Donnan equilibrium and the effective charge of sodium polyacrylate

Osmometry using an external stressor is a very useful method to measure the equilibrium osmotic pressure for dilute solutions of polyelectrolyte. By t aking into account the contribution or the ideal gas law, the excluded volu me, the solvency effect, and the Donnan equilibrium effect on the measured pressure it is possible to estimate the effective charge of sodium polyacry late 35 kgmol(-1) as a function of the polymer concentration, the pH, the i onic strength, and the presence of Ca2+ ion. The numerical resolution of st ate equations has shown that the effective charge increases with the ionic strength or with the decreasing polymer concentration, in agreement with re cent theoretical models. On the other hand, the effective charge is pH-inde pendent. This statement remains valid as long as the degree of neutralizati on or the polyacrylate is over 0.5. Above this degree of neutralization, an y further neutralization promoted by NaOH addition leads to the condensatio n of the Na+ counterion, in agreement with the general concept of ionic con densation. The effective charge represents only 10-20% of the total number of monomer units for pH within 6 and 9 and ionic strength below 0.1 M. The polymer can tolerate the presence of Ca2+ at least up to a molar ratio Ca2/-COOH = 0.222 without any influence on the effective charge.