Numerical study of H₂, CH₄, CO, O₂ and CO₂ diffusion in water near the critical point with molecular dynamics simulation

Diffusion coefficient of H₂, CH₄, CO, O₂ and CO₂ in water near the critical point (600-670K, 250atm) is numerically investigated using Molecular Dynamics (MD) simulation. Main factors determining diffusion coefficient are discussed. Arrhenius behavior of temperature can be divided into two separate parts which are subcritical region and supercritical region. The activation energy has a huge difference between two regions. Diffusion coefficient has a negative power relation with density of water through logarithmic plot. Viscosity of water has effects on diffusion coefficient by a combination with temperature that term 1/Tη has a quadratic relation with diffusion coefficient. A new empirical equation to predict diffusion coefficient in water near the critical point is developed in which the effect of solute gas and solvent water is separated to the pre-factor A₀ and the second part F_w. A₀ is a unique constant for different solutes and F_w considers temperature, density and viscosity of water. It successfully predicts diffusion coefficient near the critical point for all solute gases and average absolute relative deviation is only 7.65%. Compared to other equations, our equation shows the best accuracy and simplicity for extension and modification.