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.