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, ξ, which range from 0.12 to 2.88. Activity coefficients were determined for Na+ and K+ ions at T=25°C. The results were compared with those predicted from the theories of Manning and of Iwasa. Above the critical value, ξc=1, agreement with theory is satisfactory, but for ξ<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, θNa and θK, decrease with decreasing ξ as predicted from theory. For ξ<1, the experimental error in θNa and θK 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 ξ>1, the binding isotherms are in reasonable agreement with theory. For ξ<1, the Manning theory tends to under-estimate the degree of Mg2+ binding, θMg. Interestingly, θMg is not zero at ξ=0.28, suggesting that for divalent ions counterion binding occurs above ξc=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 ξ should be replaced by ξeff. It is observed that there is a small binding specificity for Na+ over K+ at large ξ. This vanishes when ξ 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.
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Nordmeier, E. Studies of Polyelectrolyte Solutions V. Effects of Counterion Binding by Polyions of Varying Charge Density and Constant Degree of Polymerization. Polym J 26, 539–550 (1994). https://doi.org/10.1295/polymj.26.539
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DOI: https://doi.org/10.1295/polymj.26.539