MARTINI Coarse-Grained Molecular Dynamics study of 1,4-dioxane partitioning in concentrated SLES solutions for stripping process design

1,4-dioxane is a trace impurity formed during ethoxylation processes and frequently detected at ppm levels in sodium lauryl sulfate (SLES) formulations. Industrial experience shows that its removal from concentrated SLES solutions is often more difficult than predicted by simple equilibrium models. In particular, experiments indicate that increasing temperature does not necessarily improve stripping efficiency and may even lead to higher residual dioxane concentrations, suggesting that the structured micellar environment of SLES solutions influences its retention. In this work, coarse-grained molecular dynamics simulations using the MARTINI 3 force field were performed to investigate the partitioning behaviour of 1,4-dioxane in concentrated SLES systems and its temperature dependence. A model solution containing approximately 30 wt% SLES was constructed in a 12 x 12 x 12 nm simulation box with 300 SLE₂S molecules, counterions, water beads, and ten dioxane molecules representing a dilute tracer regime. Simulations were performed at 298, 323, and 348 K, and production trajectories of up to 200 ns were analysed to characterise aggregate morphology and solute localisation. Radial distribution functions, residence statistics, and an effective free-energy profile derived from equilibrium distance distributions were used to evaluate dioxane localisation relative to the SLES aggregates. The simulations show that dioxane does not preferentially occupy the hydrophobic core of the aggregates but instead samples predominantly the hydrated ethoxylate-rich interfacial region. The reconstructed free-energy profile indicates a shallow stabilization of dioxane near the micellar interface relative to bulk water. These results suggest that the structured micellar environment of concentrated SLES solutions can stabilise dioxane at the surfactant interface, reducing its volatility and helping explain the experimentally observed decrease in stripping efficiency at elevated temperatures.