Monte Carlo simulation of H₂O-CO₂ mixtures to 1073.15 K and 30 kbar

The thermodynamic and structural properties of H₂O-CO₂ mixtures along two supercritical isotherms at 773.15 and 1073.15 K in the pressure range from 0.2 to 30 kbar have been investigated by the NVT and NPT Monte Carlo method using the TIP4P and MSM3 effective intermolecular potentials for H₂O and CO₂, respectively. Calculated excess volumes of mixing are in very good agreement with experimental data at low pressure. Results suggest that non-ideal mixing still occurs at 1073.15 K and at pressures up to 7 kbar. Although structural analysis shows that there is some clustering of H₂O molecules in the mixtures relative to the pure fluids, this does not appear to correlate with excess volumes. The most pronounced clustering occurs at high pressure and low temperature while the largest excess volumes occur at low pressure and low temperature. An equation is given to calculate excess volumes of mixing at different pressures and compositions along both isotherms. It has the following mathematical expression: V_ex = [a₁X_CO2(1 - X_CO2) + b₁ X_CO2 1n(X_CO2)] (a + bP + c√P + d/P + e/P² + fP²) and reproduces calculated V_ex with a standard deviation of 0.8 and 0.4 cm³/mol at 773.15 and 1073.15 K, respectively.