Enthalpic Driving Force for the Selective Absorption of CO2 by an Ionic Liquid

Development of a universal method for vibrational analysis of the terminal alkyne C≡C stretch

The terminal alkyne C≡C stretch has a large Raman scattering cross section in the "silent" region for biomolecules. This has led to many Raman tag and probe studies using this moiety to study biomolecular systems. A computational investigation of these systems is vital to aid in the interpretation of these results. In this work, we develop a method for computing terminal alkyne vibrational frequencies and isotropic transition polarizabilities that can easily and accurately be applied to any terminal alkyne molecule. We apply the discrete variable representation method to a localized version of the C≡C stretch normal mode. The errors of (1) vibrational localization to the terminal alkyne moiety, (2) anharmonic normal mode isolation, and (3) discretization of the Born-Oppenheimer potential energy surface are quantified and found to be generally small and cancel each other. This results in a method with low error compared to other anharmonic vibrational methods like second-order vibrational perturbation theory and to experiments. Several density functionals are tested using the method, and TPSS-D3, an inexpensive nonempirical density functional with dispersion corrections, is found to perform surprisingly well. Diffuse basis functions are found to be important for the accuracy of computed frequencies. Finally, the computation of vibrational properties like isotropic transition polarizabilities and the universality of the localized normal mode for terminal alkynes are demonstrated.

Intramolecular Vibrational Energy Relaxation of CO2 in Cross-Linked Poly(ethylene glycol) Diacrylate-Based Ion Gels

Ultrafast two-dimensional infrared spectroscopy (2D-IR) and Fourier transform infrared spectroscopy (FTIR) were used to measure carbon dioxide (CO₂) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), cross-linked low-molecular-weight poly(ethylene glycol) diacrylate (PEGDA), and an ion gel composed of a 50 vol % blend of the two. The center frequency of the antisymmetric stretch, ν₃, of CO₂ shifts monotonically to lower wavenumbers with increasing polymer content, with the largest line width in the ion gel (6 cm^-1). Increasing polymer content slows both spectral diffusion and vibrational energy relaxation (VER) rates. An unexpected excited-state absorbance peak appears in the 2D-IR of cross-linked PEGDA due to VER from the antisymmetric stretch into the bending mode, ν₂. Thirty-two response functions are necessary to describe the observed features in the 2D-IR spectra. Nonlinear least-squares fitting extracts both spectral diffusion and VER rates. In the ion gel, CO₂ exhibits spectral diffusion dynamics that lie between that of the pure compounds. The kinetics of VER reflect both fast excitation and de-excitation of the bending mode, similar to the ionic liquid (IL), and slow overall vibrational population relaxation, similar to the cross-linked polymer. The IL-like and polymer-like dynamics suggest that the CO₂ resides at the interface of the two components in the ion gel.

Evidence of Direct Dissolution of CO2 into the Ionic Liquid [C4min] [NTf2] during Their Initial Interaction

Ionic liquids (ILs) are known for their high ability to capture CO₂. However, the mechanism of CO₂ solubility into ILs during their initial interaction remains controversial. In this study, we analyzed the initial dissolution of CO₂ into an IL 1-butyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([C₄min] [NTf₂]) by measuring its solubility using a combination of a molecular beam and a flowing liquid jet sheet beam (FJSB) and the King and Wells method (KW method). The temperature dependence of the initial dissolution probability indicates that the solubility of CO₂ in the IL [C₄min] [NTf₂] increases with increasing temperature. This result is not consistent with what has been reported in an equilibrium state. The initial dissolution probability was well-fitted by the Vogel-Fulcher-Tammann (VFT) equation, which describes the dynamical cage structure in IL systems. We also find that the initial dissolution probability was correlated to the cage lifetime and correlation length. The simple model of CO₂ dissolution into an IL with the cage model was implemented to explain the experimental results in this study. Our results indicate that the initial dissolution of CO₂ into the IL corresponds to a direct solution and not an uptake process.

