Molecular Simulations Investigation of Deep Eutectic Solvent Induced Structural and Interfacial Behavior in Surfactant-Enhanced Oil Recovery Systems

Abstract

Chemically enhanced oil recovery (c-EOR) relies heavily on surfactant-induced reduction of oil–water interfacial tension; however, the molecular-level role of deep eutectic solvent (DES) chemistry in surfactant-assisted interfacial stabilization remains poorly understood. In this work, atomistic molecular dynamics simulations were employed to investigate the interfacial behavior of choline chloride-based deep eutectic solvents (DESs) in combination with the anionic surfactant sodium dodecylbenzenesulfonate (SDBS) at the oil–water interface. Two DES systems, choline chloride–ethylene glycol (ChCl/EG) and choline chloride–cresol (ChCl/cresol), were examined over a concentration range of 10–30 wt %. Density distribution, hydrogen-bonding, radial distribution function, and RDF-derived excess entropy analyses reveal that the surfactant remains stably adsorbed at the interface, while DES molecules predominantly occupy the aqueous phase and progressively enrich the interfacial region with increasing concentration. Both DES systems significantly reduce interfacial tension, with the IFT decreasing from approximately 12.4 mN/m in the DES-free system to 8.47 mN/m for ChCl/EG and 7.23 mN/m for ChCl/cresol at 30 wt % DES concentration. The ChCl/EG system exhibits a gradual reduction in interfacial tension driven by cooperative hydrogen-bond-mediated restructuring of the aqueous phase and relatively uniform intermolecular ordering. In contrast, the ChCl/cresol system produces stronger interfacial effects at higher concentrations due to the greater interfacial affinity and localized accumulation of cresol molecules near the oil–water boundary. Hydrogen-bonding and excess entropy analyses further demonstrate that the EG-based DES promotes relatively diffuse and cooperative aqueous structuring, whereas the cresol-based DES induces more localized and concentration-dependent intermolecular organization near the interface. These findings demonstrate that DES chemistry plays a critical role in governing intermolecular organization, hydration structure, and interfacial stabilization in surfactant-assisted c-EOR systems. The molecular-level insights obtained in this study provide useful guidelines for the rational design of DES–surfactant formulations for EOR applications.