Stability mechanism of oil-swollen surfactant micelles: Insights from surfactant escape free energy

The stability of oil-swollen surfactant micelles is critically important in fields such as drug delivery, environmental remediation, and crude oil extraction. Nevertheless, the microscopic mechanisms underlying their structural stability remain unclear. Here, we employ coarse-grained molecular dynamics simulations coupled with umbrella sampling to study the structural characteristics of oil-swollen surfactant micelles composed of varying numbers of sodium dodecyl sulfate molecules (N_SDS) and decane molecules (N_DEC). By analyzing the escape process of surfactant molecules from the micelles, we explore the microscopic mechanisms through which N_SDS and N_DEC influence oil-swollen micellar stability. The results show that increasing N_SDS results in structural imbalance of oil-swollen micelles, while increasing N_DEC enhances micellar compactness and stability. Both the escape free energy and escape time of surfactant molecules from oil-swollen micelles exhibit non-monotonic trends with N_DEC increasing, which are attributed to the balance between intermolecular interactions (oil-surfactant and surfactant-surfactant). Moreover, we identify the critical ranges for N_SDS and N_DEC that maintain micellar stability and determine the optimal decane-to-surfactant ratio of stable oil-swollen micelles. These results could provide effective guidance for optimizing the formulation design of swollen micellar solutions from free energy viewpoints.