Inter- and intrachain attractions in solutions of flexible polyelectrolytes at nonzero concentration

Constant temperature molecular dynamics simulations were used to study solu tions of flexible polyelectrolyte chains at nonzero concentrations with exp licit counterions and unscreened Coulombic interactions. Counterion condens ation, measured via the self-diffusion coefficient of the counterions, is f ound to increase with polymer concentration, but contrary to the prediction of the Manning theory, the renormalized charge fraction on the chains decr eases with increasing Bjerrum length without showing any saturation. Scalin g analysis of the radius of gyration shows that the chains are extended at low polymer concentrations and small Bjerrum lengths, while at sufficiently large Bjerrum lengths, the chains shrink to produce compact structures wit h exponents smaller than a Gaussian chain, suggesting the presence of attra ctive intrachain interactions. A careful study of the radial distribution f unction of the center-of-mass of the polyelectrolyte chains shows clear evi dence that effective interchain attractive interactions also exist in solut ions of flexible polyelectrolytes, similar to what has been found for rodli ke polyelectrolytes. Our results suggest that the broad maximum observed in scattering experiments is due to clustering of chains. (C) 1999 American I nstitute of Physics. [S0021-9606(99)50905-7].