Structure–Property Relationships for n‐Alkylamine and Aliphatic Carboxylic Acid‐Derived Thermomorphic Multiphase Systems

Thermomorphic multiphase systems (TMS) are liquids that display abrupt and reversible changes in miscibility with temperature. TMS based on mixtures of amines and carboxylic acids have been found to exhibit lower critical solution temperature (LCST) behavior, although prior examples have relied on clinical anesthetics as the amine component due to poor understanding of the relationship between the amine structure and LCST behavior. Here, the role of the amine partner of the TMS was systematically investigated, utilizing an array of primary, secondary, and tertiary amines in combination with aliphatic carboxylic acids. Systems featuring tripropylamine (TPA) or dibutylamine (DBA) were uniquely capable of displaying thermoswitchable behavior in the presence of a sufficiently hydrophobic aliphatic carboxylic acid, whereas primary amines and other secondary and tertiary amines investigated did not form TMS. TMS formation could be related to macroscopic hydrophilicity parameters for the amines, providing a foundational basis for future TMS design, and their ability to facilitate extractions was demonstrated. These findings highlight the potential to prepare TMS from scalable, low-cost amines toward the development of environmentally benign switchable solvent systems.