First Investigation of the Micelles Forming in a Novel Deep Eutectic Solvents-Based Aqueous Micellar Two-Phase System: Partitioning of Cationic/Neutral/Anionic Pigments

A novel aqueous micellar two-phase system (AMTPS) formed by two new types of PEG-based hydrophobic (acted as cationic surfactant or nonionic surfactant) and hydrophilic deep eutectic solvents (DESs) was first established. Three typical representative pigments, methylene blue (MB), Sudan III (Su), and sunset yellow FCF (SY) with diverse charge and hydrophobicity, were selected as the target to evaluate the possibility of the novel DES/DES-based AMTPS as a green extraction technology for partitioning various substances. The constitutions of AMTPS were systematically studied, including the effects of hydrophobic and hydrophilic DES types, molar ratio of choline chloride/ glycerol, and molecular weights of PEG. After investigation, the DES/DES-based AMTPS was prepared with [N-8881]Cl/ PEG400 as hydrophobic DES and Ch/G (molar ratio 1:1) as hydrophilic DES. The concentration of pigments, ionic strength, pH value, and temperature were comprehensively evaluated in the influence factor experiments. Also, the results showed that more than 95.00% of Su/SY distributed in the micellar-rich phase, and nearly all of MB could be transferred from micellar-poor phase to micellar-rich phase with the increase of the pH value. Response surface methodology was utilized to further optimize the extraction conditions for Su. It was found that almost 99.75% Su was separated into the micellar-rich phase. Method validation illustrated that the proposed method had the priority to get precise experimental data for pigments' separation. Mechanism studies through the conductivity, dynamic light scattering (DLS), and transmission electron microscope (TEM) proved that the formation of micelles between pigments and DESs by hydrophobic and electrostatic interaction was the dominant force in the separation. In addition, [N-8881]Cl/PEG400 could efficiently be recovered and reused in the regeneration studies. The novel DES/DES-based AMTPS proposed in this work provided an eco-friendly extraction way in the separation of various charges or hydrophobicity substances.