Utilizing Infrared Spectroscopy to Analyze the Interfacial Structures of Ionic Liquids/Al₂O₃ and Ionic Liquids/Mica Mixtures under High Pressures

Pressure-Dependent Clustering in Ionic-Liquid-Poly (Vinylidene Fluoride) Mixtures: An Infrared Spectroscopic Study

The nanostructures of ionic liquids (ILs) have been the focus of considerable research attention in recent years. Nevertheless, the nanoscale structures of ILs in the presence of polymers have not been described in detail at present. In this study, nanostructures of ILs disturbed by poly(vinylidene fluoride) (PVdF) were investigated via high-pressure infrared spectra. For 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([HEMIm][TFSI])-PVdF mixtures, non-monotonic frequency shifts of the C^4,5-H vibrations upon dilution were observed under ambient pressure. The experimental results suggest the presence of microheterogeneity in the [HEMIm][TFSI] systems. Upon compression, PVdF further influenced the local structure of C^4,5–H via pressure-enhanced IL–PVdF interactions; however, the local structures of C²–H and hydrogen-bonded O–H were not affected by PVdF under high pressures. For choline [TFSI]–PVdF mixtures, PVdF may disturb the local structures of hydrogen-bonded O–H. In the absence of the C^4,5–H⋯anion and C²–H⋯anion in choline [TFSI]–PVdF mixtures, the O–H group becomes a favorable moiety for pressure-enhanced IL–PVdF interactions. Our results indicate the potential of high-pressure application for designing pressure-dependent electronic switches based on the possible changes in the microheterogeneity and electrical conductivity in IL-PVdF systems under various pressures.

Structural Reorganization of Imidazolium Ionic Liquids Induced by Pressure-Enhanced Ionic Liquid-Polyethylene Oxide Interactions

Mixtures of polyethylene oxide (PEO, M.W.~900,000) and imidazolium ionic liquids (ILs) are studied using high-pressure Fourier-transform infrared spectroscopy. At ambient pressure, the spectral features in the C-H stretching region reveal that PEO can disturb the local structures of the imidazolium rings of [BMIM]+ and [HMIM]+. The pressure-induced phase transition of pure 1-butyl-3-methylimidazolium bromide ([BMIM]Br) is observed at a pressure of 0.4 GPa. Pressure-enhanced [BMIM]Br-PEO interactions may assist PEO in dividing [BMIM]Br clusters to hinder the aggregation of [BMIM]Br under high pressures. The C-H absorptions of pure 1-hexyl-3-methylimidazolium bromide [HMIM]Br do not show band narrowing under high pressures, as observed for pure [BMIM]Br. The band narrowing of C-H peaks is observed at 1.5 GPa for the [HMIM]Br-PEO mixture containing 80 wt% of [HMIM]Br. The presence of PEO may reorganize [HMIM]Br clusters into a semi-crystalline network under high pressures. The differences in aggregation states for ambient-pressure phase and high-pressure phase may suggest the potential of [HMIM]Br-PEO (M.W.~900,000) for serving as optical or electronic switches.

Characterization of Local Structures of Confined Imidazolium Ionic Liquids in PVdF-co-HFP Matrices by High Pressure Infrared Spectroscopy

The nanoscale ion ordering of ionic liquids at confined interfaces under high pressures was investigated in this study. 1-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][NTf₂])/poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf₂])/PVdF-co-HFP were prepared and characterized by using high-pressure infrared spectroscopy. Under ambient pressure, imidazolium C²–H and C^4,5–H absorptions were blue-shifted in frequency due to the presence of PVdF-co-HFP. However, the absorption of anionic ν_a SO₂ did not reveal any significant shifts in frequency upon dilution by PVdF-co-HFP. The experimental results suggest that PVdF-co-HFP disturbs the local structures of the imidazolium C–H groups instead of the anionic SO₂ groups. The frequency shifts of C^4,5–H became dramatic for the mixtures at high pressures. These results suggest that pressure-enhanced ionic liquid–polymer interactions may play an appreciable role in IL-PVdF-co-HFP systems under high pressures. The pressure-induced blue-shifts due to the PVdF-co-HFP additions were more obvious for the [HMIM][NTf₂] mixtures than for [EMIM][NTf₂] mixtures.

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

1-Butyl-3-methylimidazolium hexafluorophosphate ([C₄MIM][PF₆])/DNA and 1-methyl-3-propylimidazolium hexafluorophosphate ([C₃MIM][PF₆])/DNA mixtures were prepared and characterized by high-pressure infrared spectroscopy. Under ambient pressure, the imidazolium C²-H and C^4,5-H absorption bands of [C₄MIM][PF₆]/DNA mixture were red-shifted in comparison with those of pure [C₄MIM][PF₆]. This indicates that the C²-H and C^4,5-H groups may have certain interactions with DNA that assist in the formation of the ionic liquid/DNA association. With the increase of pressure from ambient to 2.5 GPa, the C²-H and C^4,5-H absorption bands of pure [C₄MIM][PF₆] displayed significant blue shifts. On the other hand, the imidazolium C-H absorption bands of [C₄MIM][PF₆]/DNA showed smaller frequency shift upon compression. This indicates that the associated [C₄MIM][PF₆]/DNA conformation may be stable under pressures up to 2.5 GPa. Under ambient pressure, the imidazolium C²-H and C^4,5-H absorption bands of [C₃MIM][PF₆]/DNA mixture displayed negligible shifts in frequency compared with those of pure [C₃MIM][PF₆]. The pressure-dependent spectra of [C₃MIM][PF₆]/DNA mixture revealed spectral features similar to those of pure [C₃MIM][PF₆]. Our results indicate that the associated structures of [C₄MIM][PF₆]/DNA are more stable than those of [C₃MIM][PF₆]/DNA under high pressures.

Pressure-Dependent Confinement Effect of Ionic Liquids in Porous Silica

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.