Molecular dynamics simulation of infinite dilution diffusivity of carbon dioxide in supercritical water

We apply equilibrium molecular dynamic simulations for investigating the infinite dilution diffusivity of carbon dioxide in near- and supercritical water (SCW) over the wide temperature ranging from 647 to 973 K at 115, 217, 426 and 663 kg/m3. The diffusion coefficients of SCW and infinite dilution diffusion coefficients (IDDCs) of carbon dioxide are calculated by using the Einstein mean-square displacement method. It is observed that the IDDCs of carbon dioxide in SCW are approximately 1–2 orders of magnitude higher than that in normal water. The analysis of the temperature dependence of the IDDCs of carbon dioxide reveals that they do not conform to the Arrhenius equation. In the near critical region, the IDDCs of carbon dioxide do not exhibit a monotonic change with a variation in temperature, but increase gradually with an increase in the temperature in a region far from the critical state. The anomaly of the temperature dependence of the diffusion of carbon dioxide in SCW can be explained using the microstructure features of the different states of the water. In addition, we observe that the IDDCs of oxygen are slightly larger than those of carbon dioxide under the same conditions.