The phase behaviour of aqueous solutions of poly(ethy1ene oxide), PEO, is analysed by means of a structural model of the system and a simple statistical-mechanical model based thereupon. The intention is to elucidate the structural questions involved in the water-PEO coupling and to gain some insight in possible consequences of this coupling. The experimental partial molar enthalpy and entropy of water can be reproduced, at least in fairly dilute solutions, if a zone with increased structuring of the water is assumed to exist around the PEO chain. The phase separation that takes place at high temperatures is traced back to the increase in total extension of the zones of enhanced water structure that occurs when the water content is increased. The chain-length dependence of the location of the solubility gap is mainly determined by the combinatorial entropy of the chains. The water solubility of PEO, which is unique in this respect among the polyethers, can be explained in terms of a good structural fit between the water and the polymer.

The cloud point of high-molecular-weight poly(ethy1ene 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. I, 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.

The nuclear magnetic relaxation rates of 7Li, 23Na, 133Cs, 36C1, and 81Br have been measured for alkali halide-poly(ethylene oxide) (PEO)-water mixtures. Addition of PEO to aqueous salt solutions increases the relaxation rates markedly for all the nuclei studied. The largest effect has been observed for 133Cs: in a 1.0 m CsBr solution containing 45% PEO by weight its relaxation rate is ~65 times larger than that in the polymer-free solution. The increases found are mainly caused by (i) asymmetric hydration of the ions, induced by the PEO, and (ii) direct cation-ether oxygen interactions of the same type-but much weaker-as in metal complexes with crown ethers and similar compounds. From the temperature dependences of the relaxation rates activation energies have been calculated. Addition of PEO raises the activation energy-typically by a factor of ~4 for a PEO-rich sample compared to that of a pure aqueous salt solution-which reflects a stabilization of the water structure, due to the presence of the polymer.

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