The effects of salts on polyelectrolyte systems.

摘要

The effects of salts on the behavior of polyelectrolyte systems were investigated. The phase behavior of polyelectrolyte solutions in the presence of added salt was calculated by combining the free energies due to Flory-Huggins mixing and Debye-Huckel electrostatics, with both terms modified for our polyelectrolyte solutions. Using the calculated phase diagrams, we found that most results give a typical polymer-solvent-nonsolvent phase diagram, with the solvent acting as a “nonsolvent” (since we assume that the polymer-solvent interaction parameter, χ, is positive) and the dissociated salt acting as a “solvent”. However, for high charges of the salt ions, we found a completely different phase diagram, one which can be explained by a “salting out effect” where the addition of salt over a certain concentration threshold causes complete phase separation over all concentrations of polymer and solvent. Also, the density and repulsive force profiles for a system comprised of two parallel, planar, uncharged surfaces uniformly covered with poly electrolyte brushes in an electrolyte solution was calculated using a computational enumeration of a one-dimensional random walk model. For large surface separations, we found three different density profiles: a Gaussian regime when κ is large, a stretched regime when κ is intermediate in value, and a “pancake” regime, with chains collapsed onto their grafted surface, when κ is small. For small surface separations, the first two regimes are replaced with an interpenetrating regime, where the density is essentially uniform across the entire region between the surfaces. For intermediate surface separations, the repulsive force scales as exp unlike what is expected using Gouy-Chapman theory.