Role of water structure on phase separation in polyelectrolyte–polyethyleneglycol based aqueous two-phase systems

Partitioning of proteins in aqueous two-phase systems (ATPS) has emerged as one of the important downstream processing techniques in bioprocess technology. The phase separation behavior of polyelectrolyte–polyethyleneglycol (PEG) based ATPS have been studied to elucidate the mechanism controlling phase behavior. The effect of various inorganic salt additives revealed the importance of water structure as a major factor controlling phase separation in these systems. Nitrate and potassium (water structure breaking ions) elevated the binodial line while sulphate, phosphate and sodium (water structure making ions) depressed the binodial line in both polyacrylic acid–PEG as well as polyethylenimine–PEG based ATPS. The effect of increase in concentration of either of the constituent polymers in both systems (at constant salt concentration) always led to a greater propensity towards phase separation. These results point to a mechanism in which salt-assisted polymer-modified water structure interactions play a central role in phase separation in ATPS.