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

Abstract

The structure of clusters in methanol-water solutions in its dependence on the methanol mole fraction cursive Greek chiM 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 cursive Greek chiM, accompanied by two inflection points at cursive Greek chiM = ∼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 < cursive Greek chiM < 0.3 the tetrahedral-like water cluster is the main species, at 0.3 < cursive Greek chiM < 0.7 chain clusters of methanol molecules gradually evolve with increasing methanol content, and finally, at cursive Greek chiM > 0.7 chain clusters of methanol molecules 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.