Limiting diffusion coefficients of ionic liquids in water and methanol: a combined experimental and molecular dynamics study

Abstract

Mutual diffusion coefficients D₁₂ of the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₂MIM][NTf₂]) and [C₄MIM][NTf₂] in highly diluted solutions of water and methanol have been measured at different temperatures between 288 K and 313 K using the Taylor dispersion technique. Tracer diffusion coefficients of the two cations [C₂MIM]⁺ and [C₄MIM]⁺ as well as the anion [NTf₂]⁻ in these solutions have been obtained by molecular dynamics (MD) simulations. For our simulations we used well established force fields for the solvents water and methanol and a recently developed force field for imidazolium-based ionic liquid [C_nMIM][NTf₂]. Mutual diffusion coefficients D₁₂ have been calculated from the tracer diffusion coefficients using the Nernst-Hartley equation strictly valid only at low ionic concentration. The agreement between the diffusion coefficients reported in the literature, the experimental data obtained in this work and the MD results is excellent.