Thermoresponsive ionic liquid/water mixtures for separation and purification technologies

Some ionic liquids (ILs) having moderate hydrophobicity show unique phase behavior after mixing them with water such as lower critical solution temperature (LCST)-type phase transition. Such IL/water mixtures are obtained as a homogeneous clear solution at a certain temperature, but it shows liquid/liquid phase separation after heating them up. From the viewpoint of water content, hydration degree of these ILs can be controlled by temperature. Even after the phase-separation, the IL phase still contains a considerable amount of water. This should be helpful in dissolving many materials including proteins. In this mini review, we introduce typical ILs to show this LCST-type phase transition to consider the requirements for component ions to show this transition. Also, we explain the way to control this thermoresponsibility. Some separation techniques are also shown as an application of this phase change.