Evaluation of the interfacial elasticity of surfactant monolayer at the CO₂-water interface by molecular dynamics simulation: Screening surfactants to enhance the CO₂ foam stability

Foam stability is adversely affected by the drainage process in the liquid film, which has been a great challenge for CO2 foam flooding in applications. The Gibbs-Marangoni effect, which is induced by the interfacial tension (IFT) gradient, provides a resisting force to the thinning of liquid films in foams under disturbance. Surfactants with higher interfacial elasticity enable rapid recovery to the equilibrium state, thereby improving the stability of the CO2 foam film. However, there is a lack of mechanistic studies aimed at elucidating the correlation between the interfacial elasticity of surfactant monolayers and the molecular structure of surfactants. In this paper, molecular dynamics simulations were employed to investigate the impact of the headgroups and linking groups of sodium polyoxyethylene alkyl ether sulfate (AES), sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (SDSn), and sodium laurate (SLA) surfactants on the interfacial morphology, monolayer properties, and surfactant behaviors at the CO2-water interface under reservoir conditions. The results show that the capacity to enhance CO2 foam stability follows the order AES > SDS > SDSn > SDBS > SLA, which is in good agreement with the available experiments. AES monolayers can reach the lowest IFT (similar to 0.2 mN/m) and the highest interfacial elasticity (30.4 mN/m). We show that the ethylene oxide groups (EO chains) can strongly interact with water molecules via hydrogen bonding and have affinity interactions with CO2 molecules, which is beneficial to the hydrophilic-CO2-philic balance (HCB). SDS monolayers show the second-lowest IFT (3.1 mN/m) and second-highest interfacial elasticity (26.0 mN/m). The linking oxygen atom in SDS facilitates bond rotation and is favorable to film stability. The phenyl group makes SDBS molecules too rigid and has poor HCB, leading to low stability with high IFT (16.0 mN/m) and low interfacial elasticity (13.1 mN/m). SLA has the highest IFT (22.1 mN/m) and lowest interfacial elasticity (6.3 mN/m), and also presents strong aggregation behaviors at the interface owing to interactions between the carboxyl groups. The simulation method and the microscopic insights delivered here are of great significance in screening surfactants to improve CO2 foam performance in oilfields.