Liquid–Liquid–Liquid Aqueous Triple-Phase System Based on DES and Dipotassium Hydrogen Phosphate

Deep eutectic solvent (DES) and inorganic salt can form a salt–salt aqueous two-phase system (ATPS) and have been widely studied recently. The DES-based ATPS extends the polarity of the analyte of the separation as well as the characteristics of environmental benignity, biocompatibility, and low toxicity. At present, aqueous two-phase systems are mainly reported, but seldom are aqueous triple-phase systems (ATrPS) found and investigated. In this paper, a clear liquid–liquid–liquid (L–L–L) triple-phase aqueous system has been developed based on 100 wt % DES (choline chloride/PEG 400/glycerol = 1:1:1) and 50 wt % dipotassium hydrogen phosphate (K2HPO4) at 298 K and under atmospheric pressure. The specific physicochemical properties of ATrPS, such as phase ratio, density, viscosity, and dissolvability, have been studied. It is shown that DES plays a key role in the formation of three-phase ATPS. When DES increases by 5 wt %, the upper and middle phases increase steadily in volume to 11.50 and 6.80 wt %, respectively, while the lower phase decreases by 2.44 wt %. The polarity of each phase in ATrPS in terms of dissolving naphthalene is less than that of ATPS and can be tuned to be flexible. By analyzing the infrared spectra of the two phases and three phases of ATPS, it is shown that a complete DES-rich phase (upper phase) in the triple-phase ATPS is observed. The preparation of three-phase ATrPS expands the separation dimension of two-phase ATPS and brings more possibilities for the extraction and separation of complex solutes.