Morphological and microstructural insights into mixed surfactant systems: a molecular dynamics study

Surfactants are the backbone of detergent formulations, playing a crucial role in determining the cleaning performance, product aesthetics and formulation stability. The strategic combination of ionic and non-ionic surfactants has been demonstrated to reduce the critical micelle concentration (CMC), modulate phase behavior, and promote the formation of tailored micellar structures, thereby optimizing detergent performance. In this study, we employ all-atom molecular dynamics simulations to investigate the morphology and microstructure of single and binary mixed surfactant systems comprising ionic surfactants sodium lauryl ether sulfate (SLES) and linear alkylbenzene sulfonate (LAS) and non-ionic surfactant poly(oxyethylene) <i>n</i>-dodecyl ether (EO7). Our results reveal concentration-dependent transitions in micellar morphology from spherical to ellipsoidal to wormlike-cylindrical aggregates. The addition of non-ionic surfactants reduces electrostatic repulsion between ionic headgroups, leading to larger and more stable aggregates. This results in an increase in the aggregation number by approximately 30% and a corresponding decrease in the solvent-accessible surface area by up to 27%. EO7 exhibits adaptive conformations, compact in ionic mixtures <i>versus</i> extended in single surfactant systems, optimizing solvation and steric interactions. The study highlights critical solvation dynamics in mixed surfactant systems, where the addition of EO7 modulates surface properties by reducing hydrogen bonding between ionic headgroups (SLES/LAS) and water compared to single surfactant systems, while enhancing water-water hydrogen bonding within the first and second solvation shells when the non-ionic concentration is increased. These findings provide molecular-level insights into the self-assembly mechanisms, aggregation behavior, and solvation properties of mixed surfactant systems, contributing to the rational design of optimized formulations for industrial applications.