Thermodynamics and structure of the {water + methanol} system viewed from three simple additive pair-wise intermolecular potentials based on the rigid molecule approximation

The ability of three additive pair-wise intermolecular potentials to reproduce both thermodynamics and structure of the {methanol + water} system is analysed in detail using Metropolis Monte Carlo simulations. The three potentials were constructed using the OPLS model for methanol and for water, in each case, were used the TIP4P, TIP4P/2005, and TIP4P/ice, respectively. For all the potentials, the Lorentz–Berthelot combining rule was considered to calculate water–methanol cross interactions. A wide set of first- and second-order excess thermodynamic derivatives and the site–site radial distribution functions were chosen for the study. The properties were obtained at room conditions over the whole composition range and were critically compared with selected experimental data. It turns out that the simulation results of the different potentials show no qualitative differences in the radial distribution functions whereas that the excess thermodynamic properties usually decreases in the sense TIP4P > TIP4P/2005 > TIP4P/ice. The best agreement with the experiments has been found for the potential that uses the TIP4P/2005 model which demonstrates that using potentials based on phase diagram calculations significantly improve the results for this mixture. However, even in this case only partial success was found. As regards thermodynamics, it has been detected problems to describe the excess compressibility and the excess molar enthalpy. As regards structure, although it predicts clustering of methanol molecules via methyl groups after mixing, it was unable to reproduce the preservation of the pure water structure in the mixture, a well-established phenomenon observed by neutron scattering experiments.