Molecular dynamics analysis of NaOH aqueous solution surface and the sum frequency generation spectra: Is surface OH^- detected by SFG spectroscopy?

We elucidated the phase-sensitive sum frequency generation (SFG) spectrum of NaOH aqueous solution by molecular dynamics simulation. The MD analysis reproduced the effects of NaOH on the imaginary χ⁽²⁾ spectrum, characterized with a positive (upward) shift in the amplitude in midfrequency region (3300-3600 cm^-1) and a negative (downward) shift in low frequency (3000-3200 cm^-1). We found that the positive shift is a consequence of electric double layers, while the negative shift is attributed to the first solvation shell of OH^-. The latter mechanism generally holds for electrolyte solutions, though it becomes conspicuous for the NaOH solution in the low-frequency tail region. These perturbation mechanisms evidence that OH^- at the surface is fully hydrated and not preferentially exposed to the surface.