Synergistic effects of betaine and anionic surfactants at the oil-water interface: A molecular dynamics simulation study

Studies demonstrated that the mixed system of betaine and anionic surfactants significantly reduces interfacial tension through synergistic effects, offering new molecular insights for improving industrial processes such as emulsion stability and oil recovery. This work employs molecular dynamics simulations to investigate the synergistic effects of alkylbenzene sulfobetaine (XSB) mixed with three different anionic surfactants: sodium dodecylsulfate (SDS), sodium dodecylbenzenesulfonate (SDBS) and sodium dioctyl sulfosuccinate (AOT) at oil-water interfaces. Through systematic analysis of parameters including charge distribution, molecular orientation, ordered arrangement characteristics, and intermolecular interactions, it was found that the structural characteristics and electrostatic distribution of surfactant molecules are the key factors determining interfacial behavior. Research has shown that the dual chain AOT/XSB binary composite demonstrated optimal synergy, forming a compact surfactant layer (thickness: 2.14 nm) with a reduced XSB tilt angle (Δ θ > 10°), achieving 48.9 % interfacial tension reduction versus single-component systems (the lowest IFT value reaches 9 × 10 −3 mN/m). Ratio optimization demonstrated/revealed that the interfacial activity reached its optimal state at an XSB:AOT ratio of 1:4 (interfacial formation energy: −220.59 kJ/mol). This discovery elucidates the synergistic mechanism of complex systems from the perspective of molecular interactions, providing theoretical guidance for the development of efficient complex surfactant systems and having important practical application value. • Molecular insights into mixed surfactants' behavior at oil-water interfaces. • Electrostatic complementarity drives tight packing of XSB and anionic surfactants. • AOT's dual chains adjust angles to fill interfacial gaps, enhancing layer density. • Charge complementarity and conformational adaptability drive XSB-AOT superiority. • 1:4 XSB/AOT ratio demonstrates optimal synergistic effect.