The validity of high pressure IR for detecting the interactions between β-cyclodextrin and imidazolium based ionic liquids

Abstract

The effects of alkyl side-chain length on the interactions between imidazolium ionic liquids (ILs) and β-cyclodextrin (β-CD) were studied at ambient pressure and up to 2.5GPa. No noticeable changes in the C-H band frequencies of 1-methyl-3-propylimidazolium hexafluorophosphate ([MPIM][PF6]) were observed upon β-CD addition under ambient pressure. Nevertheless, β-CD addition inhibited the formation of the [MPIM][PF6] aggregated structure under pressures of ≤0.7 GPa. As the pressure was raised to 1.1–2.5 GPa, [MPIM][PF6] aggregated and formed self-associated configurations. This pressure-induced dissociation was not detected in the 1-butyl-3-methylimidazolium hexafluorophosphate and β-CD mixture ([BMIM][PF6]-β-CD) due to the long side chain in the [BMIM] cation. This indicates that the alkyl side-chain length of the imidazolium ILs plays a non-negligible role in controlling the order and strength of IL-β-CD interactions. It is well known that the validity of infrared spectroscopy for detecting the inclusion complexation is questionable in the literature. However, this study demonstrates that the combination of high pressure and IR spectroscopy may provide a suitable way to monitor the IL-β-CD interactions.

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 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.…”

“…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.…”

“…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.…”

“…The electronic responses, optoelectronic properties, energy band gaps, and crystal structures of different materials, such as semiconductors [29], perovskites [30], and piezochromic materials [31], can be varied by compression. 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].…”