Aqueous two-phase systems of polyethylene glycol and inorganic salts: a statistical model

Thermal-statistical models that try to reproduce the behavior of aqueous polymer solutions have been extensively studied over the years. However, the number of articles related to aqueous systems containing polymers and added salts is still very restricted. We investigate a model based on the Flory-Huggins theory to analyze the formation of aqueous two-phase systems comprising long-polymer chains of polyethylene glycol in mixtures containing monovalent inorganic salts. In the proposed statistical model the competition between the hydrogen-bond formation among water molecules and the monomers of the polymer chain, in addition to the ion-dipole interactions between water and the dissociated salt ions play a key role in shaping the equilibrium configurations. Through numerical fits of experimental data presented in the literature, for each type of salt introduced into the mixture, with model predictions for the phase-separation temperatures, the so-called Cloud-Point Temperatures (CPT), we find a set of four phenomenological interaction parameters that enables proper theoretical representation of the continuous behavior of the CPT's as a function of the concentration of solutes. The observed behavior of the adjusted parameters can be interpreted in terms of the Hofmeister (or lyotropic) series. The reduced number of adjustable parameters needed to reproduce some experimental data is one of the advantages of the proposed model over other theoretical approaches as activity-coefficient chemical models like UNIFAC or UNIQUAC.