Ethanol Solvation in Water Studied on a Molecular Scale by Photoelectron Spectroscopy

Because of the amphiphilic properties of alcohols, hydrophobic hydration is important in the alcohol–water system. In the present paper we employ X-ray photoelectron spectroscopy (XPS) to investigate the bulk and surface molecular structure of ethanol–water mixtures from 0.2 to 95 mol %. The observed XPS binding energy splitting between the methyl C 1s and hydroxymethyl C 1s groups (BES_[CH3–CH2OH]) as a function of the ethanol molar percentage can be divided into different regions: one below 35 mol % with higher values (about 1.53 eV) and one starting at 60 mol % up to 95 mol % with 1.49 eV as an average value. The chemical shifts agree with previous quantum mechanics/molecular mechanics (QM/MM) calculations [Loytynoja, T.; J. Phys. Chem. B 2014, 118, 13217]. According to these calculations, the BES_[CH3–CH2OH] is related to the number of hydrogen bonds between the ethanol and the surrounding molecules. As the ethanol concentration increases, the average number of hydrogen bonds decreases from 2.5 for wa...