Impact of Intermolecular Forces on Structural Changes and Local Density Fluctuations of CO₂ in Liquid and Supercritical Phases

This study uses molecular dynamics simulations to investigate the structural features of carbon dioxide (CO₂) in the liquid and supercritical phases at different isobars. Density fluctuations, which mark the liquid-to-gas-like crossover, were quantified using advanced statistical tools such as nearest-neighbor distance distributions, interaction energies, and local density profiles derived from Voronoi analysis and density-based spatial clustering of applications with noise. Our findings reveal that these fluctuations arise from the temperature-dependent difference in the spatial extent of attractive contributions from electrostatic (ES) and Lennard-Jones (LJ) potentials, leading to a maximum in the contrast between the packed and loose density domains. Specifically, we demonstrate that the first and second solvation shells are characterized by neighbors experiencing maximal LJ and minimal ES attractive contributions, respectively. Within these shells, the orientation of the CO₂ molecules is governed by the maximal attractive contribution of ES interactions.