Measurement of Two Solvation Regimes in Water-Ethanol Mixtures Using X-Ray Compton Scattering

Water-ethanol mixtures exhibit interesting anomalies in their macroscopic properties. Despite a lot of research, the origin of the anomalies and the microscopic structure itself is still far from completely known. We have utilized the synchrotron x-ray Compton scattering technique to elucidate the structure of aqueous ethanol from a new experimental perspective. The technique is uniquely sensitive to the local molecular geometries at the angstrom and subangstrom scales. The experiments reveal two distinct mixing regimes in terms of geometry: the dilute 5 mol % and the concentrated $>15\mathrm{mol}\%$ regimes. By comparing with pure liquids, the former regime is characterized by an intramolecular and the latter by an intermolecular change. The findings bring new light to evaluating the hypothesis of formation of clathratelike structures at the dilute concentrations.

Figure 1

Example of a configuration (all the molecules not shown) extracted from the molecular dynamics simulation. Large arrows (blue) point to the center of the cluster and indicate the direction along which the molecules were moved for simulating a density increase in the mixture. Small arrows (gray) indicate O-H bond lengths that were modified to simulate intramolecular changes.

Figure 2

(a) Changes in the Compton profile when all the intramolecular O-H bond lengths are increased by 0.003 Å (blue solid line) and one bond [O-H of water or ethanol (red), C-O (cyan), C-C (green), or C-H (blue)] in a cluster increased by 0.01 Å (latter multiplied by 3 for better clarity). (b) Changes in the Compton profile when the density increases: intermolecular O…O distances decreased by 1% and 2% (for 15% mixture) and 2% and 3% (for 95% mixture). For both concentrations, larger features arise for larger contractions.

Figure 3

Experimental Compton profile differences from Eq. (2). The experimental data are given by red crosses (×) and blue circles (◯). The molar percentages of ethanol have been indicated, and the data have been shifted for clarity. The black dashed curves are model calculations from modified MD clusters. Uppermost curve: O-H bonds elongated by 0.003 Å in the 5% mixtures; middle curve: O…O distances decreased by 1% in the 15% mixtures; lowermost three curves: O...O distances decreased by 1%, 2%, and 3% in the 95% mixtures.