Molecular dynamics study on BS12/SDS stabilized CO2 foam: Evaluation of surfactant monolayer structure and interfacial properties

Abstract

CO2 foam flooding holds great potential for enhanced oil recovery and geological storage. However, ensuring the stability of supercritical CO2 foam systems under high temperature and pressure reservoir conditions remains a challenge. Note that the term "supercritical CO2 foam" specifically denotes a foam system created by combining an aqueous solution containing chemical additives with CO2 in a supercritical state under high-temperature and high-pressure conditions. Surfactants can reduce the surface Gibbs free energy by adsorbing onto the CO2-water interface to form a monolayer structure, which hinders CO2 diffusion and improves foam stability. In this study, molecular dynamics simulations were employed to analyze the adsorption behavior and interfacial performance of betaine surfactant BS12 and the anionic surfactant SDS at the CO2-water interface. The results indicate that BS12 can reduce interfacial tension more effectively at near-saturated concentrations. Moreover, the monolayer formed by BS12 has a lower voidage and wider interfacial thickness. The carboxyl-nitrogen interaction among BS12 molecules impedes their further aggregation, thereby maintaining the mutually perpendicular "L" orientation of the headgroup and hydrophobic tail thus forming a rigid and stable monolayer structure. Additionally, the spatial distribution function, radial distribution function, and hydrogen bond statistics indicate that although BS12 exhibits slightly weaker hydrophobicity than SDS, its hydrophobicity is less affected by concentration. Specifically, the first hydration shell of the carboxyl group remains stable even at high concentrations. Besides, the steered simulations show that the rate-limiting step in the CO2 permeation process is the dense region of the surfactant headgroups. Besides, the permeation of CO2 molecules in the BS12 monolayer is characterized by a lower diffusion coefficient and higher energy barrier to overcome. In summary, BS12 forms a thicker and more uniform monolayer structure, effectively hindering the contact between CO2 and water molecules at the interface and further enhancing foam stability. Our findings further reveal the relationship between surfactant molecular architecture and interfacial properties, providing references for the screening and structural design of surfactants used in CO2 foam flooding.