Pressure-Dependent Confinement Effect of Ionic Liquids in Porous Silica

Abstract

The effect of confining ionic liquids (ILs) such as 1-ethyl-3-methylimidazolium tetrafluoroborate [C2C1Im][BF4] or 1-butyl-3-methylimidazolium tetrafluoroborate [C4C1Im][BF4] in silica matrices was investigated by high-pressure IR spectroscopy. The samples were prepared via the sol-gel method, and the pressure-dependent changes in the C–H absorption bands were investigated. No appreciable changes were observed in the spectral features when the ILs were confined in silica matrices under ambient pressure. That is, the infrared measurements obtained under ambient pressure were not sufficient to detect the interfacial interactions between the ILs and the porous silica. However, dramatic differences were observed in the spectral features of [C2C1Im][BF4] and [C4C1Im][BF4] in silica matrices under the conditions of high pressures. The surfaces of porous silica appeared to weaken the cation-anion interactions caused by pressure-enhanced interfacial IL-silica interactions. This confinement effect under high pressures was less obvious for [C4C1Im][BF4]. The size of the cations appeared to play a prominent role in the IL-silica systems.

Pressure-Dependent Stability of Imidazolium-Based Ionic Liquid/DNA Materials Investigated by High-Pressure Infrared Spectroscopy

“…In comparison with pure [C 4 MIM][PF 6 ] at the pressure of 0.4 GPa (Figure 2b), the C 2 -H and C 4,5 -H absorption bands of [C 4 MIM][PF 6 ]/DNA mixture at 0.4 GPa (Figure 3b) show smaller frequency shifts under compression. Previous studies [26][27][28] revealed that high pressure can enhance the cluster structure interaction and lead to a band frequency shift. The hydrogen-bond network of an IL cluster structure may be disrupted, as some added molecules (such as DNA) may disturb the associations of ILs or cut large aggregations to small pieces.…”

“…Figure 1a shows two imidazolium peaks at 3124 and 3169 cm −1 , which correspond to the vibrational absorption bands of C 2 -H and C 4,5 -H on the imidazolium ring, respectively [26][27][28]. The other three bands in the region of 2850-3000 cm −1 in Figure 1a are assigned to the alkyl C-H vibrational modes on the cation tail of pure [26][27][28]. The curve fitting and deconvolution of pure ionic liquids spectra were performed with Lorentzian peaks.…”

“…(Figure 4A,B) show drastic blue shifts in the pressure range from ambient to 0.7 GPa and mild blue shifts in the pressure range from 0.7 to 2.5 GPa. Previous studies indicated [26][27][28] that high pressure can easily shorten the imidazolium C-H bonds by the enhancement of weak C-H hydrogen bonding, which leads to a blue shift. Blue shifts of the pure [C 4 MIM][PF 6 ] C-H stretching band in Figure 4A (Figure 4A,B) Figure 4C show blue shifts at the pressure increase from ambient to 0.7 GPa and slight frequency shifts in the pressure range from 0.7 to 2.5 GPa.…”

“…The ILs that can spontaneously self-assemble and specifically attach to groove and backbone structures allow to skillfully control the DNA structure [19,20,25].High pressure allows to force ions or molecules into close-range to enhance the interactions between them, which makes it a useful and effective technique. Previous studies [26][27][28] revealed that pure ILs have an anisotropic cluster structure, and the hydrogen-bonding network can be easily disturbed by adding various materials under high-pressure conditions. Deemyad et al [29][30][31] concluded that pressure can induce superconductivity and electronic properties modification of Cu 2 I 2 Se 6 by a structural change.…”

“…For example, IL-β-cyclodextrin (IL-β-CD) associated forms under low pressures can be turned to dissociated structures with the pressure increase [26]. The studies of pressure-dependent confinement effect of ILs indicated that interfacial IL-silica interactions tend to weaken the cation-anion interactions under high pressure [27]. In this study, we demonstrate that high pressure may provide a unique insight into the stabilization of DNA-IL associations and key interactions between DNA and IL.…”