Effect of Ethanol Concentration on AOT Clustering in Supercritical CO ₂ : A Molecular Dynamics Study

CO 2 foam flooding has emerged as a promising enhanced oil recovery (EOR) strategy for shale/tight reservoirs, but the intrinsically low solubility of surfactants in supercritical CO 2 (scCO 2 ) remains a major barrier to practical application. While experiments have shown that ethanol can improve surfactant solubility, the molecular mechanisms remain unclear. Here, molecular dynamics simulations were performed to investigate the effect of ethanol concentration (5–20 wt %) on the aggregation of AOT in scCO 2 at reservoir-relevant conditions (333 K, 200 bar). Across all systems, AOT assembled into clusters within 600 ns, but increasing the ethanol concentration produced smaller and more dispersed clusters, indicating enhanced solubilization. Structural analyses identified two key drivers of this behavior: (i) stronger solvation of Na + ions by ethanol and (ii) increased hydrogen bonding between ethanol and AOT headgroups, both of which suppress AOT–AOT association. Potential of mean force calculations further revealed that interactions between clusters become progressively less favorable with increasing ethanol content, confirming ethanol’s dispersive role. These molecular-level insights establish the mechanistic basis by which ethanol improves surfactant solubility in scCO 2, providing theoretical guidance for optimizing surfactant- co -solvent formulations to enhance foam stability and injectivity in low-permeability reservoirs.