The C-O Stretching Infrared Band as a Probe of Hydrogen Bonding in Ethanol-Water and Methanol-Water Mixtures

Infrared spectra of the ethanol-water and methanol-water mixtures in the mole fraction range of 0.1 to 0.9 were recorded in the attenuated reflection (ATR) mode. Traditionally, the hydrogen bonding of water with other molecules has been studied by investigation of the OH stretching band frequencies and intensities of water. However, in the case of alcohol-water mixtures, this procedure presents a problem due to the complete overlap of the hydroxyl absorptions from the alcohol and water. In the present study, we have adopted an alternative approach of understanding the ethanol-water and methanol-water hydrogen bonds through the analysis of the C-O stretching band. The intrinsic high intensity of the C-O band and nearly complete absence of its overlap with the water bands make it a good candidate for the study of hydrogen bonding interactions in alcohol-water mixtures. The integrated areas of the C-O stretching band versus mole fractions were plotted for both mixtures. In the case of methanol-water mixtures, the C-O stretching band area plot was linear, whereas such plot for the ethanol-water mixtures had two distinct slopes that switched at the 0.5 mole fraction. The C-O band plot areas were used to explain the molecular associations in the mixtures studied.