Hydrophobic hydration in aqueous acetic acid and formic acid solutions probed by high pressure

This work describes the CH- - -O interactions of acetic acid/D2O and formic acid/D2O mixtures measured as a function of pressure and concentration. The CH stretching frequency of acetic acid shows low sensitivity to dilution with D2O, while the CH vibration absorption was altered into one prominent band at ∼2955 cm−1 as its aqueous solution was compressed. The experimental results are in accord with ab initio calculation predicting a CH stretching band located at ∼2928 cm−1 originated from CH- - -O hydrogen bonding. The pressure was found to strengthen the CH- - -O interactions, while the hydration state in aqueous acetic acid transforms from van-der-Waals-like type to hydrophobic hydration upon pressure applied. Pressure-dependent studies on aqueous formic acid reveal a blueshift in CH stretching frequency analogous to aqueous acetic acid upon the formation of CH- - -O hydrogen bonding. The present study demonstrates that the pressure provides a key to investigate hydrophobic hydration originated from pressure-enhanced CH- - -O interactions.