RESUMEN
The molecular-level orientation and structure of ionic liquids (ILs) at liquid-solid interfaces are significantly different than in the bulk. The interfacial ordering influences both IL properties, such as dielectric constants and viscosity, and their efficacy in devices, such as fuel cells and electrical capacitors. Here, we report the layered structures of four ILs on unbiased, highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy. The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C4F9SO3]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl)imide ([MTBD][Tf2N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C4F9SO3]). Molecular dynamics simulations provide complementary information on the position and orientation of the ions. These ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]+ and [MTBD]+ have similar orientations at the surface, but [MTBD]+ forms a thinner layer compared to [emim]+ on both HOPG and Pt(111). In addition, [Tf2N]- shows stronger interactions with Pt(111) surfaces than [C4F9SO3]-.
RESUMEN
The extent to which cations and anions in ionic liquids (ILs) and ionic liquid solutions are dissociated is of both fundamental scientific interest and practical importance because ion dissociation has been shown to impact viscosity, density, surface tension, volatility, solubility, chemical reactivity, and many other important chemical and physical properties. When mixed with solvents, ionic liquids provide the unique opportunity to investigate ion dissociation from infinite dilution in the solvent to a completely solvent-free state, even at ambient conditions. The most common way to estimate ion dissociation in ILs and IL solutions is by comparing the molar conductivity determined from ionic conductivity measurements such as electrochemical impedance spectroscopy (EIS) (which measure the movement of only the charged, i.e., dissociated, ions) with the molar conductivity calculated from ion diffusivities measured by pulse field gradient nuclear magnetic resonance spectroscopy (PFG-NMR, which gives movement of all of the ions). Because the NMR measurements are time-consuming, the number of ILs and IL solutions investigated by this method is relatively limited. We have shown that use of the Stokes-Einstein equation with estimates of the effective ion Stokes radii allows ion dissociation to be calculated from easily measured density, viscosity, and ionic conductivity data (ρ, η, λ), which is readily available in the literature for a much larger number of pure ILs and IL solutions. Therefore, in this review, we present values of ion dissociation for ILs and IL solutions (aqueous and nonaqueous) determined by both the traditional molar conductivity/PFG-NMR method and the ρ, η, λ method. We explore the effect of cation and anion alkyl chain length, structure, and interaction motifs of the cation and anion, temperature, and the strength of the solvent in IL solutions.
RESUMEN
Separation of olefins from their paraffin analogs relies on energy-intensive cryogenic distillation. Facilitated transport-based membranes that reversibly and selectively bind olefins, but not paraffins, could save considerable amounts of energy. However, the chemical instability of the silver ion olefin-binding carriers in such membranes has been a longstanding roadblock for this approach. We discovered long-term carrier stability against extended exposure to hydrogen, a common contaminant in such streams. Based on UV/Vis absorption and Raman spectroscopy, along with XRD analysis results, certain ionic liquids solubilize silver ions, and anion aggregates surrounding the silver ion carriers greatly attenuate their reduction by hydrogen. Here, we report the stability of olefin/paraffin separation properties under continuous exposure to high pressure hydrogen, which addresses a critical technical roadblock in membrane-based olefin/paraffin separation.
RESUMEN
Virtually, every investigation and application of ionic liquids (ILs) involves gas-liquid, liquid-liquid, and liquid-solid interactions. Therefore, understanding the behavior of ILs at those interfaces is critical. In this work, we studied the interfacial properties of protic and aprotic ILs with N-alkylimidazolium and 1-alkyl-3-methylimidazolium as cations and bis(trifluoromethylsulfonyl)imide, methanesulfonate, and trifluoromethanesulfonate as anions. The surface tension of these ILs is measured with the pendant drop method in a temperature range of 293.15-343.15 K and at atmospheric pressure. The contact angle measurements are performed at 293.15 K on three solid substrates: polytetrafluoroethylene, glassy carbon, and platinum. Dispersive and nondispersive components of the IL surface energy are determined from the experimental data using Fowkes theory. The most interesting result is that the protic ILs have lower surface tension and smaller contact angles than the equivalent aprotic ILs, despite the presence of high charge density on the proton associated with one of the nitrogens of the cation. Higher charge density on the anion results in a higher surface tension, and decreasing surface tension and contact angles are observed for increasing alkyl chain length on the cation.
RESUMEN
We encapsulated six solvents with novel physical and chemical properties for CO2 sorption within gas-permeable polymer shells, creating Micro-Encapsulated CO2 Sorbents (MECS), to improve the CO2 absorption kinetics and handling of the solvents for postcombustion CO2 capture from flue gas. The solvents were sodium carbonate (Na2CO3) solution, uncatalyzed and with two different promoters, two ionic liquid (IL) solvents, and one CO2-binding organic liquid (CO2BOL). We subjected each of the six MECS to multiple CO2 absorption and regeneration cycles and measured the working CO2 absorption capacity as a function of time to identify promising candidate MECS for large-scale carbon capture. We discovered that the uncatalyzed Na2CO3 and Na2CO3-sarcosine MECS had lower CO2 absorption rates relative to Na2CO3-cyclen MECS over 30 min of absorption, while the CO2BOL Koechanol appeared to permeate through the capsule shell and is thus unsuitable. We rigorously tested the most promising three MECS (Na2CO3-cyclen, IL NDIL0309, and IL NDIL0230) by subjecting each of them to a series of 10 absorption/stripping cycles. The CO2 absorption curves were highly reproducible for these three MECS across 10 cycles, demonstrating successful absorption/regeneration without degradation. As the CO2 absorption rate is dynamic in time and the CO2 loading per mass varies among the three most promising MECS, the process design parameters will ultimately dictate the selection of MECS solvent.