Monte Carlo simulations of defect-free cross-linked gels in the presence of salt

Properties of polyelectrolyte gels in equilibrium with a reservoir of salt solution described within the primitive model have been investigated using Monte Carlo simulations. The polyelectrolyte gel was modeled as a charged defect-free three-dimensional network of diamondlike topology with explicit simple ions. The deswelling upon addition of salt was investigated for gels with different charge density, cross-linking density, chain flexibility, and counterion valence. All polyelectrolyte gels underwent a deswelling upon addition of salt. In most systems, however, even at high salt concentration, the equilibrium volumes remained significantly larger than those of the corresponding uncharged gels, and the salt concentration in the gel remained smaller than that in the reservoir. The salt content in the gel was larger as compared to that predicted by an ideal Donnan equilibrium due to a more negative excess chemical potential of the salt in the gel as compared to that in the reservoir. The predictions of the Flory-Rehner-Donnan theory were qualitatively in agreement with our results, but they displayed significant quantitative disagreement, mainly due to an overestimated contribution from the counterions to the osmotic pressure.