Chain morphology, swelling exponent, persistence length, like-charge attraction, and charge distribution around a chain in polyelectrolyte solutions: effects of salt concentration and ion size studied by molecular dynamics simulations

Abstract

Properties of polyelectrolytes in tetravalent salt solutions are intensively investigated by a coarse-grained model. The concentration of salt and the size of tetravalent counterions are found playing a decisive role on chain properties. If the size of tetravalent counterions is compatible with the one of monomers, the chains show extended structures at low and at high salt concentrations, whereas at intermediate salt concentrations, they acquire compact and prolate structures. The swelling exponent of a chain against salt concentration behaves in an analogous way as the morphological quantities. Under certain condition, the electrostatics gives a negative contribution to the persistence length, in companion with a salt-induced mechanical instability of polyelectrolytes. Nearly at the same moment, it appears like-charge attraction between chains. The equal size of the tetravalent ions and the monomers is the optimal condition to attain the strongest attraction between chains and the most compact chain structure. Moreover, the ions form a multi-layer organization around a chain and, thus, the integrated charge distribution reveals an oscillatory behavior. The results suggest that charge inversion has no direct connection with redissolution of polyelectrolytes at high salt concentrations.