Dynamical properties of EMIM-SCN confined in a SiO₂ matrix by means of ¹H NMR relaxometry

Abstract

H nuclear magnetic resonance relaxometry is applied to investigate the translational and rotational dynamics of ionogels composed of an ionic liquid (IL): 1-ethyl-3-methyl-imidazolium-thiocyanate (EMIM-SCN) confined in a nanoporous SiO matrix. The relaxation studies were performed in the frequency range of 4 kHz-40 MHz and the temperature range of 223-248 K for different concentrations of the IL; the ratio (no. of moles of IL/no. of moles of SiO) yields: 1/2, 3/5 and 7/10. A thorough analysis of this large set of experimental data was performed assuming the existence of two fractions of the liquid: a core fraction (near the pore center) and a surface fraction (near the confining walls). It was shown for all concentrations that the confinement does not significantly affect the translational motion near the pore center compared to the dynamics in bulk. The diffusion coefficients in the surface fraction are considerably smaller compared to the core fraction (from one to two orders of magnitude) and the difference becomes larger with increasing temperature. The diffusion coefficients become smaller for higher concentrations - this effect is not large, but visible. Very importantly, it was shown that, despite the interactions with the surface, the diffusion in the surface fraction remains of 3D character. As far as rotational dynamics in the surface fraction is concerned, it slows down compared to the bulk (and the core fraction), but this effect is of the order of factor 2-3.

Dynamics of Ionic Liquids in Confinement by Means of NMR Relaxometry—EMIM-FSI in a Silica Matrix as an Example

“…This implies that one is able to probe, in a single experiment, dynamical processes on the time scales from ms tons [ 10 , 12 , 13 ]. Moreover, the shape of the relaxation dispersion profile (spin–lattice relaxation rate versus the resonance frequency) unambiguously reveals the mechanism of motion [ 14 , 15 , 16 , 17 ], also allowing to differentiate between the translation diffusion pathways of different dimensionality: 3D, 2D, 1D [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 ]. Relaxation rates are given as linear combinations of spectral density functions being Fourier transforms of the corresponding time correlation functions, characterising the motion associated with the relaxation process [ 3 , 4 , 5 , 6 , 7 ].…”

Direct Correlation between Adsorption Energetics and Nuclear Spin Relaxation in a Liquid‐saturated Catalyst Material

“…For instance, Jonas and co‐workers used a series of porous silicas to explore the influence of surface interactions and confinement effects on the relaxation characteristics of different wetting and non‐wetting molecular liquids . Both fixed field, and field‐cycling, relaxation measurements have been applied to the study of silicas saturated with ionic liquids (so‐called ionogels), and Krzyżak et al . used , and experiments to characterise and compare the relaxation behaviour of water in the ordered mesoporous silicas SBA‐15 and MCM‐14 at different loadings .…”

Reorientation dynamics and ion diffusivity of neat dimethylimidazolium dimethylphosphate probed by NMR spectroscopy

“…48,86 We wish to point out that eld-cycling NMR relaxometry is successfully employed to investigate the dynamics of ILs recently. 26,45,47,63,[87][88][89][90][91][92] Although from a theoretical point of view similar in size, the [C 1 C 1 IM] + cation and the [(CH 3 ) 2 PO 4 ] À anion reveal different reorientation mobilities and diffusivities. The [(CH 3 ) 2 PO 4 ] À anion shows a three to ve times faster reorientation at room temperature compared to the cation.…”