Effect of aqueous two-phase system constituents in different poly(ethylene glycol)-salt phase diagrams

Abstract

Eight different poly(ethylene glycol) (PEG)/salt aqueous two-phase systems (ATPS's) were characterized at 23 °C. PEGs with different molecular weights (1500 and 8000) were combined with different salts (sodium or potassium citrates and formates) to study the effect of each ATPS constituent in the phase diagram. The binodal curve and several tie-lines were determined for each system using common analytical techniques described in the literature for ATPS's (cloud point method, conductivity, and gravimetric analysis after lyophilization). According to the results, it was found that each ATPS constituent produced a particular effect in the phase diagram. For the salts, changes in the anion produced larger variations in the heterogeneous region size than changes of the cation. Thus, the effect of salt type seems to be largely dependent on the anion, which makes the dominant contribution. The ability to form ATPS's is connected with the Gibbs energy of hydration, ΔG hyd, of the anions: more negative ΔG hyd values facilitate ATPS formation. Changes in the PEG molecular weight affected also the phase diagram: the size of the heterogeneous region increases with the increase in the polymer molecular weight.