Aqueous biphasic systems formed by hydrophilic and hydrophobic deep eutectic solvents for the partitioning of dyes.

Two kinds of hydrophobic deep eutectic solvents (DESs), including hexafluoroisopropanol (HFIP)-based DESs and polypropylene glycol 400 (PPG400)-based DESs, were synthesized to construct aqueous biphasic systems (ABSs) with choline chloride (ChCl)-based hydrophilic DESs for the first time, respectively. Three kinds of dyes with different hydrophobicity, including tartrazine, methylene blue and sudan Ⅲ, were chosen as the target analytes to evaluate the partitioning behaviors of the DES/DES ABSs. The effect of the types of hydrophilic DESs on dyes extraction were investigated and ChCl-glycol (ChCl-G) was selected as the phase-forming component. Then the partitioning of dyes in diverse DES/DES ABSs with different hydrophobicity was addressed by altering the carbon chain length of hydrogen bond accepter (HBA) in hydrophobic DES and changing the molar ratio of HBA: hydrogen bond donor (HBD) in hydrophilic DES. The results proved that in the ABSs of HFIP-based DES/DES, the sudan Ⅲ tended to migrate to the hydrophobic DES-rich phase with the increased carbon chain length of hydrophobic DES and was inclined to transfer to the hydrophilic DES-rich phase with the increasing proportion of G in ChCl-G. Afterwards, the methylene blue was chosen to explore the influence factors of the extraction process. And the results showed that the adjusting of pH value could achieve a complete opposite distribution of methylene blue in PPG400-based DES/DES ABSs. Besides, extraction of dyes in real samples were evaluated and recoveries of 92.3%-106.1% were achieved. Moreover, the analysis of mixed samples demonstrated that 88.64% of tartrazine and 92.63% of methylene blue were enriched into the hydrophilic phase, while nearly all of the sudan Ⅲ was moved into the hydrophobic phase. In addition, the phase-forming components could be reused according to the regeneration studies. Method validation proved the good precision, repeatability and stability of the established method. At last, the extraction mechanism was further investigated by dynamic light scattering (DLS) and transmission electron microscope (TEM). It turned out the formation of DES-dye aggregates might be responsible for the separation process. Above all, the results highlighted the possibility of the DES/DES ABSs as tunable systems for the partitioning of dyes with different hydrophobicity.