Oil-dependent stability of SDS/CTAB catanionic CO₂ foams: A combined experimental and molecular modeling study

In this study, the stability of CO₂ foams obtained by mixing anionic/cationic surfactants (catanionic surfactants) was comprehensively studied in the presence/absence of oil phase and the underlying mechanism was investigated. To this end, experimental studies were conducted, including static foam stability tests and CO₂/aqueous solution interfacial tension measurements. In parallel with the experiments, CO₂ solubility and its diffusion coefficient in the liquid phase were calculated using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, respectively. Also, density functional theory (DFT) calculations were used in conjunction with atom-in-molecule (QTAIM) analysis to identify the nature of noncovalent interactions. The results showed that mixing surfactants significantly improved the CO 2 foam stability in the absence of oil. The maximum stability was observed at the 50/50 M ratio, which coincided with the minimum interfacial tension, the lowest solubility, and the lowest CO₂ diffusion coefficient. Molecular analyses attributed this phenomenon to the formation of compact structures at this ratio, which increased the mass transfer resistance in the bulk liquid phase and the interface. DFT/QTAIM calculations at the B3LYP/6–31 + G(d) level confirmed the existence of noncovalent interactions between CO₂, water, and the mixed SDS/CTAB system and revealed the molecular basis for the decrease in CO₂ adsorption tendency in the mixed state. In the presence of n-heptane as the model oil phase, the behavior of the system changed significantly. Under these conditions, the foam formed at the previous optimal ratio (50/50) became unstable, while the ratios of 30/70 and 70/30 showed a favorable balance between stability and foaming ability. These results indicate that mixing of oppositely charged surfactants is an effective strategy for simultaneously tuning the interfacial, thermodynamic (solubility) and kinetic (diffusion) properties associated with Ostwald ripening. However, the selection of the optimal mixing ratio is strongly dependent on the presence or absence of the oil phase.