Surface Studies of Aqueous Methanol Solutions by Vibrational Broad Bandwidth Sum Frequency Generation Spectroscopy

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Abstract

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 hydrogen-bonding acceptor when the methanol molecule resides in the interfacial region.

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