Dynamics of ions in model charged porous media: Influence of polyelectrolytes

We have studied by means of Brownian dynamics simulations the dynamics of small ions in model charged porous media. We have focused on the influence on this dynamics of short polyelectrolytes with the same charge sign as the solid phase. We have compared the self-diffusion coefficients of counterions of four families of systems with different compositions (presence or not of charged obstacles and of polyelectrolytes). Our main result is that the presence of polyelectrolytes only modulates a behavior we had already observed before: In the domain where the porosity is relatively high compared to the Debye length, diffusion coefficients of counterions increase when the porosity decreases. Moreover, we have shown that, in the systems investigated here without charges on obstacles, the self-diffusion of counterions is mostly affected by the presence of polyelectrolytes and not by concentration effects, contrarily to co-ions. Also, we have seen that even if the density of the probability of presence of counterions in the vicinity of polyelectrolytes is much higher than that on obstacles, the residence time of ions around polyelectrolytes is shorter than around charged obstacles.