Non-Uniformity of Cross-linking in Ion-Exchange Polymers

Abstract

IN theories of ion-exchange, the exchanger material is usually treated as a homogeneous phase, and transfer phenomena between the exchanger and aqueous solutions have always been explained on the basis of equilibria between two homogeneous phases. Recent experiments have shown that this simple picture cannot be maintained. The exclusion of co-ions by ion-exchangers in equilibrium with an aqueous solution, for example, the exclusion of cations by anion-exchangers with fixed cationic groups, or vice versa, has been well known for many years (for numerous references see the recent paper by Freeman¹). Qualitatively this phenomenon is well understood on the basis of a Donnan equilibrium, which requires that the product of the ion activities of the electrolyte in the exchanger must be proportional to that in the solution. For a 1 : 1 electrolyte solution in contact with an anion-exchanger: Hence, at low concentrations where m̄ ≪ M, the uptake m̄ of a 1 : 1 electrolyte like sodium chloride in an anion-exchanger of high fixed charge molality M should be proportional to the second power of the concentration, or more precisely, of the ion activity (mγ) of this electrolyte in the equilibrating solution.