Surface structures of NaF and Na ₂SO ₄ aqueous solutions: Specific effects of hard ions on surface vibrational spectra

We elucidated surface structure and sum frequency generation (SFG) spectra of aqueous NaF and Na ₂SO ₄ solutions by molecular dynamics (MD) simulation. These electrolyte solutions contain hard ions, and their surfaces are considered to be void of ion species. Nevertheless, the two solutions have shown remarkably different SFG spectra, implying different surface structure. The present MD analysis confirmed that all the ionic species are repelled from the surface, though their experimental SFG spectra are well reproduced. The difference in their surface structure is observed in the subsurface region, where slight charge separation occurs in the Na ₂SO ₄ solution. The perturbation in the SFG spectra is not attributed to the topmost layer but to the water orientation in a fairly deep region of the surface.