.g., εSPCE=81.0 at 300 K and εSPCE=6. at Tc 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 nHB scales with the density all along the saturation curve. In the same way the values of nHB for orthobaric densities seems to follow a law analogous to the law of rectilinear diameter for orthobaric densities.