Molecular Insights into the Oil-Wetness Reduction of n -Decanoic-Acid-Modified Sandstone by Different Surfactants

Oil-wetness reduction is a critical mechanism in chemically enhanced oil recovery (cEOR). While surfactants are widely used to reduce oil wetness, the molecular-level interactions governing the efficacy of different surfactant charges remain ambiguous. Specifically, the distinct mechanisms by which anionic, cationic, and nonionic surfactants disrupt the adsorption of polar crude oil components on mineral surfaces require further elucidation. This study employs a synergistic approach combining macroscopic interfacial tension and contact-angle measurements with molecular dynamics (MD) simulations. An oil-wet SiO2–oil–water model was constructed, where the surface was rendered hydrophobic by the adsorption of n-decanoic acid (DA). The oil-wetness reduction performance of sodium dodecyl benzenesulfonate (SDBS, anionic), dodecyl trimethylammonium bromide (DTAB, cationic), and AEO-1 (nonionic) was evaluated both experimentally and computationally. Experimental results confirmed that SDBS achieved the most significant reduction in the contact angle, while AEO-1 and DTAB showed weaker and comparable effects. MD simulations suggested that the oil-wetness reduction is not driven by direct surfactant adsorption onto the quartz surface but rather by the competitive interaction between surfactants and the adsorbed DA molecules. Radial distribution function (RDF) analysis identified that SDBS interacts via sulfonate oxygen with the hydroxyl hydrogen of DA, AEO-1 bonds via hydroxyl hydrogen with the hydroxyl oxygen of DA, and DTAB shows preferential association via N-methyl C–H hydrogen with the oxygen atoms of DA. Within the single-trajectory MD comparisons, the superior performance of SDBS is attributed to its ability to simultaneously disrupt both Si–O coordination and hydrogen bonds between DA and the quartz surface. This study provides insights into the targeted selection of surfactants based on the specific polar components in the crude oil within this model system.