Synergistic effects in betaine based binary surfactant at water/oil interface: Insights from molecular dynamics simulations

Surfactants are critical in the enhanced oil recovery (EOR) process due to their ability to reduce oil-water interfacial tension (IFT), thereby improving EOR efficiency. While betaine surfactants have emerged as promising candidates for achieving ultralow IFT values (similar to 10(-3) mN/m), their standalone performance is often limited by inherent interfacial packing defects arising from their molecular architecture. In this study, we employ systematic molecular dynamics simulations to investigate the synergistic effects of blending betaine surfactants [alkyl sulfobetaine (ASB) and xylyl substituted alkyl sulfobetaine (XSB)] with nonionic surfactants Span80 or Tween80, aiming to explore the IFT reduction capability and to understand the interfacial behavior for potential application in EOR. The results indicate that when the two betaines are blended with Tween80, they compete for water molecule adsorption on the aqueous side and incorporate water molecules into the interfacial film. This disrupts the film's ability to isolate water molecules, thus producing an antagonistic effect. In contrast, blending betaines with Span80 yields favorable interactions: at low concentrations, Span80 primarily increases intermolecular spacing between adjacent betaines to reduce electrostatic forces, generating synergistic effects. At higher concentrations, Span80 adapts to interfacial vacancies by enhancing the rigidity and angle of its hydrophobic chains, exhibiting good interfacial effects. Notably, ASB/Span80 and XSB/Span80 systems achieved ultralow IFT values but due to structural differences, XSB has relatively weaker synergy with Span80 than ASB, nonetheless it still outperforms other single or mixed surfactant systems in IFT reduction capability.