Temperature effect on the extraction of carboxylic acids by amine-based extractants

The extraction of carboxylic acids by extractants that contain lipophilic amines is dependent on temperature. That phenomenon is used in industry, where extraction is conducted at approximately ambient temperature and back-extraction occurs at an elevated temperature. In this work, stronger temperature effects are observed in cases where the alkyl amine is relatively weak (pH_hn <5), the alkyl amine is highly substituted, the carboxylic acid is relatively weak (pK_a >3), polycarboxylic acid is extracted, the concentration of the amine and/or the carboxylic acid in the organic phase is low, the amine and/or acid are bulky, and the diluent of the amine has low polarity. This shows that the temperature effect is strong in systems where the ion-pair interaction between the protonated amine and the acid's anion is relatively weak (weak acid and amine), where ion-pair stabilization is dependent on the aggregates' formation (hydrophilic acids, low-polarity amine, low concentration) and where the formation of those aggregates is hindered (bulky amines and acids).