Molecular dynamics study on the transport and structural behaviors of supercritical water–organic mixtures under nanoscale confinement

Molecular dynamics simulations were performed to systematically investigate the transport behavior and microscopic structural characteristics of supercritical water–organic binary mixtures (alkane or aromatic) confined within carbon nanotubes. The results indicate that the molecular structure of the solute plays a crucial role in regulating the diffusion properties of the confined fluid. Compared with alkanes, aromatic solutes significantly reduce the diffusion coefficients of both solute and solvent components, and this inhibitory effect becomes more pronounced with increasing the number of aromatic rings. Energy and spatial distribution analyses reveal that, owing to the π-conjugated structure of aromatics, strong π–π interactions occur between aromatic rings and the carbon nanotube wall as well as among solute molecules, leading to preferential adsorption and cluster formation near the tube wall, which markedly restricts molecular mobility. Furthermore, increasing temperature effectively weakens the adsorption between aromatic molecules and carbon nanotube surfaces, thereby promoting the desorption and diffusion of organics from the confined region. This study elucidates, at the molecular level, the structural rearrangement and dynamic regulation mechanisms of SCW–organic systems under nanoscale confinement, providing theoretical insight for optimizing supercritical water gasification and related energy conversion processes.