Effect of ionic strength on the dynamic mobility of polyelectrolytes

The electrokinetic sonic amplitude (ESA) technique and density measurements have been used to determine the dynamic mobility of a cationic polyacrylam ide, CPAM (mol wt 5 x 10(6)), and two cationic polyamines (mol wt 5 x 104 a nd 5 x 10(5)). For all three polyelectrolytes, the ESA signal increased lin early with increasing polymer concentration up to 4 mg/mL. The dynamic mobi lity was higher for the larger polyamine than for the smaller, but after co rrection for its higher charge density, it was found that the dynamic mobil ity was essentially independent of molecular weight for the polyamines. At low ionic strength the dynamic mobility of the polyacrylamide was 5 and 6 t imes lower than those for the two polyamines, in agreement with its 5 and 6 times lower charge density, respectively. The dynamic mobility decreased w ith increasing ionic strength for all three polymers, as expected. However, the corresponding electrokinetic charge fraction of the polymers, calculat ed by modeling the polyelectrolytes as cylinders, decreased with increasing ionic strength. This in contrast to the constant charge fraction evaluated from the dynamic mobility of DNA (Rasmusson, M.; Akerman, B. Langmuir 1998 , 14, 3512), which shows that the atypical behavior of the polyacrylamide a nd the polyamines is not an inherent property of the dynamic mobility of po lymers. The apparent persistence length of the polyacrylamide was evaluated from viscosity measurements. From comparison with the electrophoretic beha vior of other polymers, it is concluded that CPAM is free-draining in ESA m easurements, which shows that the cylinder model is applicable. The decreas ing charge fraction thus most likely reflects a real change in the electrok inetic charge of the polymer with increasing salt concentration.