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

Abstract

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 alcohol, at temperatures between room temperature and the experimental freezing point of the given mixture. All-atom-type (optimized potential for liquid simulations) 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 yielded 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. A clear tendency was observed toward that an increasing proportion of water molecules participate in hydrogen bonding with exactly two donor and two 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 5-fold rings, and not 6-fold rings, could be observed.

Molecular Dynamics Simulation Studies of the Temperature-Dependent Structure and Dynamics of Isopropanol–Water Liquid Mixtures at Low Alcohol Content

“…Agreement with experiment is rather good for both water potential models applied: for the TIP4P/2005 model 44 , the match is nearly quantitative. Contrary to what was found for ethanol-water mixtures in our earlier work 26 , here this water potential works significantly better and therefore, in what follows, all results are shown for these calculations only. Fig.1 shows TSSF-s only for one composition, x ip =0.2 (i.e., 20 mol % isopropanol); graphs for the other compositions, as well as corresponding numerical data describing deviations between experiment and simulation, are provided in the Supporting Information.…”

“…In the case of pure water, the ring-size distribution has a well-defined maximum around 6 and the number of cyclic entities is significantly increasing as the temperature is decreasing 37 . A similar tendency was found in water-methanol mixtures as a function of temperature 15 , and for ethanol water-mixtures below the alcohol concentration of x e =0.1 26 .…”

“…Here, the primitive rings of oxygen atom nodes and hydrogen bond edges were sought. This method has been already used for investigating the topology of H-bonded clusters in pure water, and in water-methanol, waterethanol and water-formamide mixtures 25,26,38 . Ring size distributions (not normalized) for the three mixtures (left panel: x ip = 0.05; middle panel: x ip = 0.1; right panel: x ip = 0.2) are presented in Fig.…”

“…Recently, the temperature dependent structure of methanol/water 25 and ethanol/water 26 liquid mixtures, as well as the microscopic dynamics 27 in ethanol/water solutions has been investigated extensively in water-rich mixtures, using molecular dynamics (MD) simulations. The basis for detailed discussions of structure and dynamics has been provided by very good agreements with measured 15 X-ray diffraction data.…”

“…2-propanol, or propan-2-ol), again, at low alcohol concentrations (between molar ratios, x ip , of 0.05 and 0.2). Since findings of our previous investigations [25][26][27] have proven to be rather thought-provoking, we have decided to follow on with a detailed T-dependent MD study on isopropanol/water mixtures.…”