Influence of Charge Sequence on the Adsorption of Polyelectrolyte Solution on to Polyelectrolyte Brush

We use coarse-grained MD to elucidate the role of charge sequence on the adsorption efficacy of oppositely charged free polyelectrolytes on to a polyelectrolyte brush. We consider four different model systems wherein the free and the brush polyelectrolytes can have either brush or alternating charge sequence. Our model treats the polyelectrolytes in a bath of implicit solvent, excess salt and explicit counterions. Adsorption efficiency is highest when both free and brush polyelectrolytes possess a block charge sequence, and it is lowest when both free and brush polyelectrolytes possess an alternating charge sequence. By computing the free energy, internal energy and entropy of adsorption using umbrella sampling methods, we find that the origin of the differences in adsorption efficiency for different charge sequences is enthalpic. Additionally, equilibrium conformations for different charge sequences reinforce the results obtained from energetic calculations. When the number of brush polymers is more than that of the free polymers, a spatial heterogeneity is observed in the charge distribution within the brush region which is strongly dependent on the charge sequence of free and brush polyelectrolytes.