Molecular Structure of Alcohol-Water Mixtures

We use x-ray emission spectroscopy to elucidate the molecular structure of liquid methanol, water, and methanol-water solutions. We find that molecules in the pure liquid methanol predominantly persist as hydrogen-bonded chains and rings with six and/or eight molecules of equal abundance. For water-methanol solutions we find evidence of incomplete mixing at the microscopic level. Our results provide a new explanation for a smaller entropy increase in the solution due to water molecules bridging methanol chains to form rings.

Figure 1

Left: Experimental oxygen XE spectra of methanol recorded at 534.5 (A), 537 (B), and 560 eV (C) excitation energies in comparison with theoretical XE spectra of different structures: 6-,8-chains and rings. The XA spectrum of methanol is shown in the inset. Right: Theoretical XE spectra of a series of methanol chains of different lengths.

Figure 2

The 537 (B) and 560 eV excited (C) XE spectra of liquid water, methanol, and their equimolar mixture. The linear combination of the two former ($I_M+I_W$) reproduces the latter almost in detail (inset). Note, however, a significant difference regarding the $E_1^{\prime }$ feature (marked D), indicating a depletion of methanol chains in the solution.

Figure 3

A new prepeak appears at 532.5 eV in the XA spectrum of the water-methanol mixture (inset). The main emission peak becomes narrower in the XE spectra resonantly excited at 532.5, 531.5, and 530.6 eV. The depleted intensity at 525 eV and the opposite shift of emission peaks to the elastic peak suggest the existence of new chemical species.

Figure 4

The experimental resonant XE spectra are in good agreement with the prediction for the water clusters associated with methanol 6- and 8-chain configurations.