Water Structure at Air/Acetonitrile Aqueous Solution Interfaces

How is interfacial water organized beneath an acetonitrile monolayer at the air/acetonitrile aqueous solution interface? The method of vibrational sum frequency generation was used to address these issues. It was found that the different water structures expressed at different vibrational spectral regions, i.e. the “ice-like”, the “liquid-like”, and the non-hydrogen bonded “free” OD, responded differently as the acetonitrile bulk concentration was increased. At the bulk acetonitrile concentration that was sufficient to form an acetonitrile monolayer and at higher bulk concentrations, it was observed that there are no longer vibrational resonances that correspond to the oriented collective water vibrations, i.e. the “ice-like” and “liquid-like” structures of water as observed at the air/water interface. The story was different in the free OD vibrational region where a red-shifted resonance was found at acetonitrile concentrations sufficient to form an acetonitrile monolayer. Speculations on the origin of this resonance are presented.