Thermodynamic studies on the complete phase diagram of the ternary aqueous system containing polyvinylpyrrolidone 3500 and sodium sulfate at different temperatures: Experiment and correlation

• LLE and LLS of (polyvinylpyrrolidone + sodium sulfate + water) was studied.
• The effects of temperature and type of salt on the binodal were investigated.
• Crossing of binodals was observed at different temperatures.
• Binodal data were correlated with two temperature dependent equations.
• Tie-lines at different temperatures were fitted with different thermodynamic models.

The complete phase diagram for the {polyvinylpyrrolidone (PVP) + sodium sulfate (Na2SO4) + water} with PVP of molar mass 3500 g ⋅ mol−1 was determined at T = 298.15 K and atmospheric pressure (≈95 kPa). (Liquid + liquid) equilibria (LLE) of {PVP3500 + sodium sulfate + water} was also studied at T = (298.15, 308.15 and 318.15) K. Furthermore for this system and some previously studied systems containing PVP3500 the free energies, enthalpies and entropies of cloud points were calculated at the mentioned temperatures in order to investigate the driving force formation of these two-phase systems. In addition, the calculated free energies for phase separation of the studied systems were used to investigate the salting-out ability of the salts having different anions. The effect of temperature on the salting-out effect of polyvinylpyrrolidone by sodium sulfate has been studied. For representing the experimental binodal data the Merchuk equation in the original form and with the temperature dependency and an empirical equation were used. A temperature dependent Setschenow and the extended NRTL were used to fit the tie-line data of this system. Also, we estimated plait point, slope and the length of tie-lines at T = (298.15, 308.15 and 318.15) K.