Theoretical studies of liquids by computer simulations: the methanol-pyridine mixture

Abstract

The Monte Carlo method was used to calculate thermodynamic properties and radial distribution functions for methanol-pyridine mixtures as a function of composition. All calculations were performed in the isothermal and isobaric ensemble at T = 298.15 K and p = 1.0 atm. The optimized potentials for liquid simulations (OPLS) force field was used for pyridine and methanol molecules. In the OPLS force field the methanol molecule is represented by a three-site model, as the methyl group is considered as a united atom site. Two potential models were used for pyridine: a six-site one with carbon and hydrogen atoms represented by united atom sites, and an eleven-site model with all atoms explicitly included. The partial charges used in the eleven-site model for pyridine are derived from ab initio 6–31G∗ wavefunctions. The results obtained for the average configurational energy of the methanol-pyridine system as a function of the mole fraction are in good agreement with experimental data. The partitioning of the total configurational energy shows comparatively weak interaction between methanol and pyridine molecules. The values obtained for the methanol-pyridine interaction energy are slightly more negative when the hydrogen atoms of pyridine are considered explicitly. The radial distribution functions calculated for methanol-pyridine correlation show characteristic features indicating the formation of hydrogen-bonded dimers. The position and amplitude of the peaks observed on these distribution functions depend on the particular model used to represent the pyridine molecules. The coordination numbers obtained for methanol-methanol interaction in the pure liquid and in the equimolar methanol-pyridine mixture are very similar, indicating a small influence of pyridine in the association of methanol molecules. Molecular graphics representation confirms the association of methanol molecules through hydrogen bonding in the bulk of the methanol-pyridine mixture.