Effects of conformational flexibility of alkyl chains of cations on diffusion of ions in ionic liquids

Abstract

Molecular dynamics simulations of ionic liquids [1-alkyl-3-methylimidazolium (alkyl = ethyl, butyl and hexyl), N-butylpyridinium, N-butyl-N,N,N-trimethylammonium and N-butyl-N-methylpyrrolidinium cations combined with the (CF₃SO₂)₂N⁻ (TFSA) anion] show that the conformational flexibility of the alkyl chains in the cations is one of the important factors determining the diffusion of ions. Artificial constraint imposed on the internal rotation of alkyl chains significantly decreases the self-diffusion coefficients of cations and anions. The internal rotation of the C–N bond connecting the alkyl chain and the aromatic ring has large effects on the diffusion of ions in imidazolium and pyridinium based ionic liquids. The calculated self-diffusion coefficients of cations and anions decrease 20–40% by imposing the torsional constraint of the C–N bond. On the other hand the torsional constraint of the C–N bond does not largely change the diffusion of ions in the quaternary alkyl ammonium based ionic liquids. The conformational flexibility of the terminal C–C–C–C bond of the alkyl chains has large effects on the diffusion of ions in the quaternary alkyl ammonium based ionic liquids. The influence of the electrostatic interactions and the high density of ionic liquids on the diffusion of ions were studied. The electrostatic interactions have the paramount importance on the slow diffusion of ions in ionic liquids, while the high density of ionic liquids is also responsible for the slow diffusion. The electrostatic interactions and the high density of ionic liquids enhance the effects of the torsional constraint on the diffusion of ions, which suggests that the charge-ordering structure and small free volume originated in the strong electrostatic interactions are the causes of the significant effects of the conformational flexibility on the diffusion of ions in ionic liquids.