Abstract
Methanol-water liquid mixtures have been investigated by high-energy synchrotron X-ray and neutron diffraction at low temperatures. We are thus able to report the first complete sets of both X-ray and neutron weighted total scattering structure factors over the entire composition range (at 12 different methanol concentrations (xM) from 10 to 100 mol%) and at temperatures from ambient down to the freezing points of the mixtures. The new diffraction data may later be used as reference in future theoretical and simulation studies. The measured data are interpreted by molecular dynamics simulations, in which the all atom OPLS/AA force field model for methanol is combined with both the SPC/E and TIP4P/2005 water potentials. Although the TIP4P/2005 water model was found to be somewhat more successful, both combinations provide at least semi-quantitative agreement with measured diffraction data. From the simulated particle configurations, partial radial distribution functions, as well as various distributions of the number of hydrogen bonds have been determined. As a general trend, the average number of hydrogen bonds increases upon cooling. However, the number of hydrogen bonds between methanol molecules slightly decreases with lowering temperatures in the concentration range between ca. 30 and 60 mol% alcohol content. The same is valid for water-water hydrogen bonds above 70 mol% of methanol content, from room temperature down to 193 K.
Highlights
Alcohol-water mixtures have been, and still are, continuously in the center of attention in the physical and chemical sciences
In aqueous solutions of methanol, both methanol and water can act as H-donor and H-acceptor, and hydrogen bonded networks can be constructed via water-water, methanol-methanol and methanol-water hydrogen bonded pairs
At room temperature the height of the 2nd peak is decreasing with increasing methanol concentration, and only a shoulder can be observed for the xM ≥ 0.8 mixtures
Summary
Alcohol-water mixtures have been, and still are, continuously in the center of attention in the physical and chemical sciences (for very recent works, see e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]). Interpretation of the X-ray data suggested that the structure of dominant clusters formed in the mixtures at 298 K do not significantly change with lowering the temperatures. To extend the investigations to equimolar and methanol-rich mixtures would be desirable, but a complete diffraction data set over the whole range of methanol concentration at temperatures down to the freezing point was missing. We provide additional experimental data on methanol-water liquid mixtures for 12 methanol concentrations, from xM = 0.1 to 1.0 Both X-ray and neutron diffraction total scattering structure factors have been determined at 1 bar and several temperatures, from 300 K down to the freezing points of the mixtures. Various distributions of the number of hydrogen bonds have been calculated
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