Surfactant as selective modulator in the partitioning of dyes in aqueous two-phase systems: A strategy for separation

Abstract

In this work, aqueous two-phase systems (ATPSs) were employed to separate two model dyes, remazol golden yellow (RY) and methylene blue (MB), using the cetyltrimethylammonium bromide (CTAB) surfactant as a selective modulator in the partitioning. Experiments involving individual and simultaneous partitioning of the dyes were performed in the absence and presence of CTAB in ATPSs formed by a polymer (polyethylene glycol (PEG), polypropylene glycol (PPG), or poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer (L35)), a salt ((NH4)2SO4, MgSO4, Na2SO4, or Li2SO4), and water. In experiments of individual partitioning, RY and MB were transferred to the top phase in ATPSs formed using PEG in the absence of CTAB. However, in the presence of certain surfactant concentrations, the MB dye remained in the top phase, while the RY dye was transferred to the bottom phase. The same result was obtained in experiments of simultaneous partitioning. This was attributed to a specific interaction between CTAB and RY, which promoted the formation of RY-CTAB aggregates that were stabilized in the bottom phase. The amount of RY transferred to the bottom phase in the presence of CTAB was highly dependent on the polymer molar mass and the natures of the salt and the polymer. Interestingly, the surfactant did not promote the transfer of RY to the bottom phase in ATPSs formed by PPG or L35, due to competition between the polymer and the RY dye for the CTAB monomers. The best separation factor between the dyes was of 271.2 in the ATPS formed by PEG with molar mass of 1500 g mol−1, MgSO4, and water, hence demonstrating the potential of ATPSs for separation among dyes.