Effect of temperature on the phase equilibrium of aqueous two-phase systems containing polyvinylpyrrolidone and disodium hydrogen phosphate or trisodium phosphate

Phase diagrams and liquid–liquid equilibrium (LLE) data of the aqueous polyvinylpyrrolidone–disodium hydrogen phosphate and aqueous polyvinylpyrrolidone–trisodium phosphate systems have been determined experimentally at 298.15, 308.15, 318.15 and 328.15 K. The effect of temperature and the type of salt on the binodal and tie-lines has been studied. It was found that, for the studied systems in the polyvinylpyrrolidone (PVP)-rich region, an increase in temperature caused the expansion of one-phase area; while, for the salt-rich region, expansion of the two-phase area was observed with increase in the temperature. It was also observed that the slope of all equilibrium tie-lines increased with increasing temperature. Furthermore, it was found that, as temperature is increased, these aqueous two-phase systems exhibit a phase inversion. Specifically, the PVP-rich phase – which at low temperatures is the less dense top phase – becomes the lower phase at elevated temperatures. Finally, an extended Flory–Huggins theory proposed in our previous paper has been used for correlation of LLE data.