Unusual LCST-type behaviour found in binary mixtures of choline-based ionic liquids with ethers

Abstract

The fluid phase equilibria of binary mixtures of three cholinium-based ionic liquids-N-alkyl-N,N-dimethylhydroxyethylammonium bis(trifluoromethane)sulfonylimide, [N_1 1 n 2OH][Ntf₂]^– with nine different ethers have been investigated. The experiments were conducted at atmospheric pressure between 200 K and 360 K using the cloud-point method. Most of the systems exhibit unusual liquid–liquid demixing behavior characterized by the existence of lower critical solution temperatures (LCSTs). To the best of our knowledge this is the first report on a functionalized ionic liquid plus ether system exhibiting a LCST behavior. The mutual solubilities of the ionic liquids and ethers are larger (higher LCST values) for ionic liquids with longer alkyl side chains attached to the cations. On the other hand, the solubility of the ethers is defined by the exposure of the corresponding C–O–C dipole and the extension/compactness of the alkyl residues surrounding the ether function. Auxiliary ab initio calculations and Molecular Dynamics simulations were used to rationalize the experimental results at a molecular level.