Variations of the Hydrogen Bonding and of the Hydrogen Bonded Network in Ethanol-Water Mixtures on Cooling

TL;DR

This study uses molecular dynamics simulations to analyze how hydrogen bonding and network structures in ethanol-water mixtures change with temperature, revealing increased hydrogen bonding and cyclic structures, especially five-membered rings, as temperature decreases.

Contribution

It provides detailed structural insights into ethanol-water mixtures across temperatures, highlighting the dominance of five-membered hydrogen-bonded rings at lower temperatures.

Findings

Hydrogen bonding increases with decreasing temperature.

Number of hydrogen-bonded rings rises as temperature drops.

Five-membered rings dominate over six-membered rings in ethanol-water mixtures.

Abstract

Extensive molecular dynamics computer simulations have been conducted for ethanol-water liquid mixtures in the water-rich side of the composition range, with 10, 20 and 30 mol % of the alcohol, at temperatures between room temperature and the experimental freezing point of the given mixture. All-atom type (OPLS) interatomic potentials have been assumed for ethanol, in combination with two kinds of rigid water models (SPC/E and TIP4P/2005). Both combinations have provided excellent reproductions of the experimental X-ray total structure factors at each temperature; this provided a strong basis for further structural analyses. Beyond partial radial distribution functions, various descriptors of hydrogen bonded assemblies, as well as of the hydrogen bonded network have been determined from the simulated particle configurations. A clear tendency was observed towards that an increasing proportion of water molecules participate in hydrogen bonding with exactly 2 donor- and 2 acceptor sites as temperature decreases. Concerning larger assemblies held together by hydrogen bonding, the main focus was put on the properties of cyclic entities: it was found that, similarly to methanol-water mixtures, the number of hydrogen bonded rings has increased with lowering temperature. However, for ethanol-water mixtures the dominance of not the six-, but of the five-fold rings could be observed.