Effect of Alcohol Tail Length on Aggregate Behavior of Alcohol and AOT at the Water-scCO₂ Interface: MD Simulation Study

Alcohols are originally introduced to scCO₂ foam flooding as a cosolvent to increase the surfactant solubility in scCO₂. Other than increasing surfactant solubility, alcohols can also distribute at the interface region and further influence the water/scCO₂ (foam interface) interfacial properties. In this report, we use the molecular dynamics (MD) simulation to study the alcohol effect on the foam interface properties and their partitioning in various phases. Alcohols with varying tail lengths (C₂OH-C₁₆OH) under a wide range of concentrations are introduced to water/AOT/scCO₂ interface systems to study their effects. Temperature and pressure are set as a typical reservoir condition (333 K and 200 bar). We find that alcohols can distribute in water, interface region, and scCO₂ phases, and their partitioning in various phases is dependent on the alcohol tail length. Alcohol tail length has a negligible effect on its distributions at the interface under the same concentration in scCO₂ (). On the other hand, the alcohol concentration in the water phase () increases as tail length decrease. The ability to reduce interfacial tension (IFT) is similar for various alcohols when is relatively low (before reaching the inflection point). Longer chain alcohols reach the inflection point under lower . In other words, the lowest available IFT increases as alcohol chain length increases. The mean square displacement of AOT decreases as increases, and such a decrement trend is more significant in the systems with long-chain alcohols (C₁₆OH). In addition, long-chain alcohols (C₈OH and C₁₆OH) also help orientate the AOT tail group, while a negligible change in the AOT tail orientation is observed for the systems with short- and mid-tail alcohols.