Self-Assembly and Hydration-Dependent Structure and Dynamics of AOTSiC Reverse Micelles in Supercritical Carbon Dioxide

Supercritical carbon dioxide (scCO2) is an attractive green solvent, but its low polarity limits the solubilization of polar and ionic compounds, motivating the development of surfactants capable of forming reverse micelles. Here, reverse micelles formed by silicon-modified aerosol OT (AOTSiC) with sodium counterions in scCO2 are investigated over a wide hydration range, W0 = 1.7–20.0, using a multiscale computational approach. Density functional theory calculations were combined with classical molecular dynamics simulations to develop a modified all-atom OPLS-AA force field with scaled charges and adjusted Lennard–Jones parameters. The experimentally reported aggregation behavior at the reference hydration, W0 = 5.0, was used as a target during force-field development. At this hydration level, the simulations yield stable reverse micelles with a mean aggregation number of 6, consistent with the experimental aggregation number distribution. At low hydration (W0 = 1.7), discrete micelles persist, whereas increasing hydration leads to coacervation at W0 = 13.3. Analysis of the confined water reveals a progressive transition, with decreasing hydration, from a connected hydrogen-bond network to a fragmented, ion-dominated structure characterized by increasing local heterogeneity. Intermittent hydrogen-bond dynamics and molecular reorientation slow progressively under stronger confinement, whereas continuous hydrogen-bond lifetimes are only weakly affected. Spectral densities further indicate reduced cooperative intermolecular motions and increased localization of sodium-ion dynamics. These results provide molecular-level insight into hydration-controlled self-assembly and water confinement in AOTSiC reverse micelles in scCO2.