Surface studies of aqueous methanol solutions by vibrational broad bandwidth sum frequency generation spectroscopy

Neat methanol (CH3OH) and aqueous methanol solutions were investigated using broad bandwidth sum frequency generation (BBSFG), Raman spectroscopy, and FTIR spectroscopy. BBSFG results indicate a net orientation of methanol molecules at the air−liquid interfaces of neat methanol and aqueous methanol solutions. However, as the methanol concentration increases above 0.57 methanol mole fraction the methanol molecules become less ordered, i.e., a larger distribution of orientation angles of the CH3 transition moment about the surface normal exists. This was further verified by the BBSFG results from partially deuterated methanol (CD3OH) aqueous solutions. A red-shift of the CH3 symmetric stretch (CH3−SS) frequency with the increase of methanol concentration observed by BBSFG, Raman, and FTIR spectroscopy suggests a changing hydrogen-bonding configuration between the methanol and the water molecules at the surface and in the bulk. Moreover, the surface studies strongly suggest that methanol is a more efficient ...