Physical-chemical properties of newly synthesized tetraalkylammonium alkyl ether carboxylate ionic liquids

Abstract

Thirty-nine different Ionic Liquids were synthesized, with symmetric quaternary ammonium cations and alkylethercarboxylates as anions. Similar to the recently published Concept of Melting Point Lowering due to Ethoxylation, here propoxyethylene groups, PO groups, in the anions allow a wide range of salts being liquid at or close to room temperature. The physical chemical properties, density, viscosity, conductivity, thermal stability, melting behaviour and hygroscopy of such Ionic Liquids were determined. Density was linearly dependent on the temperature and was affected by the molecular weight of the Ionic Liquid. Higher viscosity of longer carbon chains resulted from stronger intermolecular Van der Waals forces. Viscosity of the Ionic Liquids was lowered by shorter carbon chains in the anions, bulkier cations or lower propoxyethylene degrees. The conductivity of the Ionic Liquids ranged from 1 to 3000 μS/cm and was dependent on the structure of the compound. Melting points of Ionic Liquids were dependent on the length of the carbon chain. At high temperatures, they decomposed in a two-step mechanism. First, the cationic part decomposed, and, at a higher temperature, the anionic part of the Ionic Liquid. Moreover, the Ionic Liquids showed a strong hygroscopic behaviour that was observed for an extended time range up to 5 days.