Osmotic Properties of Charged Cylinders: Critical Evaluation of Counterion Condensation Theory

Abstract

The osmotic coefficient of B-DNA in water may, in moderately dilute solutions, deviate as much as 100 % from predictions based on a simple 'counterion condensation' theory. We determine the results for osmotic properties via a cell model description of the ionic atmosphere near a cylindrical polyelectrolyte. The cell model predictions for the osmotic properties disagree with predictions based on simple condensation theory, but are in surprisingly good harmony with experimental findings. We argue that the neglect of finite-radius effects makes simple condensation theory inapplicable at all but impractically low polyelectrolyte concentrations and, unable to reproduce osmotic properties of polyelectrolytes such as DNA.