The Effect of Lengthening Cation Ether Tails on Ionic Liquid Properties

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Ionic liquids (ILs) are attractive alternative solvents in a variety of applications such as in supercapacitors, batteries, solar energy conversion and for the storage and clean-up of nuclear waste. However, their high viscosities impede their adoption in many large scale processes. Ether side chains have been shown to lower the viscosity of ILs. In this work, we have synthesized a series of ten pyrrolidinium ionic liquids and ten imidazolium ionic liquids bearing ether and alkyl side chains of varying lengths (4 to 10 atoms in length). Their physical properties, such as viscosity, conductivity and thermal profile were measured and compared. Results reveal a dramatic decrease in viscosity with the substitution of alkoxy (ether) side chains for alkyl side chains on the pyrrolidinium cation. However, as the length of the ether chain increases there is negligible change in the viscosity (of about 50 cP). In contrast, a consistent increase in the viscosity was observed as the length of the alkyl side chain increases. Preliminary results of the imidazolium series show a similar trend. These results provide significant insight on the choice of starting materials for researchers designing ILs for specific applications. The work at BNL was supported by the BNL Office of Educational Programs and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under contract DE- SC0012704.