Atomistic molecular dynamics simulations of CO₂ diffusivity in H₂O for a wide range of temperatures and pressures

Molecular dynamics simulations were employed for the calculation of diffusion coefficients of CO₂ in H₂O. Various combinations of existing force fields for H₂O (SPC, SPC/E, and TIP4P/2005) and CO₂ (EPM2 and TraPPE) were tested over a wide range of temperatures (283.15 K < T < 623.15 K) and pressures (0.1 MPa < P < 100.0 MPa). All force-field combinations qualitatively reproduce the trends of the experimental data; however, two specific combinations were found to be more accurate. In particular, at atmospheric pressure, the TIP4P/2005-EPM2 combination was found to perform better for temperatures lower than 323.15 K, while the SPC/E-TraPPE combination was found to perform better at higher temperatures. The pressure dependence of the diffusion coefficient of CO ₂ in H₂O at constant temperature is shown to be negligible at temperatures lower than 473.15 K, in good agreement with experiments. As temperature increases, the pressure effect becomes substantial. The phenomenon is driven primarily by the higher compressibility of liquid H₂O at near-critical conditions. Finally, a simple power-law-type phenomenological equation is proposed to correlate the simulation values; the proposed correlation should be useful for engineering calculations.