The dynamic behavior of a liquid ethanol-water mixture: A perspective from quantum chemical topology

Quantum Chemical Topology (QCT) is used to reveal the dynamics of atom-atom interactions in a liquid. A molecular dynamics simulation was carried out on an ethanol-water liquid mixture at its azeotropic concentration (X _ethanol = 0.899), using high-rank multipolar electrostatics. A thousand (ethanol)₉-water heterodecamers, respecting the water-ethanol ratio of the azeotropic mixture, were extracted from the simulation. Ab initio electron densities were computed at the B3LYP/6-31+G(d) level for these molecular clusters. A video shows the dynamical behavior of a pattern of bond critical points and atomic interaction lines, fluctuating over 1 ns. A bond critical point distribution revealed the fluctuating behavior of water and ethanol molecules in terms of O-H⋯O, C-H⋯O and H⋯H interactions. Interestingly, the water molecule formed one to six C-H⋯O and one to four O-H⋯O interactions as a proton acceptor. We found that the more localized a dynamical bond critical point distribution, the higher the average electron density at its bond critical points. The formation of multiple C-H⋯O interactions affected the shape of the oxygen basin of the water molecule, which is shown in three dimensions. The hydrogen atoms of water strongly preferred to form H⋯H interactions with ethanol's alkyl hydrogen atoms over its hydroxyl hydrogen.