Exploring the Use of Ionic Liquid Mixtures to Enhance the Performance of Dicationic Ionic Liquids

Abstract

Dicationic ionic liquids (DILs) of diverse structural architectures (including symmetrical and asymmetrical ammonium, phosphonium and heterodications and the bis(trifluoromethylsulfonyl)amide (NTf 2 − ) anion) have been prepared and used as additives to N-methyl-N-ethoxyethylpyrrolidinium (P 1 EOE) NTf 2 , a relatively high-performing IL in terms of its transport properties (viscosity 53 mPa s). The three-ion, binary IL mixtures were characterized for their thermal and transport properties using differential scanning calorimetry, temperature dependent viscosity, conductivity and Pulsed Gradient Spin Echo (PGSE) NMR. Variable temperature 1 H, 19 F and 31 P self-diffusion coefficients were determined at 25, 60 and 75 • C. The order of the diffusion coefficients was D(, and the composition of the dication had a strong effect on the degree to which diffusion of all three species is more or less coupled. IL mixtures containing about 30 mol % of the dicationic NTf 2 and 70 mol % of P 1 EOE NTf 2 resulted in a significant decrease in glass transition temperatures and viscosities compared to the pure DIL. The mixtures extended the liquid range and potential for practical applications significantly. The data obtained here provides insight into the future design of dicationic salts tailored to exhibit lower viscosity and higher conductivities.

Mixing divalent ionic liquids: effects of charge and side-chains

“…Since mixing DILs with less viscous monovalent ILs with the same side-chains yield virtually ideal mixtures, this could be an interesting route to predictably lowering the viscosity of these ILs similarly to asymmetric DILs with linkers longer than a single atom. 23 Alternatively, combining mismatched sidechains of different functionality appears to be a route to generating slightly positive excess molar volumes, which often indicate an increase in free volume within the mixture. This effect is nevertheless difficult to predict using group contribution methods as they are not yet accurate enough for DILs.…”

“…The majority of room temperature ILs that comprise a formal divalent ion contain two formal monovalent ions that are joined by an aliphatic or aromatic linker. 22,23 Accordingly, the two charged sites of the cation are distant from one another and do not localise the charge density.…”

“…These investigations represent the first calorimetric studies of mixtures of divalent salts with monovalent ILs and present a valuable insight into the properties of ILs with high charge densities. The ion dynamics and glass transition temperatures of mixtures containing divalent and a monovalent IL have been studied previously 23 but only at a few compositions and including families of ILs that contained divalent salts having large spacers between the charged head-groups.…”

Structure and Dynamics of an Ionic Liquid Mixture Film Confined by Mica

“…Ionic liquids (ILs) have unique chemical and physical properties that make them excellent candidates for various applications in chemical synthesis, catalysis, lubrication, and electrochemistry, among other areas, and they can be designed with specificity for particular problems. − The design for particular purposes needs not be restricted to the combination of a single cation with a single anion, and many studies have gone beyond this by considering mixtures with conventional solvents or multiple ions. Significant research has recently been devoted to binary and ternary mixtures of ILs. − Because this is relevant to applications, when dealing with multiple salt components, we are not only interested in their bulk behavior but also in their preferential arrangement at interfaces.…”