Hydration of Liquid Ethanol Probed by Raman Spectra

In order to study the concentration dependence of the molecular interactions in ethanol-water hydrogen bonded system, Raman spectra of ethanol-water mixtures with different water contents were obtained at room temperature. It was found that the positions of the 3 C-H stretching vibration bands of ethanol molecule located in the range of 2 800 similar to 3 050 cm(-1) would generally present blue shifts when more water was added into the mixture; at the same time, however, the C-O stretching vibration band located at around 1 048 cm(-1) showed an opposite behavior. The different hydration, which is induced by hydrogen bonding, which happened in different concentration mixtures, was thought to be responsible for this situation, the hydration process of liquid ethanol was thus deduced: when adding a small amount of water into pure ethanol, and clusters containing one water molecule and several ethanol molecules were formed instead of ethanol self-association short chain clusters existing in pure ethanol; the clusters would combine more water molecules to form ringlike clusters through hydrogen bond association when adding more water into the mixture, then a temporary saturation would be seen when the volume percent of water reached 50%, and this saturation state would last until the water content reached 70 vol%; after that, the large number of water molecules would dissociate the ringlike clusters to smaller clusters and then associate to the ends of these dissociated clusters through hydrogen bonding; in addition, the improper hydrogen bonding between oxygen atom of water molecule and C-H bond of ethanol molecule is considered to be formed after the content of water reached a high value.