Controlling the Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization

The structural dynamics of planar thin films of an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf₂) as a function of surface charge density and thickness were investigated using two-dimensional infrared (2D IR) spectroscopy. The films were made by spin coating a methanol solution of the IL on silica substrates that were functionalized with alkyl chains containing head groups that mimic the IL cation. The thicknesses of the ionic liquid films ranged from ∼50 to ∼250 nm. The dynamics of the films are slower than those in the bulk IL, becoming increasingly slow as the films become thinner. Control of the dynamics of the IL films can be achieved by adjusting the charge density on substrates through multilayer network surface functionalization. The charge density of the surface (number of positively charged groups in the network bound to the surface per unit area) is controlled by the duration of the functionalization reaction. As the charge density is increased, the IL dynamics become slower. For comparison, the surface was functionalized with three different neutral groups. Dynamics of the IL films on the functionalized neutral surfaces are faster than on any of the ionic surfaces but still slower than the bulk IL, even for the thickest films. These results can have implications in applications that employ ILs that have electrodes, such as batteries, as the electrode surface charge density will influence properties like diffusion close to the surface.

Comparing ion transport in ionic liquids and polymerized ionic liquids

Polymerized ionic liquids (polyILs) combine the unique properties of ionic liquids (ILs) with macromolecular polymers. But anion diffusivities in polyILs can be three orders of magnitude lower than that in ILs. Endeavors to improve ion transport in polyILs urgently need in-depth insights of ion transport in polyILs. As such in the work we compared ion transport in poly (1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly ([BVIM]-[BF₄])) polyIL and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF₄]) IL. The diffusivities of ions in the polyIL and IL were measured and computed. According to the results of the molecular dynamics simulations performed, in the IL the coupling motion between an anion and the ions around determines the ion diffusivities, and the ion association lifetime gives the time scale of ion transport. But in the polyIL, the hopping of an anion among cages composed of cationic branch chains determines the diffusivity, and the associated anion transport time scale is the trap time, which is the time when an anion is caught inside a cage, not the ion association lifetime, as Mogurampelly et al. regarded. The calculation results of average displacements (ADs) of the polyIL chains show that, besides free volume fraction, average amplitudes of the oscillation of chains and chain translation speed lead to various diffusivities at various temperatures.

Computer simulation study of fluorocarbon phosphate surfactant based aqueous reverse micelle in supercritical CO2: roles of surfactant functional groups, ionic strength, and phase changes in CO2

Structural and dynamic properties of an aqueous micelle organized from fluorocarbon phosphate surfactant molecules in supercritical carbon dioxide (CO₂) are investigated via molecular dynamics computer simulations. The roles of the functional groups and ionic strength of the surfactants on the formation of reverse micelles in supercritical CO₂, and related water dynamics characterized as translational and reorientational dynamics, are systematically demonstrated by employing three different phosphate-based surfactants paired with sodium cations. The strong electrostatic interactions between the phosphate head groups and sodium cations result in formation of an aqueous core inside the surfactant aggregates, where water molecules are bonded together with loss of the tetrahedral hydrogen bonded network found in bulk water. It is found that all the three surfactants with CO₂-philic fluorocarbon double tails build up well-stabilized reverse micelles in supercritical CO₂, avoiding direct contacts between CO₂ and water molecules. Despite this, the surfactant with a carboxylic ester linkage between the phosphate head and fluorocarbon tail group tends to coordinate water molecules toward sustaining the inter-water hydrogen bonds, indicating better efficiency at covering the aqueous core with hydrophobic groups compared to one without a carboxylic ester group. As for water molecules confined in the reverse micelle, their translational and reorientational motions, and fluctuating dynamics of the inter-water hydrogen bonds, significantly slow down compared to bulk water at ambient temperature. The water dynamics become more restricted with an increase in ionic strength of the anionic surfactant; this is attributed to divalent surfactant heads and sodium cations being more tightly bound together with bonding to water compared to monovalent ones. Lastly, the structural and dynamic changes of the reverse micelle caused by a phase change in CO₂ are monitored with gradually decreasing temperature and pressure from the supercritical to gaseous state for CO₂. The average reverse micelle structure equilibrated in supercritical CO₂ is found to remain stable over a time period of 0.2 ms through a depressurization process to gaseous CO₂. We note that the diverse pathways of surfactant self-aggregation in gaseous CO₂ could be controlled by the preceding solvation procedure in the supercritical regime which governs the final aggregated structures in gaseous CO₂.

