Preferential Ionic Interactions and Microscopic Structural Changes Drive Nonideality in Binary Ionic Liquid Mixtures as Revealed from Molecular Simulations

Abstract

Thermophysical and structural properties of two binary mixtures of ionic liquids were determined in this study using molecular dynamics simulations at 353 K. The mixtures contains common cation and different anions, combining 1-n-butyl-3-methylimidazolium [C 4 mim] + chloride [Cl] − with two other ionic liquids, namely, [C 4 mim] + methylsulfate [MeSO 4 ] − and [C 4 mim] + bis(trifluoromethanesulfonyl)imide [NTf 2 ] − . Each mixture was characterized in terms of thermodynamic quantities such as densities and excess molar volumes and transport properties specifically selfdiffusion coefficients and ionic conductivities, using seven molar compositions (0:00, 10:90, 25:75, 50:50, 75:25, 90:10, 100:0). Excess molar volumes for the two binary ionic liquid mixtures exhibited small deviations from ideality; the Cl-[MeSO 4 ] system showed negative deviation while positive deviation was observed for the Cl-[NTf 2 ] system. Structural analysis elucidated in terms of radial distribution functions, orientations of the anions around cation through angular distribution functions, and spatial distribution functions revealed that the significant changes in the ionic interactions occur within the first solvation shell of the cations and these changes are responsible for the observed nonideality. The self-diffusion coefficients of the ions were found to decrease monotonically with Cl − concentration for both the mixtures. Further, predictions of ionic conductivities using both the Nernst−Einstein formalism and Einstein relationship pointed to the presence of correlated ionic motion which was confirmed by the long ion-pair relaxation time constants especially for the anion present as a minor component.

Insights on the mixtures of imidazolium based ionic liquids with molecular solvents

“…4a and 4b show an excellent agreement between experimental and MD predicted density in the full composition range and for all the studied mixtures. Excess molar volume was also predicted by MD, and although the prediction of this property by MD is difficult (because packing, steric and intermolecular forces, and their evolution with composition are involved) [74,75], an excellent qualitative and quantitative agreement is obtained, Figs. 4c and 4d; thus, the reported force field parameterization may give an accurate microscopic description of the fluids properties and structuring.…”

Thermophysical Properties of Imidazolium-Based Binary Ionic Liquid Mixtures Using Molecular Dynamics Simulations

“…The apparent discrepancy is probably related to the difference in the size of the two anions and efficient molecular packing in the case of [C 4 mim]Cl due to the spherical shape of the anion. The densities for the ionic liquid mixtures vary smoothly between the densities of the pure ionic liquids, indicating gradual transition in the molecular structures observed in these systems . For all the systems, except [C 4 mim] Cl x [OAC] 1– x , a positive curvature from the mole fraction averaged densities can be discerned (Figure S1 of the Supporting Information).…”

“…For the rest of the paper, these ionic liquids are designated as (9) [C 2 mim] Cl x [NTf 2 ] 1– x , (10) [C 6 mim] Cl x [NTf 2 ] 1– x , and (11) [C 8 mim] Cl x [NTf 2 ] 1– x . These anion combinations were chosen based on differences in their hydrogen bonding ability with the cation, size, and the molar volumes of pure ionic liquids. ,, …”

“…Similarly, insight into the characteristics of the ionic liquids that lead to nonideal behavior is key to developing ionic liquid mixtures that exhibit higher self-diffusion coefficients and ionic conductivity, and lower viscosity than those of the parent ionic liquids. Toward this end, a number of binary ionic liquid mixture systems have been investigated, both, experimentally − and with simulation-based ,,− approaches. For more examples of a number of ionic liquid mixture properties, the reader is directed to excellent reviews published by Welton and co-workers, and Rogers and co-workers .…”

“…The authors hypothesized that, in addition to nonrandom mixing of the ions, additional attractive interactions must be present to explain the viscosity maximum. Our previous study has also illustrated that dominance of the hydrogen-bonding interactions between 1- n -butyl-3-methylimidzolium [C 4 mim] + and chloride Cl – leads, in its mixture with [C 4 mim] + bis­(trifluoromethanesulfonyl)­imide [NTf 2 ] − , to molecular structures that are not found in either pure [C 4 mim]Cl or [C 4 mim]­[NTf 2 ] . It is possible that the presence of such non-native structures is responsible for the experimental observation that CO 2 solubility is in excess of the ideal mixing predictions for the binary mixtures of [C 4 mim]­Cl–[C 4 mim]­[NTf 2 ], [C 2 mim] ethylsulfate [C 2 H 5 SO 4 ]–[C 2 mim]­[NTf 2 ], and [C 4 mim]­[C 2 H 5 SO 4 ]–[C 2 mim]­[NTf 2 ] mixtures. , Welton and co-workers also concluded that nonideality in binary ionic liquid systems is likely to arise when there is a substantial difference in the hydrogen bonding ability of the anions, as this can result in preferential interaction at distinct cationic locations.…”