Decomposition of deep eutectic solvents based on choline chloride and phenol in aqueous phase

Abstract

Deep eutectic solvents (DESs) are compounds formed by donor and acceptor of hydrogen bond in a liquid state at ambient temperature. They are non-volatile, with high thermal stability and readily dissolve many organic and inorganic compounds. DESs based on choline chloride and phenol have recently appeared in analytical practice and have already found wide application in various fields. The numerous articles have been reported efficient liquid-liquid extraction using these DESs as extraction solvents. In this paper the process of liquid-liquid extraction using DESs based on choline chloride and phenol was investigated. It was established that the DES decomposition in aqueous phase takes place due to the dissociation of choline and phenol, which leads to the destruction of the hydrogen bond. In addition, with the addition of an aprotic solvent, tetrahydrofuran, the extraction of organic phase is observed. This phase was mistakenly used as DES, but in our study we showed that this phase consists of tetrahydrofuran, water and phenol, which is not the initial phase of DES. This phenomenon was investigated by gas chromatography with mass spectrometric detection and coulometric Karl-Fischer titration It was confirmed that these DESs cannot be considered as extraction solvents in liquid-liquid extraction from aqueous solutions. The main goal of the present work is to pay attention of researchers to a wrong conception in liquid-liquid extraction using DESs based on choline chloride and phenol to avoid mistakes in the future.