Coupled diffusion of weakly ionized polyelectrolytes. Polyacrylic acids in water

Ternary diffusion coefficients have been measured conductimetrically for polyacrylic acid (90,000 g-mol^–1) + polyacrylic acid (5,000 g-mol^–1) + water at 25°C. Binary diffusion coefficients for each fraction of polyacrylic acid have also been measured. The conductimetric results support previously reported Gouy optical data that indicated strong coupling between the diffusing polyacrylic acids. Although the carboxylic groups are only weakly ionized (pK_a 4.3), each polyacrylate anion carries a substantial electric charge due to the large number of carboxylic groups per polyion. An approximate analysis suggests that the electric field generated by diffusion of the polyacrylate ions with hydrogen counterions is responsible for the strongly coupled diffusion. The theory provides an explanation for the previously reported experimental results that indicate heavier fractions of polyacrylic acid can diffuse more rapidly than lighter fractions. To help interpret the polyacrylic acid data, ternary diffusion coefficients have also been measured for n-butyric acid + acetic acid + water. The diffusion of the monocarboxylic acids is weakly coupled.