Thermodynamic properties and dielectric constants in water/methanol mixtures by integral equation theory and molecular dynamics simulations

A dielectrically adjusted version of the RISM equation for the situation of ions in mixed solvents is applied to model mixtures of water with methanol in concentrations covering the complete concentration range. Calculated structures from the integral equation theory are compared to corresponding results from NVT molecular dynamics simulations. The structure calculated from the integral equation is generally in good agreement with the molecular dynamics results. The agreement in calculated energies between integral equation theory and molecular dynamics simulations are also generally good over the entire concentration range for the specific model applied. For the first time this type of integral equation theory is also applied to the direct calculation of free energy. For the specific water model the free energy is lower than the experimental value for pure water. Under-predictions of the structure and corresponding higher entropy for the specific water model might be one of the reasons for these results. The dielectric constant in a uniform liquid mixture can be described in terms of the dipole moments of the molecules involved and the molecular correlation functions for the different molecular pair combinations in the mixture. In this work I reformulate the theory into a corresponding interaction site formulation. The reformulated theory is illustrated for the water/methanol model mixtures.