Study of thermodynamic properties of imidazolium-based ionic liquids and investigation of the alkyl chain length effect by molecular dynamics simulation

Abstract

In this paper, structural and dynamical properties of five imidazolium-based ionic liquids (ILs) [amim]Br (a = methyl, ethyl, butyl, pentyl, hexyl) were studied by molecular dynamics simulations. United atom force field (UAFF) has been used for the representation of the interaction between ions. Good agreement with experimental data was obtained for the simulated density based on the UAFF. The calculated densities gradually decrease with an increase in the length of alkyl side chain, which is a result of weakening the electrostatic interaction between ions. The simulated heats of vaporisation are higher than that of non-ILs and decrease with an increase in temperature. Radial distribution function (RDF) was employed to analyse the local structure of ILs. Cation–anion RDFs show that the anions are well organised around the cation in two shells (0.41 and 0.6 nm). The velocity autocorrelation functions of the anion and cations show that the relaxation time increased with an increase in the length of the alkyl side chain. The diffusion coefficients of ions were calculated by mean square displacement of the centre of mass of the ions at 400 K. The calculated diffusion coefficients using UAFF agree well with other all atom force fields. Also diffusion coefficients decrease with an increase in the length of the alkyl side chain. The calculated transference numbers show that the cation contributes more than anion in the electrical current. The diffusion coefficients increase with temperature.