Counterion Condensation in Strong, Flexible Polyelectrolytes
  We present results of molecular dynamics simulations of strong, flexible polyelectrolyte chains in solution with added salt. The effect of added salt on the polyelectrolyte chain structure is fully treated for the first time as a function of polymer density. Systems above and below the Manning condensation limit are studied. The chain contraction due to added salt is weaker than expected from simple screening arguments. The chain structure is intimately tied to the ion density near the chain even for chains in the counterion condensation regime. In contradiction to Manning counterion condensation theory, the ion density near the polymer chain depends on the amount of added salt, and above the condensation limit the chains significantly contract due to added salt. 
Mark J. Stevens and Steve Plimpton (Sandia National Laboratories)