Molecular Origins of the Apparent Ideal CO2 Solubilities in Binary Ionic Liquid Mixtures

Aqueous solutions of binary ionic liquids: insight into structure, dynamics, and interface properties by molecular dynamics simulations and DFT methods

The behavior of aqueous solutions of mixtures of ionic liquids (ILs) is of special interest because of their amphiphilic character, from both a fundamental and application viewpoint. In this work, we conducted molecular dynamics (MD) simulations and density functional theory (DFT) calculations to understand the effect of water on the intermolecular interactions in three IL binary mixtures [C4mim]/[Cl]/[BF4], [C4mim]/[Cl]/[PF6] and [C4mim]/[BF4]/[PF6] containing the well-characterized cation, 1-n-butyl-3-methylimidazolium [C4mim]+ and the anions chloride [Cl]-, tetrafluoroborate [BF4]-, and hexafluorophosphate [PF6]-. The perturbation of the structures in the binary IL mixture by water molecules was analyzed in the bulk and at the liquid/vacuum interface using distribution functions, hydrogen-bond statistics, and density profiles. Interactions between anions and cations change drastically when the IL mixtures are dissolved in water. In particular, anion-water interactions are stronger than anion-cation interactions. H-Bonds are the dominant interactions. They are prevalently electrostatic and strong for the two [Cl]-containing systems in both the water-free and the water-containing systems. The very hydrophobic [C4mim]/[BF4]/[PF6] system gains stability from dispersive interactions and consequently segregates water markedly when admixed. The most probable orientations of IL cations in the bulk and at the vicinity of the interface were examined using bivariate distribution calculations and show [PF6]- segregating to the surface in keeping with its highly hydrophobic nature. DFT calculated structures, energies, dipole moments, global hardness and solvation energies using model ion pairs [C4mim][X] or complexes [C4mim]2[X][Y], with [X/Y]- = [Cl]-, [BF4]-, or [PF6]- are completely consistent with the findings for the bulk.

Macroscopic Differentiators for Microscopic Structural Nonideality in Binary Ionic Liquid Mixtures

Combining two ionic liquids to form a binary ionic liquid mixture is a simple yet effective strategy to not only expand the number of ionic liquids but also precisely control various physicochemical properties of resultant ionic liquid mixtures. From a fundamental thermodynamic point of view, it is not entirely clear whether such mixtures can be classified as ideal solutions. Given a large number of binary ionic liquid mixtures that emerge, the ability to predict the presence of nonideality in such mixtures a priori without the need for experimentation or molecular simulation-based calculations is immensely valuable for their rational design. In this research report, we demonstrate that the difference in the molar volumes (ΔV) of the pure ionic liquids and the difference in the hydrogen-bonding ability of anions (Δβ) are the primary determinants of nonideal behavior of binary ionic liquid mixtures containing a common cation and two anions. Our conclusion is derived from a comparison of microscopic structural properties expressed in terms of radial, spatial, and angular distributions for binary mixtures and those of the corresponding pure ionic liquids. Molecular dynamics simulations of 16 binary ionic liquid mixtures, containing a common cation 1-n-butyl-3-methylimidazolium [C₄mim]⁺ and combinations of (less basic) fluorinated {trifluoromethylacetate [TFA]^-, trifluoromethanesulfonate [TFS]^-, bis(trifluoromethanesulfonyl)imide [NTf₂]^-, and tris (pentafluoroethyl) trifluorophosphate [eFAP]^-} versus (more basic) nonfluorinated {chloride Cl^-, acetate [OAC]^-, methylsulfate [MeSO₄]^-, and dimethylphosphate [Me₂PO₄]^-} anions, were conducted. The large number of binary ionic liquid mixtures examined here enabled us to span a broad range of ΔV and Δβ values. The results indicate that binary mixtures of two ionic liquids for which ΔV > 60 cm³/mol and Δβ > 0.4 are expected to be microscopically nonideal. On the other hand, ΔV < 60 cm³/mol and Δβ < 0.4 will lead to molecular structures that are not differentiated from those of their pure ionic liquid counterparts.

Ion speciation: a key for the understanding of the solution properties of ionic liquid mixtures

Recently, combinations of two (or more) ionic liquids, known as ionic liquid mixtures, have become popular and have a broad range of applications. However, the fundamental knowledge on the molecular interactions that exist in ionic liquid mixtures is far from being understood. In this work, the experimental measurement of the water activity coefficient and computational modelling using Conductor-like Screening Model for Real Solvent (COSMO-RS) were carried out to get an insight into the molecular interactions that are present in ionic liquid mixtures in aqueous solution. The results show that the combination of two ionic liquids of different basicity in aqueous solution allows fine tuning of the water activities, covering a wide range of values that could replace several pure fluids. This is an important feature resulting from the unexpected ion speciation of the ionic liquid mixtures in aqueous solution.

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.