Constructing a phase-controllable aqueous biphasic system by using deep eutectic solvent as adjuvant

Aqueous biphasic systems (ABSs) have drawn much attentions in separation fields for their biocompatibility and sustainability. However, it is difficult to control the distribution of analytes in a certain phase, which leads to some problems in achieving selective extraction, recovery and reutilization of extraction agents, and separation of various complex analytes. Herein, we make a breakthrough to realize the precise control of pigments distribution in the PPG400/Na2SO4-based ABSs by an addition of trace deep eutectic solvent (DES). With an increase of DES volume ranging from 0.10 to 0.30 mL, all the amaranth in the top phase gradually shifts to the bottom phase. Amaranth in the upper phase showed a decreased trend from 95.45% to 4.09%. It is found that the addition of DES affects the distribution of phase components in the two phases, resulting in the different polarity of each phase which determines the distribution of pigments. Based on this discovery, we have changed the mass of phase-forming Na2SO4, the opposite transformation of amaranth from the bottom phase to the top phase has also been accomplished. Afterwards, the phase-forming process and the distribution of amaranth has been observed by optical microscope. Finally, the two-phase and three-phase systems have been constructed to separate complex compounds. This study demonstrates the controllable distribution of analytes in the ABSs, opening up a chance to realize selective, recyclable and sustainable separation of essential analytes.