Phase separation of poly(ethylene glycol)-water-salt systems

Abstract

Phase separation temperatures, each corresponding to lower critical solution temperature ( LCST) for solutions of poly(ethylene glycol) (PEG) in water-sodium chloride (NaCl) and in water-propionic acid-sodium salt (Pro-Na), have been determined for PEG with molecular weights of M η = 2.18 × 10 3, 8 × 10 3 and 719 × 10 3 over concentration ranges from 0–1.09 M (mol/1000 g solvent) NaCl and 1.02 M Pro-Na. The phase separation temperature decreases with an increase of salt concentration and depends on polymer molecular weight. The thermal pressure coefficient, thermal expansion coefficient, and density have been determined from 20° to approximately 60°C for ethylene glycol-water solutions over the entire concentration range and also for aqueous salt solutions over the concentration ranges from 0–1.7 M NaCl and 0–0.5 MPro-Na. The excess thermal pressure coefficient, γ E V , excess thermal expansion coefficient, α E , and excess of temperature dependence of γ V , [ ( ∂ γV ∂T ) E ϱ ], for the EG-water system are all positive, while the excess volume of mixing V E is negative. The thermal pressure coefficient and thermal expansion coefficient for aqueous salt solutions water-Pro-Na and water-NaCl increase with an increase of salt concentration. The behaviour of the two polymer-salt-water solutions is discussed in terms of a thermodynamic equation of state, and a shortcoming of the usual formulation of the corresponding states theory of polymer solutions is pointed out.