Role of Competitive PEO−Water and Water−Water Hydrogen Bonding in Aqueous Solution PEO Behavior

The behavior of aqueous solutions of poly(ethylene oxide) (PEO) is studied theoretically by applying a mean field-like approach which includes the effect of the competition of PEO and water as proton acceptors in hydrogen bond formation. Accounting for this effect is of crucial importance for a correct description of all solution properties. We calculate the temperature and concentration dependence of the average fraction of hydrogen bonds between PEO and water and find very good agreement between our predictions and experimental or MD simulation data. We also make predictions concerning the temperature behavior of the second virial coefficient A₂ and the effective interaction parameter χ_eff and compare it with experimental data. We found that the decrease of A₂ with temperature is caused by the delicate balance of the opposing effects of water−PEO and water−water hydrogen bonding. The phase diagram for PEO of different molecular weights in water is calculated using experimentally reported data for the energy and entropy of association. We achieved very good quantitative agreement with most of the experimental data reported, in particular reproducing the closed loop regions of phase coexistence. We also compare our findings with results of other theoretical models.