Counterion-Mediated Assembly of Fluorocarbon-Hydrocarbon Surfactant Mixtures at the Air-Liquid Interface: A Molecular...

This study employs molecular dynamics simulations to investigate counterion effects (Li⁺, Na⁺, K⁺) on the interfacial aggregation of mixed short-chain fluorocarbon, Perfluorohexanoic acid (PFH_XA), and Sodium dodecyl sulfate (SDS) surfactants. Motivated by the need for greener surfactant alternatives and a fundamental understanding of molecular interactions governing their behavior, we demonstrate that counterion hydration radius critically modulates system organization. K⁺ ions induce superior monolayer condensation and interfacial performance compared to Li⁺ and Na⁺ counterparts, as evidenced by threefold analysis: (1) RMSD/MSD-confirmed equilibrium attainment ensures data reliability; (2) 1D/2D density profiles and surface tension measurements reveal K⁺-enhanced packing density (lower solvent-accessible surface area versus Na⁺ and Li⁺ systems); (3) Electrostatic potential analysis identifies synergistic complementarity between SDS's hydrophobic stabilization via dodecyl chain interactions and PFH_XA's charge uniformity, optimizing molecular-level charge screening. Radial distribution function analysis demonstrates K⁺'s stronger affinity for SDS head groups, with preferential sulfate coordination reducing surfactant-water hydration interactions. This behavior correlates with hydrogen-bond population reduction, attributed to SDS groups functioning as multidentate ligands-their tetrahedral oxygen arrangement facilitates cooperative hydrogen-bond networks, while counterion-specific charge screening competitively modulates bond formation. The resultant interfacial restructuring enables ordered molecular arrangements with lower system curvature than those observed in Li⁺ and Na⁺-containing systems. These findings elucidate counterion-mediated interfacial modulation mechanisms and establish K⁺ as an optimal candidate for enhancing PFH_XA/SDS mixture performance through hydration-radius screening. The work provides molecular-level guidelines for designing eco-friendly surfactant systems with tailored interfacial properties.