Surface Potential at Electrolyte/Air Interfaces: A Quantitative Analysis via Sum-Frequency Vibrational Spectroscopy.

We conduct a quantitative phase-sensitive sum-frequency vibrational spectroscopic investigation on the air/water interface with various atmospherically relevant ions in water in submolar concentrations. At electrolyte concentrations below 0.1 M, the spectral changes of the OH-stretching resonance induced by ions exhibit no ion specificity and resemble the lineshape of the third-order nonlinear optical susceptibility of bulk water. These findings, along with the result of invariant free OH resonance, indicate that the primary effect of the electric double layer of ions on the interfacial structure arises from the mean-field-induced molecular alignment in a subsurface bulklike hydrogen-bonding network. Analysis of the spectra allows us to determine quantitatively the surface potentials for six electrolyte solutions (MgCl, CaCl, NHCl, NaSO, NaNO, and NaSCN). Our results agree well with the predictions of Levin's continuum theory, implying fairly small electrostatic correlations for the studied divalent ions.