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

Abstract

1H and 19F spin–lattice relaxation studies for 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide in bulk and mesoporous MCM-41 silica matrix confinement were performed under varying temperatures in a broad range of magnetic fields, corresponding to 1H resonance frequency from 5Hz to 30MHz.A thorough analysis of the relaxation data revealed a three-dimensional translation diffusion of the ions in the bulk liquid and two-dimensional diffusion in the vicinity of the confining walls in the confinement. Parameters describing the translation dynamics were determined and compared. The rotational motion of both kinds of ions in the confinement was described by two correlation times that might be attributed to anisotropic reorientation of these species.

Correlated Dynamics in Ionic Liquids by Means of NMR Relaxometry: Butyltriethylammonium bis(Trifluoromethanesulfonyl)imide as an Example

“…These interactions fluctuate in time as a result of rotational dynamics of the ions. Consequently, the relaxation contributions R intra 1,H (ω H ) and R intra 1,F (ω F ) can be expressed as [6,8,9,12,13,21]:…”

“…The inter-ionic dipole-dipole couplings fluctuate in time due to the relative translation diffusion of the interaction ions. The relaxation contributions R inter,HH 1,H (ω H ) and R inter,FF 1,F (ω F ) originate from 1 H-1 H (cationcation) and 19 F-19 F (anion-anion) interactions and can be expressed as [6,9,12,21,31,32]:…”

“…Equations ( 1) and ( 2) clearly show that the 1 H and 19 F relaxation processes are not independent, and in this sense, they are both affected by the 1 H-19 F (cation-anion) interactions. The corresponding relaxation contributions can be expressed as [6,12]:…”

“…The second question concerns diffusion paths, especially the dimensionality of the diffusion process. For ionic liquids in bulk, the translation diffusion is isotropic (three-dimensional) [6][7][8][9][10][11], but for ionic liquids in confinement (e.g., ionogels), one can expect geometrical restrictions that reduce the dimensionality of the motion [12,13].…”

“…Consequently, one can probe in a single experiment molecular (ionic) motion on the time scale from ms to ns. NMR relaxometry not only gives access to the value of the diffusion coefficients but also allows identification of the mechanism (dimensionality) of the motion [12,13,[17][18][19][20][21].…”