Eighteen protic ionic liquids containing different combinations of cations and anions, hydrophobicity, viscosity, and conductivity have been synthesized and their physicochemical properties determined. In one series, the diethanolammonium cations were combined with acetate, formate, hydrogen sulfate, chloride, sulfamate, and mesylate anions. In the second series, acetate and formate anions were combined with amine bases, triethylamine, diethylamine, triethanolamine, di-n-propylamine, and di-n-butylamine. The electrochemical characteristics of the eight protic ionic liquids that are liquid at room temperature (RTPILs) have been determined using cyclic, microelectrode, and rotating disk electrode voltammetries. Potential windows of the RTPILs have been compared at glassy carbon, platinum, gold, and boron-doped diamond electrodes and generally found to be the largest in the case of glassy carbon. The voltammetry of IUPAC recommended potential scale reference systems, ferrocene/ferrocenium and cobaltocenium/cobaltocene, have been evaluated and found to be ideal in the case of the less viscous RTPILs but involve adsorption in the highly viscous ones. Other properties such as diffusion coefficients, ionic conductivity, and double layer capacitance also have been measured. The influence of water on the potential windows, viscosity, and diffusion has been studied systematically by deliberate addition of water to the dried ionic liquids. The survey highlights the problems with voltammetric studies in highly viscous room temperature protic ionic liquids and also suggests the way forward with respect to their possible industrial use.

Low-molecular-weight Brønsted acids and amine bases were used to reproducibly prepare very dry, high-purity room-temperature protic ionic liquids (PILs). A series of eight amine bases and six Brønsted acids were combined to produce 48 mixtures, of which 18 were liquid at room temperature. The phase transitions and thermal decomposition temperatures were determined for each mixture; whereas viscosity, density and conductivity were determined for the room-temperature liquids. By utilising (15)N NMR it was possible to distinguish between neutral and ionised amine bases (ammonia vs. ammonium-type ion), which indicated that the protic ionic liquids were completely ionised when made as a stoichiometric mixture. However, a Walden plot comparison of fluidity and molar conductivity indicated the majority of PILs had much lower conductivity than predicted by viscosity unless the base contained excess proton-donating groups. This disparity is indicative of protic ionic molecules forming neutral aggregates or non-Newtonian fluid hydrogen-bonded networks with a secondary Grotthuss proton-hopping mechanism arising from polyprotic bases.

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