STUDIES OF POLYELECTROLYTE SOLUTIONS .5. EFFECTS OF COUNTERION BINDING BY POLYIONS OF VARYING CHARGE-DENSITY AND CONSTANT DEGREE OF POLYMERIZATION

Abstract

The effect of the charge density on the behaviour of polyelectrolytes in contact with counterions was studied by activity measurements, Donnan dialysis, dye-spectrophotometry, and potentiometric titration. The polyelectrolytes investigated were poly(acrylic acid), i.e., PAA, and acrylic acid/acrylamide copolymers, i.e., PAM-y, where y is the degree of hydrolysis. This gave charge density parameters, xi, which range from 0.12 to 2.88. Activity coefficients were determined for Na+ and K+ ions at T = 25-degrees-C. The results were compared with those predicted from the theories of Manning and of Iwasa. Above the critical value, xi(c) = 1, agreement with theory is satisfactory, but for xi < 1 there are significant deviations. The probable reason is change in the polyion flexibility. Binding isotherms for the univalent counterions, Na+ and K+, were determined by utilizing the dialysis technique. It is found that the degrees of binding, theta(Na) and theta(K), decrease with decreasing xi as predicted from theory. For xi < 1, the experimental error in theta(Na) and theta(K) becomes very large (> 20%), but within this error no counterion binding is observed. The binding of the divalent counterion Mg2+ was examined by a dye spectrophotometric method, where Na+ or K+ ions were added in excess. The results were analyzed within Manning's two-variable theory. For xi > 1, the binding isotherms are in reasonable agreement with theory. For xi < 1, the Manning theory tends to under-estimate the degree of Mg2+ binding, theta(Mg). Interestingly, theta(Mg) is not zero at xi = 0.28, suggesting that for divalent ions counterion binding occurs above xi(c) = 0. 5 as well as below. Potentiometric titration shows that there is some polyion folding for PAM-y. The degree of folding increases with decreasing y. This leads to increase of the effective polyion charge, so that xi should be replaced by xi(eff). It is observed that there is a small binding specificity for Na+ over K at large xi. This vanishes when xi becomes small. The reason is not clear yet. It may be due to increase in the content of helical segments of PAM-y as y decreases.