Phase Transitions, Decomposition Temperatures, Viscosities, and Densities of Phosphonium, Ammonium, and Imidazolium Ionic Liquids with Aprotic Heterocyclic Anions

Ionic liquids (ILs) with aprotic heterocyclic anions (AHAs) have been developed primarily for CO2 capture applications. However, they have also been considered for cofluid CO2/IL vapor compression refrigeration cycles and for various electrochemical applications. In all of these cases, reducing the viscosity of the IL is of primary importance. Therefore, the focus of this work is tuning the cation to produce AHA ILs with both low viscosities and low melting points. Toward this goal we have synthesized 40 AHA ILs paired with phosphonium, ammonium and imidazolium cations, as well as a number of ILs with the bis(trifluoromethylsulfonyl)imide anion to use for comparison. The azolide anions investigated were 2-cyanopyrrolide, 4-nitropyrazolide, various substituted imidazolides, 1,2,3-triazolide, and tetrazolide. Melting points, glass transition temperatures, and decomposition temperatures were measured for all ILs. Viscosities and densities were measured from 278.15 to 343.15 K and 283.15 to 353.15 K, respectively, for all ILs except those with high melting points or excessively high viscosities. Shortening the alkyl chains on tetra-alkylphosphonium and tetra-alkylammonium cations reduces viscosity, but eventually results in unacceptably higher melting points. For equivalent alkyl chain lengths and anions, ammoniums have higher melting points and lower decomposition temperatures than phosphoniums. The introduction of an ether chain on a phosphonium cation lowers viscosity but reduces thermal stability. Di- and trialkylimidazolium with sufficiently low melting points have relatively high viscosities.