Switchable Hydrophobicity and Hydrophilicity
Advances in the design, synthesis, and application of ionic liquid (IL)-based functional materials that show a reversible change in hydrophobicity and hydrophilicity in response to external stimuli, specifically temperature, are summarised in this chapter. Precisely-designed ILs with suitable hydrophobicity can afford the possibility to show a dynamic phase change after mixing with water. Apart from the static phase behaviour of IL/water mixtures, in which either a homogeneous or phase-separated state is kept intact regardless of the temperature, two types of dynamic phase changes exist, namely upper critical solution temperature (UCST)-type and lower critical solution temperature (LCST)-type phase changes. The ILs showing these phase behaviours are considered to have moderate solubility in water. Similar to general inorganic salts, the solubility of many ILs in water increases with heating, and the resulting mixtures show complete miscibility at a certain temperature. This behaviour is classified as a UCST-type phase change. Conversely, the LCST-type phase change, where homogeneous mixtures split into two phases upon heating, has recently been found in several IL/water mixed systems. Furthermore, LCST-type thermal responsiveness can also be realised in IL-based polyelectrolytes upon controlling the hydrophobicity/hydrophilicity balance of the starting IL monomers. Given the unique set of properties originating from ILs, one can conceive a wide variety of applications of the thus-prepared smart IL materials from both scientific and industrial perspectives.