Distribution of 1-butyl-3-methylimidazolium bistrifluoromethylsulfonimide in mesoporous silica as a function of pore filling

Rotational dynamics of the ionic liquid (IL) 1-butyl-3-methylimidazolium bistrifluoromethylsulfonimide, [C₄mim][Tf₂N], 1, as a neat liquid, and confined in mesoporous silica were investigated by ¹H spin-spin (T₂) and spin-lattice (T₁) relaxation measurements and ¹³C NMR spectroscopy. Translational dynamics (self-diffusion) were monitored via the diffusion coefficient, D, obtained with ¹H pulsed field gradient NMR measurements. These data were used to determine the distribution of 1 in the pores of KIT-6, a mesoporous silica with a bicontinuous gyroid pore structure, as a function of filling fraction. Relaxation studies performed as a function of filling factor and temperature reveal a dynamic heterogeneity in both translational and rotational motions for 1 at filling factors, f = 0.2-1.0 (f = 1 corresponds to fully filled pores). Spin-lattice and spin-spin relaxation times reveal that the motion of 1 in silica mesopores conforms to that expected for a two-dimensional relaxation model. The relaxation dynamics are interpreted using a two-state, fast exchange model for all motions; a slow rotation (and translation) of molecules in contact with the surface and a faster motion approximated by the values for bulk relaxation and diffusion. Compound 1 retains liquid-like behavior at all filling factors and temperatures that extend to ca. 50 degrees below the bulk melting point. Translational motion in these systems, interpreted with MD-simulated diffusivity limits, confirms the high propensity of 1 to form a monolayer film on the silica surface at low filling factors. The attractive interaction of 1 with the surface is greater than that for self-association of 1. The trends in diffusion data at short and long diffusion time suggest that the population of surface-bound 1 is in intimate contact with 1 in the pores. This condition is most easily met at higher filling fractions with successive additions of 1 increasing the layer thickness built up on the surface layer.