Distribution of potassium perfluoro (2-ethoxyethane) sulfonate in NaCl solution: insights from molecular dynamics simulation

The distribution of fluorocarbon surfactants potassium perfluoro (2-ethoxyethane) sulfonate (PESK) in 3.4% NaCl solution was studied using molecular dynamics (MD) simulation method. During the MD simulation, it was observed that the PES⁻ in the bulk solution spontaneously move to the water/gas interface. According to the change in the number of water molecules within 0.35 nm of PES⁻, it can be inferred that the solution has basically reached equilibrium when the simulation reaches 40 ns. At this moment, the vast majority PES⁻ are distributed at the air/water surface, the fluorocarbon chain is facing toward the gas phase, while the sulfonic acid radical faces toward the water, and there are about 12 water molecules within 0.35 nm of each PES⁻, a very small quantity of PES⁻ still in the bulk solution. Compared with K⁺, Na⁺ is more likely bound to PES⁻, and this is confirmed by RDF and number density distribution analysis. The weak intermolecular interactions were analyzed by the IGMH method, and the interaction energy between PES⁻ and water mainly comes from the h-bonds formed by the oxygen atom in the sulfonic acid group and hydrogen atom in water molecules; there is only van der Waals interaction between fluorine atoms and water molecules.