Molecular dynamics study on surfactants regulating CO₂-water interface stability

• MD deciphers foam stability mechanism in harsh reservoirs. • Superior salt tolerance of DTAOH vs. SDS. • DTAOH’s "L-shaped" orientation with perpendicular heads resists ion interference. • DTAOH’s charge distribution minimizes long-range electrostatic screening. CO 2 foam flooding effectively suppresses gas channeling and plugs high-permeability pathways through its high apparent viscosity, significantly enhancing oil recovery while enabling geological storage. However, the stability of CO 2 foam in high-salinity reservoirs remains a critical challenge. The results demonstrated that under the conditions of 323.15 K, 20 MPa, and near-saturation concentrations, sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium hydroxide (DTAOH) reduced the CO₂-water interfacial tension to 5.28 mN/m and 8.25 mN/m, respectively. Further analysis indicated that the DTAOH monolayer exhibited lower porosity and greater interfacial thickness: the lower porosity could reduce the contact probability between CO₂ molecules and water molecules, thereby delaying the foam decay process; while the greater interfacial thickness could enhance the anti-interference capability of the interfacial film and improve its structural stability. In terms of salt tolerance in high-salinity environments, DTAOH showed significantly superior performance compared to SDS. Specifically, the single anionic head group of SDS molecules was prone to strong electrostatic adsorption with cations in the system, which further induced molecular aggregation and precipitation, and ultimately led to the loss of its interfacial activity. In contrast, DTAOH could mitigate the interference of ions on its interfacial adsorption behavior through a charge compensation effect, effectively maintaining its own interfacial activity and ensuring the stability of CO₂ foam under complex reservoir conditions.