Amine-free CO2-switchable hydrophilicity solvents and their application in extractions and polymer recycling

Abstract

Volatile organic solvents are used in almost every industry currently in operation. Their key advantage, being easily removed from products, is overshadowed by the issues associated with their vapour emissions, inhalation risks, smog formation, and flammability. There is, therefore, a dire need for replacement solvents that are much less volatile and yet are as easily removed from products. Switchable-hydrophilicity solvents (SHS) meet this need. SHS are solvents that alternate between water-miscible and water-immiscible states upon the application of a trigger. Carbon dioxide functions as the trigger, as it acidifies the solution and causes protonation of the switchable solvent, which in the past has almost always been a liquid amine. Anionic SHS (ASHS) systems, a recently discovered variation on the idea of an SHS, consist of a hydrophobic carboxylic acid paired with a water-soluble base. In these systems, the low miscibility carboxylic acid (in its neutral state) is combined with a base dissolved in water. A very small number of ASHS have been reported, but they can only function in the presence of an amine and an unacceptably large excess of water. We sought an SHS that has as little toxicity as possible, and for that reason, we preferred one that contains no amine at all. We now report that the combination of branched carboxylic acids and a substoichiometric quantity of either NaHCO₃ or NaOH forms an SHS with very little toxicity and does not require a large excess of water. Two applications of these new switchable solvents are described.