Monte Carlo computer simulations in the canonical and grand canonical statistical ensemble were employed to study the structural and thermodynamic properties of the central force model of water (CF1). We studied both bulk water and water in a crowded environment. The latter was modelled with a stationary configuration of Lennard-Jones particles with a given density, where the interaction of the adsorbent particles with the oxygen of the water was repulsive. The results for bulk water were also compared with the results of the Ornstein-Zernike integral equation with the HNC approximation and bridge functions based on a reference system of hard spheres.
Monte Carlo simulations describe the geometry of bulk water molecule well (the coordination numbers for OH and HH are 2 and 1, respectively), the OH bond distance is comparable to experiment, and the HOH angle is slightly smaller than the experimentally determined value for water in the vapour phase. The integral equation in the HNC approximation does not give satisfactory results for water coordination and does not describe the structure of the system well. With the inclusion of bridge functions, we can obtain good agreement with the results of simulations at a temperature of 300 K, but the functions are not transferable (at higher temperatures, the results are worse compared to the HNC approximation). We focused on hydrogen bonding, tetrahedral and translational order parameters, which we used to investigate the structural anomalies of the model. Defining a hydrogen bond based on the pair energy distribution function of water gives better results than a one-parameter definition based on distance. At 300 K, the average value of the number of hydrogen bonds per water was calculated to be about 3.8. It was observed that the model water at 300 K has a structural anomaly in the density range between 0.95 and 1.15 g/ml. With the help of the grand canonical Monte Carlo simulations, we were able to reproduce the density anomaly with a simple linear dependence of the chemical potential on temperature, and the dependence ρ(T) is in good agreement with experimental data. We have also studied the phase diagram of the model in the μ−ρ projection. At sufficiently low temperatures (below 400 K), the coexistence of two phases (gas-liquid) was observed.
In the second part, we studied the behaviour of model water in a disordered porous material (presence of Lennard-Jones-type obstacles). In the presence of the adsorbent, the intermolecular geometry of the water did not change. We found that the size of the adsorbent particles has a significant effect on other quantities (hydrogen bonding, order parameters) for a given fraction of the space occupied by adsorbent particles. When the adsorbent has a comparable particle size to water (σ = 2 ˚A), we did not observe a large change in the number of waters coordinated by hydrogen bonds. As the density of the adsorbent particles increased, the tetrahedral and translational order parameters decreased slightly. Larger adsorbent particles (σ = 5 ˚A) had a greater effect on CF1 water: compared to bulk water, the number of hydrogen bonds increased with the density of adsorbent particles, the tetrahedral order parameter decreased more sharply, however the translational order parameter exhibited non-monotonous behaviour. At high adsorbent density, the structurally anomalous region disappeared. We also investigated the effect of confinement on the μ − ρ projection of the equation of state. We found that the presence of the adsorbent displaces the water from the system and at a certain temperature for dense adsorbents with large particles the phase transition is no longer observed in contrast to pure water.
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