The solvation of Na+ ions by ethoxylate moieties enhances adsorption of sulfonate surfactants at the air-water interface.

Hypothesis: Experiments show pronounced synergy in the reduction of surface tension when the nonionic surfactant Poly(oxy-1,2-ethanediyl), .alpha.-tris(1-phenylethyl)phenyl-.omega.-hydroxy- (Ethoxylated tristyrylphenol, EOT) is mixed with the anionic surfactant Sodium 4-dodecylbenzenesulfonate (NaDDBS). We hypothesize that the synergism is due to counterion (cation) effects. This would be unusual as one of the surfactants is nonionic. To test this hypothesis, the molecular mechanisms responsible need to be probed using experiments and simulations.

Approach: The interfacial properties of mixtures comprising EOT and NaDDBS are investigated using equilibrium molecular dynamics (MD) simulations. Free energy calculations using thermodynamic integration and umbrella sampling methods are employed to analyze the molecular interactions at surface and reveal the role of counterion solvation on the results observed. Simulation snapshots and trajectories are interrogated to confirm the findings.

Findings: Simulation results indicate that the ethoxylate moieties solvate Na⁺ ions, forming long-lasting cation-EOT complexes. Free energy calculations suggest that these complexes are more stable at the interface than in the bulk, likely because of changes in the dielectric properties of water. The cation-EOT complexes, in turn, cause a stronger affinity between the interface and NaDDBS when EOT is present. Similar studies conducted for mixtures of EOT and cationic surfactant Dodecylammonium chloride (DAC) do not show evidence of Cl^- ions solvation via the ethoxylate moieties, while the DAC headgroup was found to form hydrogen bonds with the EOT headgroup. This suggests that the mechanisms observed are likely ion specific.