Synergistic effects of cardanol anionic/nonionic surfactants at the oil/water interface: a molecular dynamics study

Enhancing oil recovery is a critical challenge in petroleum exploitation, and surfactant flooding improves displacement efficiency by lowering oil–water interfacial tension and stabilizing emulsions. In this study, molecular dynamics simulations were performed to examine the interfacial behavior of sulfonate-type (S10PO10EOSO3) and nonionic-type (S8PO8EOH) surfactants derived from saturated cardanol at various molar ratios. The system exhibited the most favorable interfacial performance at a molar ratio of n(S10PO10EOSO3)/n(S8PO8EOH) = 2:8; the interfacial tension decreased from 9.7 to 3.86 mN/m, the interfacial thickness reached 1.52 nm, and the diffusion coefficients increased by 1.8-fold, which shortened the equilibration time by 35%. Density profile and radial distribution function analyses revealed that hydroxyl substitution redirected the diffusion pathways and increased the coordination number of water around sulfonate groups from 4.2 to 6.1. These results indicate that the co-adsorption of sulfonate and hydroxyl groups enhances interfacial packing density and amphiphilicity, thereby reducing interfacial tension and providing a theoretical basis for designing surfactant systems with improved oil displacement efficiency.