Dynamics of a Room Temperature Ionic Liquid in Supported Ionic Liquid Membranes vs the Bulk Liquid: 2D IR and Polarized IR Pump–Probe Experiments

Abstract

Supported ionic liquid membranes (SILMs) are membranes that have ionic liquids impregnated in their pores. SILMs have been proposed for advanced carbon capture materials. Two-dimensional infrared (2D IR) and polarization selective IR pump-probe (PSPP) techniques were used to investigate the dynamics of reorientation and spectral diffusion of the linear triatomic anion, SeCN, in poly(ether sulfone) (PES) membranes and room-temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf). The dynamics in the bulk EmimNTf were compared to its dynamics in the SILM samples. Two PES membranes, PES200 and PES30, have pores with average sizes, ∼300 nm and ∼100 nm, respectively. Despite the relatively large pore sizes, the measurements reveal that the reorientation of SeCN and the RTIL structural fluctuations are substantially slower in the SILMs than in the bulk liquid. The complete orientational randomization, slows from 136 ps in the bulk to 513 ps in the PES30. 2D IR measurements yield three time scales for structural spectral diffusion (SSD), that is, the time evolution of the liquid structure. The slowest decay constant increases from 140 ps in the bulk to 504 ps in the PES200 and increases further to 1660 ps in the PES30. The results suggest that changes at the interface propagate out and influence the RTIL structural dynamics even more than a hundred nanometers from the polymer surface. The differences between the IL dynamics in the bulk and in the membranes suggest that studies of bulk RTIL properties may be poor guides to their use in SILMs in carbon capture applications.

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

“…24 Early MD simulations noted few linear hydrogen bonds with water 25 and many arrangements of dipolar character, while more recent MD simulations of SeCN -26 found an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy has used the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 23,[27][28][29][30][31][32] concentrated ion solutions, [33][34][35] ILs, [36][37][38][39][40] supported IL membranes, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment.…”

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

“…24 Early MD simulations noted few linear hydrogen bonds with water 25 and many arrangements of dipolar character, while more recent MD simulations of SeCN -26 found an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy has used the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 23,[27][28][29][30][31][32] concentrated ion solutions, [33][34][35] ILs, [36][37][38][39][40] supported IL membranes, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment.…”

The study of ionic liquid behavior at solid-liquid interfaces

“…59 The long-range ordering in ILs as shown by two of our previous publications were supported by recent experimental and computational studies on various ILs using different techniques. 112,[114][115][116] The reported increase in density and van der Waal's energy in this interfacial region implies solid-like behavior of the film. 127 All these above described studies characterize interfacial layers extending up to few tens of angstroms.…”

“…100 nm) of the interfaces. 115 The triflate anion, which is essentially one half of the TFSI anion, also contains C-F and S-O bonds but is smaller and has a more localized charge than TFSI. We predicted that this would lead to a stronger interaction between anion and cation in triflate based IL and reasoned this might induce the cation to reorient with the anion to maintain this favorable interaction.…”

“…This lamellar assembly spreads throughout the entire simulation slab and is expected to extend over longer dimensions if the simulation were expanded 114. Work by Fayer and co-workers on supported ionic liquid membranes (SILM) reported different dynamics in ILs under confinement as compared to the bulk with the interfacial influence proving to extend up to 100 nm 115. Additionally a recent study to understand the interfacial behavior of RTIL showed liquid to solid-like transition on various substrates with the thickness of the film depending on the substrate material.…”