We have recently developed efficient simulation methods to calculate interaction free energies between charged surfaces in polyelectrolyte solutions [1-3]. Previous simulation studies were limited to "canonical" models [4], where the polyions perfectly neutralize the surfaces. This corresponds to a vanishing bulk concentration. A consequence of imposing bulk equilibrium conditions, in contrast to the "canonical" model, is the possibility of surface charge inversion. The chains may adsorb strongly enough to overcompensate the nominal surface charge. This generates a double-layer repulsion at long range and a barrier against flocculation. Contrary to the well-known "correlation attractions", we here observe a long-ranged repulsion, as a direct consequence of correlation effects (i.e. overcharging). This can be nicely illustrated by comparisons with polyelectrolyte Poisson-Boltzmann calculations.
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