Microscopic origin of pressure-dependent diffusion in supercritical carbon dioxide from ab initio molecular dynamics

The pressure-dependent structural and diffusion properties of supercritical carbon dioxide (scCO2) were systematically investigated using ab initio molecular dynamics simulations. The results reveal two distinct molecular populations in scCO2: mobile isolated species and relatively rigid clustered counterparts. At low densities, the structural response of scCO2 to pressure is characterized by void region contraction, which reduces the ratio of mobile isolated to rigid clustered molecules and thereby limits molecular mobility. At high densities, however, void regions are largely exhausted, and the local packing of CO2 molecules is, therefore, strengthened. Considering the validity of the Stokes-Einstein relation in this regime, the reduced diffusion is attributed to the increased effective friction arising from strong intermolecular interactions and enhanced momentum transfer. Notably, the structural and diffusion crossovers coincide around a density of 0.8 g/cm3, suggesting clear structure-diffusion coupling. The predicted crossover falls within the reported Frenkel line range. In addition, a CO2-optimized hard-sphere model was developed to quantitatively describe and predict the self-diffusion coefficient of scCO2. Overall, this work provides deeper insight into scCO2 at the microscopic level. The findings facilitate future research on identifying suitable operating conditions in carbon capture, utilization, and storage processes and improving the application of scCO2 solvent in industry.