Combined Application of High-Field Diffusion NMR and Molecular Dynamics Simulations To Study Dynamics in a Mixture of Carbon Dioxide and an Imidazolium-Based Ionic Liquid

Abstract

Self-diffusion and related short-time dynamic and structural properties were investigated for mixtures of carbon dioxide and the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [bmim](+)[Tf2N](-) for a broad range of carbon dioxide molar fractions and at different temperatures. The studies were performed by a novel multinuclear pulsed field gradient (PFG) NMR technique, which combines the advantages of a high magnetic field (17.6 T) and a high magnetic field gradient (up to 30 T/m), in combination with molecular dynamics simulations. In general, a satisfactory agreement was observed between the experimental and simulation diffusion data. Under all conditions examined, the self-diffusion coefficients of carbon dioxide were found to be approximately an order of magnitude larger than the corresponding self-diffusion coefficients of the ions. It was observed that an increase in temperature and in the amount of carbon dioxide in the ionic liquid led to an increase in the ion self-diffusivities without changing the relationship between the self-diffusion coefficients of the cations and anions. An observation of a slightly higher diffusivity of the cations in comparison to that of the anions is attributed to the preferential mobility of the cations in the direction of the ring plane. The diffusion activation energies of the ions were found to decrease gradually with an increase of the carbon dioxide content in the ionic liquid. The activation energy of the carbon dioxide diffusion in all cases was found to be smaller than those of the ions.