Thermodynamic properties and microstructures of the mixture of N-butylpyridinium tetrafluoroborate with acetonitrile studied by molecular dynamics simulation

Abstract

The binary system of the ionic liquid (IL) N-butylpyridinium tetrafluoroborate ([BPy][BF4]) and acetonitrile (MeCN) was investigated by molecular dynamics simulation method. The thermodynamic properties of the mixture were obtained by simulation including density, self-diffusion coefficients, viscosity, and vaporization enthalpy. This study focused on the relationship between the microscopic structure and the proportion of [BPy][BF4] in the mixed system. Radial distribution functions (RDFs) for different components were also compared. Results showed that the first peak of the nonpolar regions of [BPy][BF4] increased with increasing mole fraction xi of IL, whereas that of polar regions decreased. The coordination number as a function of distance was calculated by integrating RDFs to the first shell. Results indicated that a wide distribution of possible configurations in ion pairs formed by anions and cations of the IL in mixtures. Spatial distribution functions (SDFs) indicated that the anions [BF4]− were mainly distributed around the pyridine ring of the cation [BPy]+ and the methyl of MeCN. Moreover, the excess molar volume was negative and showed a minimum at xi=0.3, consistent with the change law of SDFs.