Effect of Ionic Strength on the Dynamic Mobility of Polyelectrolytes

Abstract

The electrokinetic sonic amplitude (ESA) technique and density measurements have been used to determine the dynamic mobility of a cationic polyacrylamide, CPAM (mol wt 5 × 106), and two cationic polyamines (mol wt 5 × 104 and 5 × 105). For all three polyelectrolytes, the ESA signal increased linearly with increasing polymer concentration up to 4 mg/mL. The dynamic mobility was higher for the larger polyamine than for the smaller, but after correction for its higher charge density, it was found that the dynamic mobility was essentially independent of molecular weight for the polyamines. At low ionic strength the dynamic mobility of the polyacrylamide was 5 and 6 times lower than those for the two polyamines, in agreement with its 5 and 6 times lower charge density, respectively.The dynamic mobility decreased with increasing ionic strength for all three polymers, as expected. However, the corresponding electrokinetic charge fraction of the polymers, calculated 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.; Åkerman, B. Langmuir 1998, 14, 3512), which shows that the atypical behavior of the polyacrylamide and the polyamines is not an inherent property of the dynamic mobility of polymers. The apparent persistence length of the polyacrylamide was evaluated from viscosity measurements. From comparison with the electrophoretic behavior of other polymers, it is concluded that CPAM is free-draining in ESA measurements, which shows that the cylinder model is applicable. The decreasing charge fraction thus most likely reflects a real change in the electrokinetic charge of the polymer with increasing salt concentration.