Investigation of Dynamic Behavior of Confined Ionic Liquid [BMIM]+[TCM]- in Silica Material SBA-15 Using NMR

Abstract

The molecular dynamics of 1-butyl-3-methyl imidazolium tricyanomethanide ionic liquid [BMIM]⁺[TCM]^- confined in SBA-15 mesoporous silica were examined using ¹H NMR spin-lattice (T₁) relaxation and diffusion measurements. An extensive temperature range (100 K-400 K) was considered in order to study both the liquid and glassy states. The hydrogen dynamics in the two states and the self-diffusion coefficients of the cation [BMIM]⁺ above the glass transition temperature were extracted from the experimental data. The results were then compared to the corresponding bulk substance. The effects of confinement on the dynamic properties of the ionic liquid clearly manifest themselves in both temperature regimes. In the high-temperature liquid state, the mobility of the confined cations reduces significantly compared to the bulk; interestingly, confinement drives the ionic liquid to the glassy state at a higher temperature T_g than the bulk ionic liquid, whereas an unusual T₁ temperature dependence is observed in the high-temperature regime, assigned to the interaction of the ionic liquid with the silica-OH species.

Keywords: NMR; confined ionic liquids; mesoporous silica SBA-15; relaxation and diffusion.

Figure A1

Results of the derivatives of the heat flow in the relevant Differential Scanning Calorimetry (DSC) experiments. As observed, there is a difference of approximately 4.1 K between the two glass transition temperatures (Tg for the bulk IL is 186.4 K and Tg for the confined IL inside SBA-15 is 190.5 K).

Figure A2

Replicate of the data shown in Figure 2 of the main text. Here, spin-relaxation times are shown versus reciprocal of temperature for both the bulk and confined ionic liquids. The lines are fit using the BPP model (refer to the main text for details). The two vertical dashed lines are the glass transition temperatures for the bulk (Tg = 186.4 K) and confined ILs (Tg = 190.5 K) obtained using the DSC technique (shown in Figure A1). The inset of the figure shows the spin-spin relaxation times $T_2$ vs. T.

Figure 1

Left: A single molecule of the ionic liquid [BMIM]⁺[TCM]⁻ used in this study. The [BMIM]⁺ cation is the 1-butyl-3-methylimidazolium (C₈H₁₅N₂) molecule, and the [TCM]⁻ is the tricyanomethanide (C₄N₃) molecule. Right: The molecular arrangement of a cluster of IL molecules optimized using the ORCA software package [44,45]. Blue, grey, and white circles are nitrogen, carbon, and hydrogen atoms, respectively.

Figure 2

Spin-lattice T₁ relaxation time as a function of temperature for the bulk IL and confined IL in SBA-15. The two vertical dashed lines are the glass transition temperatures for the bulk ($T_g=186.4 K$) and confined ILs ($T_g=190.5 K$), obtained using the DSC technique. The arrows indicate the temperatures at which T₁ minima occur above T_g (Region I). The solid curves are the theoretical fits to the experimental data using the BPP model (refer to the text). The inset of the figure shows the spin-spin relaxation time $T_2$ versus temperature.

Figure 3

(a,c) 2D ¹H NMR D-T_2eff contour plots of the bulk ionic liquid and confined [BMIM]⁺[TCM]⁻ ionic liquid in SBA-15 at 292 K. (b,d) Distribution of the T_2eff values obtained from the 2D contour plots.

Figure 4

¹H NMR SED plots of the bulk ionic liquid and confined [BMIM]⁺[TCM]⁻ ionic liquid in SBA-15 at room temperature and 400 K at the Larmor frequency of 100 MHz and in a constant magnetic field gradient of G = 34.7 Tesla/m. In the case of a homogeneous diffusion process with a single self-diffusion coefficient D, the spin echoes decay according to the law M/M0 = exp(−D α), where α = (2/3)γ²G²τ³.

Figure 5

Self-diffusion coefficients D as a function of the inverse temperature obtained from ¹H NMR measurements for the bulk ionic liquid and confined [BMIM]⁺[TCM]⁻ ionic liquid in SBA-15. The black and red lines are the fit curves obtained using the VFT law. The dashed lines are the relevant Arrhenius curves.