Molecular dynamics study of congruent melting of the equimolar ionic liquid-benzene inclusion crystal...

We use molecular dynamics simulations to study the structure, dynamics, and details of the mechanism of congruent melting of the equimolar mixture of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide with benzene, [emim][NTf(2)]C(6)H(6). Changes in the molecular arrangement, radial distribution functions, and the dynamic behavior of species are used to detect the solid to liquid transition, show an indication of the formation of polar islands by aggregating of the ions in the liquid phase, and characterize the melting process. The predicted enthalpy of melting DeltaH(m)=38+/-2 kJ mol(-1) for the equimolar inclusion mixture at 290 K is in good agreement with the differential 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-square displacement (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 a stoichiometric number of benzene molecules does not affect the nearest neighbor ionic association between [emim](+) and [NTf(2)](-), but increases the MSDs of both cations and anions compared to pure liquid [emim][NTf(2)], showing that second shell ionic associations are weakened. We monitored the rotational motion of the alkyl chain sides of imidazolium cations and also calculated the activation energy for rotation of benzene molecules about their C(6) symmetry axes in their lattice sites prior to melting.