Monte Carlo simulation of water-pyridine mixtures

Abstract

Thermodynamic properties and radial distribution functions for a water-pyridine mixture as a function of composition were calculated with the Monte Carlo method in the isothermal and isobaric ensemble at T = 298.15K and p = 1.0 atm. The TIP4P model was used for water and optimized potentials for liquid simulations (OPLS) force field parameters were used for pyridine. The effect of including the hydrogen atoms of pyridine was explicitly investigated by comparing the results from calculations with six and eleven site models. The results obtained for the average configurational interaction energy as a function of the mole fraction are in good agreement with experimental data. The partitioning of the total configurational energy of the water-pyridine mixture is presented and compared with similar results from other systems. Comparatively low energy values were obtained for the water-pyridine interaction. This interaction becomes slightly more negative when the hydrogen atoms of pyridine are explicitly considered. The radial distribution functions for the water-pyridine interaction show characteristic features indicating the formation of hydrogen-bonded dimers. The amplitude of the peaks observed for these distribution functions depends on the particular model used to represent the pyridine molecule. Radial distribution functions and molecular graphic representations for water-water interaction indicate the formation of small clusters of water molecules in the bulk of the water-pyridine system. This clustering enhances the water-water coordination numbers and interaction energy. The contribution from solute-solvent electrostatic interaction to the free energy of solvation of water in pure water, pure pyridine and in the equimolar water-pyridine and water-methanol mixtures were calculated using statistical perturbation theory (SPT). The results obtained for the electrostatic contribution to the free energy of solvation, in absolute values, are pyridine < water-pyridine < water-methanol < water. These free energy results indicate the migration of water molecules towards an environment rich in hydrogen bonds and explain the formation of water clusters in the bulk of water-pyridine mixtures.