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

Abstract

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 C4,5–H absorptions were blue-shifted in frequency due to the presence of PVdF-co-HFP. However, the absorption of anionic νa 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 C4,5–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.

Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis

“…Several experimental methods have been applied to study the structure and dynamics of both neat IL and its mixtures with molecular solvents, particularly for phase-behaviour of water-IL mixtures, from DFT calculations [129][130][131] and molecular dynamics (MD) simulations [132][133][134], to spectroscopic techniques like NMR [32,106,[135][136][137][138], Small-Angle Neutron Scattering (SANS) [139,140], infrared and RAMAN spectroscopy [141][142][143][144][145].…”

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

“…Figure 1 shows the IR spectra of (a) pure 1-butyl-3-methylimidazolium bromide ([BMIM]Br), (b) PEO (M.W.~900,000) with 75 wt% [BMIM]Br, and (c) pure PEO (M.W.~900,000) at ambient pressure. The absorption profile of pure [BMIM]Br ( Figure 1 a) exhibits five major bands at 3138, 3067, 2958, 2934, and 2869 cm −1 , corresponding to two imidazolium C–H bands (C 4,5 –H and C 2 –H) and three cation alkyl C–H peaks (methyl and butyl groups) [ 29 , 30 , 31 , 32 , 33 , 34 ]. Figure 1 c shows the IR spectrum of pure PEO exhibiting multiple C–H stretching absorptions in the 2950–2850 cm −1 region [ 7 , 35 ].…”

“…High-pressure IR spectroscopy is a direct method to investigate the pressure-induced changes in non-covalent interactions and intermolecular (or interionic) distances [ 29 , 30 , 31 , 32 , 33 , 34 ]. The local structures of the ILs can be disturbed by the polymers (PVdF-co-HFP [ 29 ], DNA [ 30 ]), β-cyclodextrin [ 31 ], porous silica [ 32 ], mica [ 33 ], and nano-alumina [ 33 , 34 ], as determined by the high-pressure method. For example, high-pressure studies indicate that the presence of PVdF-co-HFP mainly affects the local structures of the imidazolium C–H groups instead of the anionic groups [ 29 ].…”

“…The local structures of the ILs can be disturbed by the polymers (PVdF-co-HFP [ 29 ], DNA [ 30 ]), β-cyclodextrin [ 31 ], porous silica [ 32 ], mica [ 33 ], and nano-alumina [ 33 , 34 ], as determined by the high-pressure method. For example, high-pressure studies indicate that the presence of PVdF-co-HFP mainly affects the local structures of the imidazolium C–H groups instead of the anionic groups [ 29 ]. Pressure-enhanced IL-polymer interactions are also observed [ 29 , 30 ].…”

“…For example, high-pressure studies indicate that the presence of PVdF-co-HFP mainly affects the local structures of the imidazolium C–H groups instead of the anionic groups [ 29 ]. Pressure-enhanced IL-polymer interactions are also observed [ 29 , 30 ]. In general, a variation in temperature may result in changes in both thermal energy and volume.…”