Molecular dynamics study of congruent melting of the equimolar ionicliquid-benzene inclusion crystal [emirn][NTf_2]·C_6H_6

摘要

We use molecular dynamics simulations to study the structure, dynamics, and details of themechanism of congruent melting of the equimolar mixture of 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl) imide with benzene, [emim][NTf_2]C_6H_6. Changes in the moleculararrangement, radial distribution functions, and the dynamic behavior of species are used to detectthe solid to liquid transition, show an indication of the formation of polar islands by aggregating ofthe ions in the liquid phase, and characterize the melting process. The predicted enthalpy of melting ΔH_m =38 ± 2 kJ mol~(-1)for the equimolar inclusion mixture at 290 K is in good agreement with thedifferential scanning calorimetry experimental results of 42± 2 kJ mol~(-1). The dynamics of the ions and benzene molecules were studied in the solid and liquid states by calculating the mean-squaredisplacement (MSD) and the orientational autocorrelation function. The MSD plots show strong association between ion pairs of the ionic liquid in the inclusion mixture. Indeed, the presence of astoichiometric number of benzene molecules does not affect the nearest neighbor ionic associationbetween [emim]~+and [NTf_2]~-, but increases the MSDs of both cations and anions compared to pure liquid [emim][NTf2], showing that second shell ionic associations are weakened. We monitored therotational motion of the alkyl chain sides of imidazolium cations and also calculated the activation energy for rotation of benzene molecules about their C_6symmetry axes in their lattice sites prior tomelting.