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 taking into account the contribution of the ideal gas law, the excluded volume, the solvency effect, and the Donnan equilibrium effect on the measured pressure it is possible to estimate the effective charge of sodium polyacrylate 35 kgmol⁻¹ as a function of the polymer concentration, the pH, the ionic strength, and the presence of Ca²⁺ ion. The numerical resolution of state equations has shown that the effective charge increases with the ionic strength or with the decreasing polymer concentration, in agreement with recent theoretical models. On the other hand, the effective charge is pH-independent. This statement remains valid as long as the degree of neutralization of the polyacrylate is over 0.5. Above this degree of neutralization, any further neutralization promoted by NaOH addition leads to the condensation of the Na⁺ counterion, in agreement with the general concept of ionic condensation. 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. Received: 11 July 2000/Accepted: 23 October 2000