Atomistic simulations of the solid-liquid transition of 1-ethyl-3-methyl imidazolium bromide ionic liquid

Abstract

Achieving melting point around room temperature is important for applications of ionic liquids. In this work, molecular dynamics simulations are carried out to investigate the solid-liquid transition of ionic liquid 1-ethyl-3-methyl imidazolium bromide ([emim]Br) by direct heating, hysteresis, void-nucleation, sandwich, and microcanonical ensemble approaches. Variations of the non-bonded energy, density, diffusion coefficient, and translational order parameter of [emim]Br are analyzed as a function of temperature, and a coexisting solid-liquid system is achieved in the microcanonical ensemble method. The melting points obtained from the first three methods are 547 ± 8 K, 429 ± 8 K, and 370 ± 6 K; while for the sandwich method, the melting points are 403 ± 4 K when merging along the x-axis by anisotropic isothermal-isobaric (NPT) ensemble, 393 ± 4 K when along the y-axis by anisotropic NPT ensemble, and 375 ± 4 K when along the y-axis by isotropic NPT ensemble. For microcanonical ensemble method, when the slabs are merging along different directions (x-axis, y-axis, and z-axis), the melting points are 364 ± 3 K, 365 ± 3 K, and 367 ± 3 K, respectively, the melting points we get by different methods are approximately 55.4%, 21.9%, 5.1%, 14.5%, 11.6%, 6.5%, 3.4%, 3.7%, and 4.3% higher than the experimental value of 352 K. The advantages and disadvantages of each method are discussed. The void-nucleation and microcanonical ensemble methods are most favorable for predicting the solid-liquid transition.