Polyampholyte-induced repulsion between charged surfaces: Monte Carlo simulation studies
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The force between two planar charged surfaces in the presence of polyampholytes (PAs) is investigated as a function of the surface separation. The model system contains PA molecules with zero net charge adsorbing onto the charged surfaces from a dilute surrounding solution without salt. We compare the results obtained on three levels of approximation: (i) polyampholytes moving in the mean field due to the counterions, i.e., in the Poisson-Boltzmann field, (ii) PAs in a self-consistent field generated by both counterions and PA monomers, and (iii) with all interactions treated explicitly. Either the amount of PA is kept constant for varying slit widths or chemical equilibrium with a bulk solution is considered. The PA adsorption and the surface force are found to strongly depend on the charge sequence along the chain. That is, polyampholytes with alternating charges do not adsorb, and their effect on the force is similar to that of neutral polymers. For PAs with long blocks oppositely-charged to the surfaces, however, the adsorption is more favorable and the monomer distribution for these blocks resembles that of polyelectrolytes. The counterions are in this case efficiently displaced from the surfaces, which leads to a significant extension of the electric double layer. Thus, our main conclusion is that adsorbing polyampholytes always increase the double layer repulsion between planar charged surfaces, and, the major cause for this phenomenon is the counterion redistribution. Yet, PA chains are capable of establishing "bridges" at short surface separations, but the resulting attraction can only slightly reduce the repulsive net pressure. The simplest approximation (i) is in principle only valid in the limit of zero PA density. At a finite concentration, it is sufficient, though essential, to include a linear correction to the Poisson-Boltzmann solution in order to accurately predict the interfacial force or the amount of PA in the slit at chemical equilibrium.