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 xM has been investigated by mass spectrometry on clusters isolated from submicron droplets by adiabatic expansion in vacuum and by X-ray diffraction on the bulk binary solutions. The mass spectra have shown that the average hydration number, (nm), of m-mer methanol clusters decreases with increasing xM , accompanied by two inflection points at xM = ~0.3 and ~0.7. The X-ray diffraction data have revealed a similar change in the number of hydrogen bonds per water and/or methanol oxygen atom at ~2.8 Å. On the basis of both results, most likely models of clusters formed in the binary solutions are proposed: at 0 < xM < 0.3 the tetrahedral-like water cluster is the main species, at 0.3 < xM < 0.7 chain clusters of methanol molecules gradually evolve with increasing methanol content, and finally, at xM > 7 chain clusters of methanol become predominant. The present results are compared 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.