Structure of salt-free linear polyelectrolytes in the Debye-Huckel approximation

We examine the effects of the common Debye-Huckel approximation used in theories of polyelectrolytes. Molecular dynamics simulations using the Debye-Huckel pair potential of salt-free polyelectrolytes have been performed. The results of these simulations are compared to earlier ''Coulomb'' simulations which explicitly treated the counterions. We report here the comparisons of the osmotic pressure, the end-to-end distance and the single chain structure factor. In the dilute regime the Debye-Huckel chains are more elongated than the Coulomb chains implying that the counterion screening is stronger than the Debye-Huckel prediction. Like the Coulomb chains the Debye-Huckel chains contract significantly below the overlap density in contradiction to all theories. Entropy thus plays an important and sorely neglected role in theory.