Effect of Salt on the Adsorption Affinity of an Aromatic Carbonyl Molecule to the Air-Aqueous Interface: Insight for Aqueous Environmental Interfaces

The interface-selective second harmonic generation (SHG) nonlinear optical spectroscopy and the surface tension method have been used to measure the effect of electrolyte on the adsorption isotherm of 4-hydroxyacetophenone (4-HA) at the air-solution interface. It was observed that high NaCl concentration favors adsorption of organic molecules to the air-solution interface. Measurements of the polarization of the SHG field showed that the orientation of the organic molecule at the air-solution interface is close to that in the neat air-water interface and does not vary with organic concentration. The free energy of adsorption of the molecule to the air-solution interface becomes significantly more negative with an increase in salt concentration, and good agreement was found between the results from the two different techniques. We conclude that the increased affinity of 4-HA for the air-water interface in the presence of NaCl is due to the salting-out effect.