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.