Structure of clusters in methanol-water binary solutions studied by mass spectrometry and X-ray diffraction

The structure of clusters in methanol-water solutions in its dependence on the methanol mole fraction x(M) has been investigated by mass spectrometry on clusters isolated from submicron droplets by adiabatic expansion in vacu um and by X-ray diffraction on the bulk binary solutions. The mass spectra have shown that the average hydration number, [n(m)], of m-mer methanol clu sters decreases with increasing x(M), accompanied by two inflection points at x(M) = similar to 0.3 and similar to 0.7. The X-ray diffraction data hav e revealed a similar change in the number of hydrogen bonds per water and/o r methanol oxygen atom at similar to 2.8 Angstrom. On the basis of both res ults, most likely models of clusters formed in the binary solutions are pro posed: at 0 < x(M) < 0.3 the tetrahedral-like water cluster is the main spe cies, at 0.3 < x(M) < 0.7 chain clusters of methanol molecules gradually ev olve with increasing methanol content, and finally, at x(M) > 0.7 chain clu sters of methanol molecules become predominant. The present results are com pared with clusters previously found in ethanol-water binary solutions and discussed in relation to anomalies of the heat of mixing of methanol-water binary solutions.