Solid-liquid phase behavior of mixtures of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides involving long alkyl side chains

Abstract

The solid-liquid equilibria of four binary mixtures consisting of different pairs of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids with variable lengths of the alkyl substituent were investigated. For the binary system resulting from the mixing of the ionic liquids with the ethyl and octadecyl substituents, the solid-liquid phase behavior was found to correspond to a non-eutectic system. On the contrary, for the three systems generated by mixing two 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids with long alkyl substituent (14–18 carbon atoms), a eutectic behavior was identified. The experimentally determined compositions and temperatures of the corresponding eutectic points were relatively close to those modelled by the Schröder-van Laar equation for systems with an ideal solution behavior and complete immiscibility of the species in the solid state. All three eutectic compositions had a melting temperature near the ambient one, meaning a decrease of up to ca. 20 °C with regard to the melting temperature of the lower melting parent ionic liquid. The thermal stability of the eutectic compositions was found to be equivalent to that of the parent ionic liquids. Excess molar volumes and changes of property (viscosity or surface tension) by mixing for the eutectic compositions were found to be acceptably small as to get fair estimations of the properties of the eutectics from simple combinations of the properties of the parent ionic liquids.