Molecular dynamics and self-diffusion in supercritical water

Abstract

Supercritical water (SCW) which exists beyond the critical point (Tc=647.2K, Pc=221bar) is an innovative solvent to dissolve organic material. Many applications of this new solvent such as oxidation of organic wastes and separation of metals have been researched; however, the properties of SCW have not sufficiently been understood due to the difficulty in experimental measurements at high temperatures and high pressures. Computer simulation is one of the best tools to predict and analyze the properties which are difficult to be experimentally obtained. The goals of this research is to gain a better understanding as to self-diffusivity of SCW, of which experimental data is limited, through molecular dynamic simulation. Extended Simple Point Charge (SPC/E) model reproduced the self-diffusion coefficient of water in the range of the temperature, 673-873K and the density, 0.125- 0.7g/cm3. In contrast, Simple Potential Charge (SPC) model was not relevant to calculate selfdiffusivity. In order to investigate self-diffusivity in the near critical region, the critical point of SPC/E model was calculated from direct simulation method of two coexisitng phases. Interpolating the simulated orthobaric densities by the scaling law ([beta]=0.325) approximation, Tc=616K and pc=0.296g/cm 3 were obtained. This value was explicitly lower than the expected critical point. By comparing reduced pressure/reduced density relationship from simulated data with that of experimental data, the critical point was revised to around 646K and 0.290g/cm3 which are very close to real water's data ( Tc=647K and rc=0.322g/cm3 ). Based on this obtained critical point, the self-diffusion coefficient of water in the near critical region was studied. The small drop of self-diffusion coefficient near critical point was observed from simulation results. The drop point was a little different from the critical point.