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Acetone as a Polar Cosolvent for Pyridinium-Based Ionic Liquids

The work reports transport and structure properties of the three pyridinium-based ionic liquids (RTILs) - N-butylpyridinium hexafluorophosphate [BPY][PF6], N-butylpyridinium trifluorosulfonate [BPY][TF] and N-butylpyridinium bis(trifluoromethanesulfonyl)imide [BPY][TFSI] - in their mixtures with acetone (ACET). The ionic conductivity maximum occurs at 10 mol% RTIL, irrespective of the anion. The absolute ionic conductivity value of [BPY][TF] is higher than those of other RTILs. This is explained by a weaker cation-anion binding than that in [BPY][PF6] and smaller anion size than that of [BPY][TFSI]. All the investigated RTILs are infinitely miscible with ACET, which boosts their diffusivity and conductivity. A small viscosity of ACET favors a drastic viscosity decay in the RTIL-ACET mixtures. Structure analysis (radial distribution functions, ionic clusters) of the mixtures are in line with their transport properties providing a reliable microscopic interpretation of the observed macroscopic properties. As a molecular co-solvent, ACET constitutes an interesting alternative and competitor to more intensively investigated liquids.


Introduction
Room-temperature ionic liquids (RTILs) constitute an actively developing field thanks to their versatility and an interesting set of physical chemical properties. [1][2][3][4][5][6][7] Tunability of RTILs due to the many available functionalization pathways and readily combined organic ions make them 'designer solvents'. Ideally, a virtually continuous range of physical chemical properties can be achieved to target each particular application. RTILs exhibit wide electrochemical windows, 8,9 relatively high ionic conductivities, 8,10 low volatilities, 5,11 excellent solvation and gas adsorption properties. 12,13 Amino functionalized ILs provide an efficient carbon dioxide capture opportunities through chemisorption, 14 behaving in a similar way to aqueous amine solutions.
Attaching different functional groups to the same cation or anion opens a bright avenue to develop ever better scavengers and more universal solvents.
Ionic conductivity of RTILs can be further increased by adding a polar molecular cosolvent, such as water, alcohols, acetonitrile, ketones, etc. 10,15 A dependence of ionic conductivity on solvent molar fraction exhibits a maximum, [16][17][18] which is located below 20 mol % RTIL, irrespective of the particular ionic liquid. Therefore, a significant amount of a molecular admixture is required to immobilize charge carriers. Shear viscosity of the resulting ion-molecular mixtures exponentially (vs. molar fraction) decays upon dilution. An absolute value of the conductivity maximum is determined by (1) diffusivity of a co-solvent; (2) sizes and shapes of ions; (3) cation-anion binding; (4) IL-co-solvent binding. The two latter contributions appear simultaneously relevant for the homogeneity of the mixtures. A significant collection of experimental data is available 10,19,20 confirming that smaller ions and mobile co-solvent molecules favor a higher ionic conductivity, whereas bulky ions and relatively (e.g. dimethyl sulfoxide and propylene carbonate) result in lower values. It is important for manifold electrochemical applications of ILs (solar cells, supercapacitors, lithium-ion batteries) [21][22][23][24] to have a properly conductive electrolyte.
While aqueous, alcoholic and acetonitrile solutions/mixtures of popular RTILs have been addressed in the past, acetone (ACET) seems somewhat underestimated as a co-solvent.
Relatively few works are available devoted to the imidazolium-based RTILs in ACET and some other RTILs sporadically. [25][26][27][28][29][30][31][32][33][34][35][36][37] Based on its molecular and physical chemical properties, ACET excellently mixes with most ILs thanks to the carbonyl group. Furthermore, ACET exhibits a low viscosity, 0.307 cP, 38 which correlates with its low normal-pressure boiling point. Unlike most other solvents, the dipole moment of ACET is perpendicular to its backbone. This feature is expected to give rise to a different local structure pattern. ACET does not possess a suitable hydrogen atom to get involved into hydrogen bonding. Therefore, diffusion of pure ACET is notably high, while viscosity is notably low, despite a relatively big molecular mass.
In the present work, we report a detailed investigation of the ion-molecular mixtures consisting of the three N-butylpyridinium-based ILs and acetone ( Figure 1

Simulation Details
Pairwise-potential-based MD simulation method is a powerful computer simulation tool that is widely employed nowadays to describe systems containing thousands to millions atoms. If parametrization of the interaction potentials is performed carefully, the results provide highly reliable data, whose discrepancies from the experimental data rarely exceed 10%.
The force field (FF) for the pyridinium family of RTILs was developed by our group recently. 20 This FF accurately reproduces diffusivity, conductivity, viscosity and thermodynamic quantities of the pure pyridinium-based RTILs at room and elevated temperatures. The ACET molecule was represented using the classical model proposed by Jorgensen and coworkers. 39 Both models originate from the OPLS-AA methodology. 40 Thus, these models are fully compatible. Table 1 provides an overview of the studied MD systems, whereas  We simulated electrostatic energy between each two charged interaction centers belonging to molecules and ions directly using the Coulomb law if the distance between the two interaction centers did not exceed 1.2 nm. In the case of larger distances, the particle-mesh Ewald method was used. 45 Lennard-Jones (12,6) forces were modified between 1.1 and 1.2 nm using the shifted force scheme. An accurate treatment of the interaction potentials in combination with a sufficiently small trajectory propagation time-step preserves total energy of the MD system.
Intra-molecular interactions were reproduced using harmonic potentials for bonds, covalent angles, and covalent dihedrals. The Coulombic and Lennard-Jones (12,6) potentials for atoms separated by one, two, and three bonds were omitted, whereas 1-4 interactions were decreased twice. This is in accordance with the OPLS-AA FF. 40 Radial distribution functions (RDFs) were computed using their classical definition, in which height of RDF is a ratio of local density to average density of a given atom. Self-diffusion coefficients and ionic conductivity were calculated using the Einstein-type equations for these properties, i.e. based on the mean-squared displacements of atoms. Shear viscosity was calculated based on the energy dissipation. 46 An ionic cluster was defined as a stable collection of ions, which exhibit strong electrostatic binding to one another. The clusterization threshold was set to a position of the first minimum of the HA-FP or HA-OB or HA-OS RDFs, where applicable.

Results and Discussion
The ionic conductivity dependence vs. molar fraction (Figure 3) of the selected RTILs is our major research scope. The addition of ACET significantly increases conductivity of all ILs.
The conductivity maximum occurs at 10 mol% RTIL, irrespective of the RTIL identity. The          Nevertheless, the ratio of interaction energies in the mixtures helps to hypothesize compositions of the first coordination shells of the ions. If an ion is preferentially surrounded by ACET molecules, its diffusivity is higher, as evidenced by the above discussed, Figures 5-6  The pyridinium-based RTILs constitute practical interest due to a relatively weak cation-anion binding and absence of strong hydrogen bonds in these ion-molecular systems.