Microscopic structure of methanol-water mixtures: Synchrotron X-ray diffraction experiments and molecular dynamics simulations over the entire composition range

A new set of synchrotron X-ray diffraction data has been measured for water-methanol mixtures, at ambient temperature and pressure, over the entire composition range (methanol content: 0, 20, 40, 60, 70, 80 and 100%). Molecular Dynamics (MD) computer simulation has been used for the interpretation of diffraction data via calculating the total scattering structure factor from the pair distribution functions in the isothermic-isobaric ensemble. It is found that the combination of either the SPC/E or the TIP4P/Ew water models and the OPLS/AA (all atom) methanol force field yields an at least semi-quantitative agreement with measured structure factors for each composition. Based on this finding, the sets of particle coordinates obtained from MD have been exploited for calculating fine details of the structure in terms of the partial radial distribution functions and angular distributions. It is shown that while angular distributions calculated for the first (predominantly, H-bonded) neighbors around methanol molecules vary significantly with the composition, water-centered distribution hardly change with the alcohol concentration. Analysis of the hydrogen bonding between molecules of both species is performed. It shows that both methanol and water molecules prefer water as hydrogen bonded neighbor.