Properties of pure water and sodium chloride solutions at high temperatures and pressures: a simulation study

The structural and thermodynamic properties of pure water and sodium chloride solutions at high temperatures and pressures are studied by using molecular dynamics simulations and quantum molecular simulations. Properties are calculated as functions of temperature and pressure. The results show that the structure of pure water becomes looser as temperature increases from 298 to 400K, with the collapse of the traditional tetrahedral structure. For sodium chloride solutions, the same collapse is also found with the increased concentration. Moreover, a critical point for the diffusion coefficient of [GRAPHICS] and [GRAPHICS] is evident when the temperature increases linearly under 100atm. At the critical points, the ions are abnormally clustered, due to the weakly hydrogen-bonded water molecules. The cluster size of the ions is synchronous with the corresponding diffusion coefficients.