Change of Surface Structure upon Foam Film Formation

Competing Segregation of Br- and Cl- to a Surface Coated with a Cationic Surfactant: Direct Measurements of Ion and Solvent Depth Profiles

Ion-surface scattering experiments can be used to measure elemental depth profiles on the angstrom scale in complex liquid mixtures. We employ NICISS (neutral impact collision ion scattering spectroscopy) to measure depth profiles of dissolved ions and solvent in liquid glycerol containing the cationic surfactant tetrahexylammonium bromide (THA⁺/Br^-) at 0.013 M and mixtures of NaBr + NaCl at 0.4 M total concentration. The experiments reveal that Br^- outcompetes Cl^- in its attraction to surface THA⁺, and that THA⁺ segregates more extensively when more Br^- ions are present. Intriguingly, the depths spanned by THA⁺, Br^-, and Cl^- ions generally increase with Br^- bulk concentration, expanding from ∼10 to ∼25 Å for both Br^- and Cl^- depth profiles. This broadening likely occurs because of an increasing pileup of THA⁺ ions in a multilayer region that spreads the halide ions over a wider depth. The experiments indicate that cationic surfactants enhance Br^- and Cl^- concentrations in the surface region far beyond their bulk-phase values, making solutions coated with these surfactants potentially more reactive toward gases that can oxidize the halide ions.

Quantifying the Counterion-Specific Effect on Surfactant Adsorption Using Modeling, Simulation, and Experiments

Ionic surfactants behave differently in the presence of various counterions, which plays an important role in many scientific and engineering processes. Previous work has shown that the counterion-specific surface tension can be reproduced with classical adsorption models, but the underlying origin of this effect has not been explained. In this paper, we extend our previously developed adsorption model to account for the specific counterion adsorption. This model can accurately predict the surface tension of surfactant solutions like sodium dodecyl sulfate (SDS) in the presence of the monovalent salts LiCl, NaCl, KCl, and CsCl. The predicted surface excess and surface potential are validated by corresponding sum-frequency generation (SFG) spectroscopy experiments. We also used molecular dynamic (MD) simulation to explain the origin of the counterion-specific effect for surfactant behavior. Our study shows that for SDS, binding of the counterion to both the headgroup and a few CH₂ fragments close to the surfactant head contributes to the counterion-specific effect. In general, SDS behaves like a large ion, and it prefers to bind with large counterions such as Cs⁺, which is consistent with Collins's law of matching water affinity. Therefore, large counterions enhance the surface adsorption and lower the surface tension the most.

Adsorption of ionic surfactants at the air-water interface: The gap between theory and experiment

We review the experimental and theoretical results for the adsorption and structure of ionic surfactants at the air-liquid interface. The results show that ionic surfactants form thick adsorption layers at the interfacial region. We also review several adsorption models for ionic surfactants, which become increasingly complex as they capture the many features of adsorption layers. However, the adsorption layer structures determined by experiments and the structures predicted by models do not match because most models assume very thin adsorption layers. We show the discrepancies between measured and predicted surface properties and provide several explanations. We conclude that the mismatch in the adsorption layer structure provided by experiments and the structure provided by adsorption models is the main reason for the discrepancies in the surface excess and the surface potential.