Collapse of polyelectrolyte macromolecules by counterion condensation and ion pair formation: A molecular dynamics simulation study

The conformational properties of single polyelectrolyte chains of various lengths in the presence of counterions are investigated by molecular dynamics simulations. For Coulomb interaction strengths below the critical value for Manning condensation, the molecular chain exhibits an increase of the radius of gyration and of the end-to-end distance with increasing interaction strength. Above this critical value, counterions condense on the chain and ion pairs are formed. The ion pairs possess a net attraction such that beyond a certain interaction strength the chain with the condensed ions collapses into a dense coil. The scaling behavior of the radius of gyration and the end-to-end distance with changes in the number of bonds is discussed for various Coulomb interaction strengths.