Molecular dynamics simulation of potassium perfluorooctanesulfonate at the oil/water interface

In this research, we used molecular dynamics simulation to study the transformation of a mixture containing oil, water, and potassium perfluorooctanesulfonate (KPFOS) from a “disordered” state to an “aggregated” state. During the simulation, we observed that the PFOS − molecules spontaneously migrated to the interface between the oil and water phases. The hydrophilic sulfonate groups were oriented toward the water phase, while the lipophilic fluorocarbon chains were oriented toward the oil phase. By analyzing the number density and charge density distribution, we found that PFOS − and K + ions predominantly accumulated at the oil–water interface, with some K + ions dispersed within the solution. Moreover, PFOS − formed a stable monomolecular film at the interface, creating a distinct “transition region” with a specific thickness. The mean square displacement (MSD) results indicated that self-assembled micelles composed of PFOS − -facilitated efficient migration of oil molecules within the system, displaying robust migration abilities. Further analysis of the radial distribution function revealed a high probability of K + ions being found near the oxygen atoms in PFOS − due to charge attraction. Separating K + ions from PFOS − at the interface required overcoming very strong interaction forces, which limited their migration. Weak van der Waals interactions were observed between the fluorocarbon chains and toluene, while hydrogen bonding interactions occurred between the sulfonate groups and water molecules, as identified through independent gradient model based on Hirshfeld partition analysis. These findings shed light on the complex kinetic processes governing the behavior of oil–water-KPFOS mixtures, providing valuable insights for future studies in this field.