Hydrogen bond connectivities in water-ethanol mixtures: On the influence of the H-bond definition

Simulated particle configurations of water-ethanol liquid mixtures, as a function of the composition (between 0 and 100 mol % of ethanol), have been analyzed, in order to determine connectivities between molecules. The particle arrangements have closely followed the experimental total scattering X-ray structure factors at each concentration. Applying four different geometric definitions for the hydrogen bond, cluster formation has also been studied. The majority of both ethanol and water molecules (i.e., their 'hydroxylic' hydrogen atoms) have an ideal (i.e., two) connectivity. A small ratio of molecules with higher than ideal connectivities was detected for the less strict hydrogen bond conditions. This is an indication that the strictest hydrogen bond condition, setting O-Oo and O-H distance ranges and an O-H-O angle of >150 degrees as criteria, may need to be used for completely excluding irregular hydrogen bonds. On the other hand, for obtaining an at least semi-quantitative picture of the hydrogen bonded network, sensibly chosen distance based criteria are sufficient The majority of ethanol oxygen atoms have been found to have lower than ideal connectivities for all concentrations and HB conditions, whereas for water molecules the majority of oxygen atoms have ideal connectivities. This finding is interpreted in terms of steric hindrance, caused by the (hydrophobic) ethyl group of ethanol molecules. (C) 2016 Elsevier B.V. All rights reserved.