What Determines CO₂ Solubility in Ionic Liquids? A Molecular Simulation Study

Abstract

Molecular dynamics (MD) simulations of 10 different pure and CO2-saturated ionic liquids are performed to identify the factors that determine CO2 solubility. Imidazolium-based cations with varying alkyl chain length and functionalization are paired with anions of different hydrophobicity and size. Simulations are carried out with an empirical force field based on liquid-phase charges. The partial molar volume of CO2 in ionic liquids (ILs) varies from 30 to 40 cm(3)/mol. This indicates that slight ion displacements are necessary to enable CO2 insertions. However, the absorption of CO2 does not affect the overall organization of ions in the ILs as demonstrated by almost equal cation-anion radial distribution functions of pure ILs and ILs saturated with CO2. The solubility of CO2 in ILs is not influenced by direct CO2-ion interactions. Instead, a strong correlation between the ratio of unoccupied space in pure ILs and their ability to absorb CO2 is found. This preformed unoccupied space is regularly dispersed throughout the ILs and needs to be expanded by slight ion displacements to accommodate CO2. The amount of preformed unoccupied space is a good indicator for ion cohesion in ILs. Weak electrostatic cation-anion interaction densities in ILs, i.e., weak ion cohesion, leads to larger average distances between ions and hence to more unoccupied space. Weak ion cohesion facilitates ion displacement to enable an expansion of empty space to accommodate CO2. Moreover, it is demonstrated that the strength of ion cohesion is primarily determined by the ion density, which in turn is given by the ion sizes. Ion cohesion is influenced additionally to a smaller extent by local electrostatic interactions among ion moieties between which CO2 is inserted and which do not depend on the ion density. Overall, the factors that determine the solubility of CO2 in ILs are identified consistently across a large variety of constituting ions through MD simulations.

Self-diffusion, velocity cross-correlation, distinct diffusion and resistance coefficients of the ionic liquid [BMIM][Tf2N] at high pressure

“…2 for a listing) and data for simple high-temperature molten salts. 1, 84 One may speculate that this change in ∆ is due to the "existence of a polar network formed by the charged moieties of the molecular ions" and "separations between different strands of the polar network caused by a continuous nonpolar sub-phase formed by the alkyl side chains that may be present in some of the ions" 85 as found in molecular simulations of [RMIM] + salts due to Canongia Lopes et al, 83,86 and from small-wide angle X-ray scattering work, 87 demonstrated.…”

“…The different order for these last two quantities for [Tf 2 N]salts on the one hand and [BF 4 ]and [PF 6 ]salts on the other, as for the VCC, has yet to be explained, though it could be due to greater cation-cation contact when the anions are small as suggested by the molecular dynamics simulations of Klähn and Seduraman. 83 Linear and angular momentum exchange through translationalrotational coupling and caging of the smaller ions are other effects to be considered.…”

The addition of CO2to four superbase ionic liquids: a DFT study

“…Table S1 in supporting information). These are typical values for CT in ionic pairs as calculated accurately using a QM/MM approach for a variety of ILs in the liquid phase 32 .…”

In Situ Generated Platinum Catalyst for Methanol Oxidation via Electrochemical Oxidation of Bis(trifluoromethylsulfonyl)imide Anion in Ionic Liquids at Anaerobic Condition

“…This interpretation is supported by a recent study that molecular dynamics simulations of ten different pure and CO 2 saturated ionic liquids have shown a strong correlation between the ratio of unoccupied space in pure ILs and their ability to absorb CO 2 . 50 …”

“…16 Both of the cation and anion of [Bmpy][NTf 2 ] are large with weak electrostatic cation–anion interaction densities which allow a large number of unoccupied space in the IL for methanol adsorption but not for larger molecules. 50 …”