Group Interactions in Polyelectrolytes. III. The Electrostatic Effect of Neighboring Groups on the Ionization of Polyelectrolytes

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The two parameters in the modified Henderson-Hasselbach equation, pH=pKa−n′log-[(1−α)⁄α], where pKa and n′ are empirical constants and where α is a degree of the ionization of a polymeric acid, were evaluated on the assumption that the ionization of polyelectrolytes is electrostatically affected in the main by a few neighboring ionized groups and that the over-all ionization is composed of only a few elementary ionization. The titration curves of various polyelectrolytes in the absence of salt could be classified into two types: ΔpK′≥2 and ΔpK′<2, where ΔpK′ is the difference between the succeeding ionization constants. The maleic acid-styrene copolymer and polyvinyl amine belong to the former type, while polyacrylic acid and pectinic acid belong to the latter. In the former, a plot of pH against log[(1−α)⁄α] nearly falls on a line in only a limited range and n′ is substantially equal to the number of elementary ionizations. In the latter, both pKa and n′ are related to the distance between the groups. The titration curve of polyacrylic acid, drawn by means of the above equation with the parameters calculated on the basis of the kinetic data of the alkaline hydrolysis of polymethyl acrylate, was in good agreement with the observed titration curve. The titration data of other polyelectrolytes also fall nearly on their calculated titration curves.
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