Use of surfactants as selective modulators in the partition in aqueous two-phase systems: An alternative for speciation of dyes

The inappropriate disposal of effluents containing dyes in water bodies is one of the greatest environmental concerns in the last decades. An efficient alternative for the removal of those compounds from aqueous matrices is the use of aqueous two-phase systems (ATPSs). However, there are no investigation on the use of these systems for speciation of dyes. Here, we evaluated the use of ATPSs for the speciation of remazol yellow (RY) and methylene blue (MB) using the dodecyltrimethylammonium bromide (CTAB) surfactant as selective modulator in the partition. Experiments of individual partition of RY and MB were performed in the presence of different concentrations of CTAB in ATPSs formed by poly(ethylene glycol) (PEG), salt, and water. In the absence of CTAB, RY and MB partitioned to the top phase of the systems. However, in the presence of the surfactant, while the MB dye remained in the top phase, the RY dye was transferred to the bottom phase. This result was attributed to a specific interaction between the CTAB cationic surfactant and the RY anionic dye, and was highly dependent on the PEG molar mass (Figure 1a) and nature of salt (Figure 1b). Furthermore, when the CTAB concentration increased, the removal of RY to the bottom phase increased. From this finding, experiments of simultaneous partition of the dyes were performed in high CTAB concentration. In ATPSs formed by PEG 6000, (NH4)2SO4, and water, 92.99% of the MB dye remained in the top phase, while 78.34% of RY dye was transferred to the bottom phase. This result showed the potential of ATPSs for the speciation among dyes of opposite charges.