Counterion-Mediated Assembly of Fluorocarbon–Hydrocarbon Surfactant Mixtures at the Air–Liquid Interface: A Molecular Dynamics Study
This study employs molecular dynamics simulations to investigate counterion effects (Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>) on the interfacial aggregation of mixed short-chain fluorocarbon, Perfluorohexanoic acid (PFH<sub>X</sub>A), and Sodium...
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| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
MDPI AG
2025-06-01
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| Series: | Molecules |
| Subjects: | |
| Online Access: | https://www.mdpi.com/1420-3049/30/12/2592 |
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| Summary: | This study employs molecular dynamics simulations to investigate counterion effects (Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>) on the interfacial aggregation of mixed short-chain fluorocarbon, Perfluorohexanoic acid (PFH<sub>X</sub>A), 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<sup>+</sup> ions induce superior monolayer condensation and interfacial performance compared to Li<sup>+</sup> and Na<sup>+</sup> 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<sup>+</sup>-enhanced packing density (lower solvent-accessible surface area versus Na<sup>+</sup> and Li<sup>+</sup> systems); (3) Electrostatic potential analysis identifies synergistic complementarity between SDS’s hydrophobic stabilization via dodecyl chain interactions and PFH<sub>X</sub>A’s charge uniformity, optimizing molecular-level charge screening. Radial distribution function analysis demonstrates K<sup>+</sup>’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<sup>+</sup> and Na<sup>+</sup>-containing systems. These findings elucidate counterion-mediated interfacial modulation mechanisms and establish K<sup>+</sup> as an optimal candidate for enhancing PFH<sub>X</sub>A/SDS mixture performance through hydration-radius screening. The work provides molecular-level guidelines for designing eco-friendly surfactant systems with tailored interfacial properties. |
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| ISSN: | 1420-3049 |