Effect of molecular structure of surfactants on its behavioral characteristics at the CO2/water interface

CO 2 foam plays a pivotal role in the synergistic integration of enhanced oil recovery and carbon geological storage, which is attributed to its ability to control CO 2 mobility and accelerate CO 2 dissolution-mineralization. However, foams are inherently thermodynamically unstable systems. Notably, CO 2 not only undergoes inter-bubble diffusion but also dissolves in aqueous phases, collectively exacerbating foam destabilization. Therefore, it is of great significance to optimize the structure of the foam agent for constructing a highly stable CO 2 foam system. In this paper, the properties of CO 2 bubble film and the arrangement of molecules on the liquid film were studied through experiments and molecular simulations in a series of conventional surfactant solutions. Specifically, the effects of hydrophobic carbon chains and hydrophilic groups of surfactants on liquid film permeability, interfacial adsorption process, bubble stability, and CO 2 molecular behavior were analyzed. The lab results show that the CO 2 bubbles in systems containing C16-based quaternary ammonium (TAC-16) and betaine (S16) surfactants exhibited lifespans of 186 and 69 min, respectively, representing 85 and 29 times longer than that of C12-based counterparts (TAC-12 and S12)‌. The liquid film permeability of CO 2 bubbles in TAC-16 and S16 were 5.02 and 11.99 cm/min, corresponding to 1/7 and 1/3 of TAC-12 and S12. Furthermore, the adsorption capacities of TAC-16 and S16 were 1.63 and 3.06 mol/m 2 at the gas-liquid interface, which were greater than those of TAC-12 and S12. Long-chain surfactants had strong ‌hydrophobic interactions‌, which enhanced their ‌adsorption‌ and ‌micelle formation‌ capabilities‌. Long-chain surfactants showed a certain affinity for CO 2 molecules and could form a dense adsorption layer at the gas-liquid interface. The results indicate that the long-chain surfactants can delay the diffusion and dissolution of CO 2 in the liquid film and have obvious advantages as CO 2 foam agents. This work preliminarily reveals the ‌molecular states‌ and ‌interactions‌ within CO₂ foams, providing critical insights for designing and developing CO 2 foam agents.