Composition dependence of thermodynamic, dynamic and dielectric properties of water-methanol model mixtures. Molecular dynamics simulation results with the OPLS-AA model for methanol

We have studied thermodynamic and dynamic properties, as well as the dielectric constant, of water-methanol model mixtures in the entire range of composition, by using constant pressure molecular dynamics simulations at room temperature and ambient pressure. The SPC-E and TIP4P-Ew water models are explored in combination with the OPLS-AA (all atom) model of methanol. Changes of the average number of hydrogen bonds between particles of different species and of the fractions of differently bonded molecules are put in correspondence with the evolution of excess mixing properties, of self-diffusion coefficients, reorientational relaxation times and power spectra. We were looking for the existence of correlations between properties of a different kind, in order to find reasons of satisfactory or dissatisfactory performance of the models applied. It seems that any improvement of the description of water-methanol mixtures requires a more sophisticated, carefully parametrized model for methanol and possibly, certain deviations in the combination rules.