Salt effects on the cloud point of the poly(ethylene oxide)+ water system

Abstract

The cloud point of high-molecular-weight poly(ethylene oxide), PEO, in aqueous salt solution has been determined as a function of the salt concentration for all potassium halides, alkali-metal chlorides and alkali-metal hydroxides. Our theoretical model for the pure PEO+water system (J. Chem. Soc., Faraday Trans. 1, 1981, 77, 2053) has been extended to include the effects of salt on the phase separation. Basic features of the present model are a hydration shell with enhanced structuring of water as well as a zone with decreased salt concentration surround each chain. Overlaps of such regions are involved in polymer–polymer contacts and imply transfer of water molecules and ions from the proximity of the chains to the bulk solution, which gives important contributions to the free energy of interaction. The existence of the salt-deficient zone is explained as a consequence of asymmetric hydration of the ions near the polymer. The effects of the zone are large enough to account for the influence of salts on the clouding. The experimental differences found for the alkali-metal halides have been rationalized mainly in terms of varying degrees of salt penetration into the region around the chain.