Dynamics and Vibrational Spectroscopy of Alcohols in Ionic Liquids: Methanol and Ethanol

The structure, dynamics, and vibrational spectroscopy of dilute HOD, methanol, and ethanol in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf₂], ionic liquid (IL) are investigated with molecular dynamics (MD) simulations. The structure of the ILs around the solutes is qualitatively similar, where the OD bond of the deuterated alcohols donates an interaction to an [NTf₂] anion and the [emim] cations interact with the oxygen atom of the OD group. The slowest time scale for the reorientational dynamics of the OD bond varied considerably for HOD, methanol, and ethanol (27, 71, and 87 ps, respectively). In contrast, the slowest time scales for spectral diffusion of the OD vibrational frequency were 11 ps for each of the three solutes, which indicates that the dynamics of the IL is relatively unchanged by the presence of the alcohols at dilute concentration. The theoretical results for the reorientational and spectral diffusion dynamics compare favorably with prior two-dimensional infrared (2D IR) spectroscopic measurements.

Modeling solubility of CO2 gas in room temperature ionic liquids using the COSMOSAC-LANL model: a first principles study

In this paper we present a thermodynamic model for asymmetric solutions with a special emphasis on solute-solvent interactions. The new "COSMOSAC-LANL" activity coefficient model is rooted in first principles calculations based on the COSMO model where the microscopic information passes to the macroscopic world via a dielectric continuum solvation model followed by a post statistical thermodynamic treatment of self-consistent properties of the solute particle. To model the activity coefficient at infinite dilution for the binary mixtures, a 3-suffix Margules (3sM) function is introduced to model asymmetric interactions and, for the combinatorial term, the Staverman-Guggenheim (SG) form is used. The new "COSMOSAC-LANL" activity coefficient model has been used to calculate the solubility of CO₂ in room temperature ionic liquids and to model the selectivity between CO₂ and CH₄ gases. We have shown improved solubility and selectivity prediction of CO₂ and CH₄ gas in room temperature ionic liquids using the ADF-COSMOSAC-2013 model with the new "LANL" activity coefficient model. The calculated values have been compared with experimental results where they are available.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: IV. Temperature Dependence

In previous papers in the series, the vibrational spectroscopy of CO₂ in ionic liquids (ILs) was investigated at ambient conditions. Here, we extend these studies to understand the temperature dependence of the structure, dynamics, and thermodynamics of CO₂ in the 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF₆], IL. Using spectroscopic mapping techniques, the infrared absorption spectrum of the CO₂ asymmetric stretch mode is simulated at a number of temperatures, and the results are found to be consistent with similar experimental studies. Structural correlation functions are used to reveal the thermodynamics of complete CO₂ solvent cage breakdown. The enthalpy and entropy of activation for solvent cage reorganization are found to be 6.9 and 7.6 (kcal/mol)/K, respectively, and these values are similar to the those for spectral, orientational, and translational diffusion. Caging times for CO₂ are calculated, and it is shown that the short time dynamics of CO₂ are unaffected by temperature, even though the long-time dynamics are highly sensitive to temperature.

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: III. Dynamics and Spectroscopy

In recent years, interest in carbon capture and sequestration has led to numerous investigations of the ability of ionic liquids to act as recyclable CO₂-sorbent materials. Herein, we investigate the structure and dynamics of a model physisorbing ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁Im][PF₆]), from the perspective of CO₂ using two-dimensional (2D) IR spectroscopy and molecular dynamics simulations. A direct comparison of experimentally measured and calculated 2D IR line shapes confirms the validity of the simulations and spectroscopic calculations. Taken together, the simulations and experiments reveal new insights into the interactions of a CO₂ solute with the surrounding ionic liquid and how these interactions manifest in the 2D IR spectra. In particular, higher CO₂ asymmetric stretch vibrational frequencies are associated with softer, less populated solvent cages and lower frequencies are associated with tighter, more highly populated solvent cages. The CO₂ interacts most strongly with the anions, and these interactions persist for more than 1 ns. The second strongest interactions are with the imidazolium cation ring that last 100 ps, and the weakest interactions are with the cation butyl tail that persist for 10 ps. The principal contributors to spectral diffusion of the CO₂ asymmetric stretch vibrational frequency due to the dynamical evolution of the solvent are through Lennard-Jones interactions at short times and electrostatics at long times.