Temperature, Polymer Molecular Weight, and Salt Effects on Phase Equilibria of PVP + Formate Salts + Water: Experimental Measurements, Correlations, and Thermodynamic Modeling

The phase diagram and liquid–liquid equilibrium data for poly(vinyl pyrrolidone) (PVP) of different molecular weights (10,000, 24,000, or 40,000) + potassium formate (KCHO2) or sodium formate (NaCHO2) aqueous two-phase systems were determined experimentally at different temperatures (298.15, 308.15, and 318.15 K). The phase composition and the tie-line ends were obtained by measuring and correlating the density (ρ) and refractive index (nD) of binary and ternary mixtures within the range of 0–46.6 wt % of PVP and 0–30 wt % of used salts. Furthermore, the binodal data were fitted to both the Pirdashti and Merchuk equations. The cloud point of PVP solutions was determined in the presence of salt at different concentrations. Additionally, the influence of the polymer's molecular weight and temperature on the binodal curve, tie-line length, and slope of tie-line is discussed and compared with the literature data. Moreover, the impact of the Hofmeister series on the separation factor of formate ion was determined. Finally, the extended NRTL and modified UNIFAC-NRF were used to compute the phase compositions of the studied systems, and the adjusted parameters of the models were reported. Results show that both models provide satisfactory agreement with experimental data.