Molecular dynamics simulation of synergistic behavior at the air-water interface: mixed cationic-anionic fluorocarbon-hydrocarbon surfactants

Ecological concerns surrounding conventional aqueous film-forming foam extinguishing agents, predominantly composed of long-chain fluorocarbon surfactants, have spurred the need for innovation in eco-compatible substitutes, such as short-chain fluorocarbon surfactants. Molecular dynamics simulations are a valuable tool for studying the behavior of mixed surfactant systems at the air-water interface. We have conducted molecular dynamics simulations to investigate the interfacial behavior of a mixed cationic-anionic surfactant system, including N-[3-(dimethylamino)propyl]perfluorobutanesulfonamide hydrochloride (PFB-MC) and 1-octanesulfonic acid (1-OA). The simulations explored the effects of varying PFB-MC and 1-OA ratios on aggregation and adsorption. The results indicate that the equimolar 1 : 1 ratio produced more compact aggregates at the interface and achieved the most effective reduction in surface tension and the formation of a dense interfacial film. The study highlights the competitive adsorption phenomena between surfactants and counterions at the interface, providing insights through 1D and 2D density analyses into the impact of counterbalancing ions on aggregation. An increased PFB-MC concentration resulted in decreased hydrogen bonding with water, while 1-OA showed a higher tendency for hydrogen bonding, underscoring its hydrophilicity. These findings provide valuable insights into surfactant interfacial behavior and are instrumental in the development of advanced foam extinguishing agents suitable for environmental and industrial use.