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

We conduct a quantitative phase-sensitive sum-frequencyvibrationalspectroscopic investigation on the air/water interface with variousatmospherically relevant ions in water in submolar concentrations.At electrolyte concentrations below 0.1 M, the spectral changes ofthe OH-stretching resonance induced by ions exhibit no ion specificityand resemble the lineshape of the third-order nonlinear optical susceptibilityof bulk water. These findings, along with the result of invariantfree OH resonance, indicate that the primary effect of the electricdouble layer of ions on the interfacial structure arises from themean-field-induced molecular alignment in a subsurface bulklike hydrogen-bondingnetwork. Analysis of the spectra allows us to determine quantitativelythe surface potentials for six electrolyte solutions (MgCl2, CaCl2, NH4Cl, Na2SO4, NaNO3, and NaSCN). Our results agree well with the predictionsof Levin's continuum theory, implying fairly small electrostaticcorrelations for the studied divalent ions.