Phase separation of polyethylene glycol/salt aqueous two-phase systems

Salt in polyethylene glycol (PEG)/salt aqueous two-phase systems was exclud ed by PEG and concentrated in the solvent volume available for dissolution of salt (PEG-free solvent). The concentration of salt in the PEG-free solve nt of the PEG-rich phase was the same as that at the critical point regardl ess of the compositions of the PEG/salt two-phase systems. This explained t hat the phase separation of PEG/salt two-phase systems occurs when the conc entration of salt in the PEG-free solvent reaches its solubility limit. The concentration of salt required in the PEG-free solvent for the phase separ ation was lower with higher molecular weight of PEG. The solubility of salt in the PEG-free solvent decreased with increases in the molal surface tens ion increment of salt. The solubility limit of salt in the PEG-free solvent was 0.93 M for ammonium sulfate, 0.77 M For potassium phosphate, 0.75 M fo r sodium tartrate, 0.67 M for sodium phosphate, and 0.53 M for potassium ci trate.