Chapter 5: Switchable-polarity Solvents

Unlike conventional organic solvents, switchable-polarity solvents (SPS) change between low-polarity and high-polarity states whenever a trigger such as CO₂ is applied. In a multistep chemical process, a conventional solvent that performs well for one step is unlikely to be satisfactory for the next step because each process step has different requirements. However, a switchable solvent can meet the requirements of one process step and then change its properties in order to meet the requirements of the subsequent step. For example, a desirable solute can be extracted by one state of an SPS, because that solute is soluble in that state of the SPS. After the extraction, the SPS can be switched to its other state, in which the solute is insoluble. This triggers the precipitation of the solute. The solvent can then be switched back and used again. Thus the entire process can be performed without the solvent being distilled, reducing energy costs, avoiding volatile solvent emissions, and eliminating the need for a second solvent. Many types of SPS have been reported, including single and two-component SPS using various chemistries. Applications include use of the SPS as reaction media, extraction solvents, sensors, CO₂-capture agents, and especially pretreatments and separation solvents for biomass conversion.