Molecular Insights into Oil–Water Interfacial Behavior of Gemini Sulfonate Surfactants: The Critical Role of Spacer Group

Gemini surfactants are extensively employed in enhanced oil recovery (EOR) owing to their low critical micelle concentration (CMC), excellent interfacial activity, and strong self–assembly capability. However, molecular–level understanding of how spacer chemistry regulates the interfacial behavior of sulfonate–containing gemini surfactants remains limited. All–atom molecular dynamics simulations were employed to investigate four gemini sulfonate surfactants with distinct spacers (carbon–chain spacer (CA), ester containing spacer (EG), benzene–ring spacer (BR), and nitrogen–containing (NA)) at the n–decane/water interface. Based on its superior interfacial performance, the EG system was further selected to evaluate the effects of polyacrylamide (PAM) and partially hydrolyzed polyacrylamide (HPAM). Increasing surfactant concentration markedly altered interfacial assembly, and at 2.33 μmol/m2, pronounced interfacial curvature indicated a tendency toward microemulsion-like structures. Spacer chemistry strongly regulated interfacial thickness, hydration, and molecular organization. The rigid aromatic spacer of BR broadened the interfacial region through its strong oil–phase affinity, whereas the ester–containing EG spacer enhanced hydration and hydrogen–bond formation and promoted favorable molecular packing, resulting in the best overall interfacial performance. Further simulations showed that PAM accumulated near the EG interfacial layer and strongly interacted with EG, leading to a pronounced reduction in interfacial tension. In contrast, hydrolysis strengthened HPAM–water interactions and promoted its distribution in the bulk aqueous phase, thereby limiting its influence on the interface. This study provides molecular insights into how different spacer groups regulate interfacial behavior, offering theoretical guidance for the design and formulation of gemini surfactant systems for EOR.