HYDROGEN-BONDING CLUSTER FORMATION AND HYDROPHOBIC SOLUTE ASSOCIATION IN AQUEOUS-SOLUTION OF ETHANOL

Hydrogen bonding (HE) cluster formation of ethanol and water molecules in diluted aqueous solutions (x(A) less than or equal to 0.03) is investigated on the bases of infrared (LR) absorption spectroscopy, mass spectrometric analyses of the clusters, and X-ray diffraction measurement. With increasing alcohol molar fraction (x(A)) up to 0.03, IR spectra showed the intensity decrease on the high-frequency side of the O-H stretching band of water. The s.i(s) curve at x(A) = 0.02 showed striking resemblance to the reported pure water curve at 1 kbar. The X-ray radial distribution function exhibited the decrease of linear hydrogen bonds (LHB) at 2.85 Angstrom and the new peaks at 3.3 and 3.8 Angstrom. The new peaks correspond to the O-O distance of an angular hydrogen bond and the distance from an ethyl carbon to a water oxygen of the hydrogen-bonding cage, respectively. Mass spectra of the clusters revealed the dimerization of ethanol in a very diluted solution (x(A) approximate to 0.001) at 35 degrees C. The clusters isolated from an x(A) = 0.02 solution were composed of water and ethanol molecules with an average molecular number ratio (water / ethanol) of 2 (+/- 1). This indicates that the solute-solute association is highly preferable even at such a diluted concentration. A hydrophobic core structure composed of coherent ethyl groups with a strong water hydrogen-bonding cage is presented to explain the observed results.