New Insights into the Partitioning of Phenothiazine Dyes in Aqueous Two-Phase Systems

Aqueous two-phase systems (ATPSs) have proven to be efficient and environmentally safe methods for extracting chemical species. However, the driving forces behind the partition of solutes in these systems are still little understood. A complete thermodynamic partitioning of phenothiazine dyes was investigated in poly(ethylene oxide) (PEO) + salt + water ATPSs. Standard transfer parameters (Gibbs free energy change (ΔtrGº), enthalpy change (ΔtrHº) and entropy change (TΔtrSº)) were evaluated, and their dependence on the dye structure, electrolyte nature and tie line length (TLL). All phenothiazine dyes are concentrated predominantly in the polymer enriched phase, with ΔtrGº values ranging from -4.1 up to -13.4 kJ mol-1. Due to the dye-PEO attractive interactions that occur mainly via benzene condensed ring present in the structures of phenothiazine dyes, the partitioning of these dyes was enthalpically driven, with -11.4 ≥ ΔtrHº ≥ -52 kJ mol-1 and -4.93 ≥ TΔtrSº ≥ -38 kJ mol-1.