Anionic Structural Variation in Tween 80-Based Mixed Surfactant Systems: Oil-Water Interfacial Coassembly and Winsor III Microemulsion Behavior.

Mixed nonionic/anionic surfactant systems are widely used to regulate complex oil-water/solid interfaces, yet the molecular role of anionic structural variation in interfacial coassembly and Winsor III microemulsion behavior remains insufficiently understood. Herein, Tween 80/SDBS (TS) and Tween 80/AES (TA) were selected as representative Tween 80-based mixed surfactant systems to clarify how different anionic structures influence oil-water interfacial organization. Molecular dynamics simulations were combined with middle-phase (Winsor III) microemulsion experiments to examine interfacial electrostatics, molecular packing, local hydration, local hydrogen-bond associations, counterion association, and the resulting microemulsion response. The simulations showed that electrostatic interactions made the dominant contribution to the nonbonded interfacial interactions, while local hydration and local hydrogen-bond associations further modulated interfacial organization. At a nonionic/anionic molar ratio of 4:7 under the investigated conditions, TS tended to exhibit relatively compact Tween 80 conformations and a locally concentrated interfacial organization, whereas TA exhibited a more hydrated and spatially coordinated interfacial arrangement. These differences were associated with the benzenesulfonate structure of SDBS and the EO-containing sulfate structure of AES, which gave rise to distinct charge distributions, counterion association, local hydration, and packing characteristics. Although the theoretical interfacial tensions of the TS and TA Winsor III microemulsions were both very low and comparable, their formulation-dependent behaviors remained distinct. Overall, the results establish a structure-interaction-response relationship linking anionic structural variation with local interfacial organization and macroscopic Winsor III microemulsion behavior.