Molecular Dynamics Insights into the Interactions Between Gemini Surfactant CSGS and Heavy Oil SARA Components

This study investigates the molecular interactions and interfacial behaviors of a carboxylate-sulfonate gemini surfactant (CSGS) with four heavy-oil components (SARA: saturates, aromatics, resins, and asphaltenes) using molecular dynamics (MD) simulations. To provide a comprehensive analysis, two distinct systems were constructed: a homogeneous bulk oil phase (System 1) and a solid–liquid interfacial system containing a calcite (CaCO3) substrate (System 2). In System 1, results showed that CSGS remained well dispersed in the bulk heavy-oil phase and promoted a more uniform distribution of the SARA components. The differences in mobility among the components were mainly determined by molecular structure, resulting in a consistent diffusion trend in the CSGS-containing bulk system. In contrast, the introduction of a calcite substrate (System 2) shifted the distribution from a largely disordered bulk-like state to a confined interfacial organization, with clear layering and enrichment near the mineral surface. Compared with the CaCO3-free system, molecular migration was noticeably restricted, indicating that the carbonate layer imposed additional constraints on mass transport. At the same time, CSGS preferentially accumulated in the SARA components–CaCO3 region, consistent with competitive adsorption at the carbonate interface, and further reorganized the local interfacial structure.