Adsorption of Semiflexible Polyelectrolytes on Charged Planar Surfaces: Charge Compensation, Charge Reversal, and Multilayer Formation

We study theoretically the adsorption of semiflexible charged polymers on an oppositely charged flat substrate. We restrict ourselves to polymers with relatively high line charge density τ and bare bending stiffness 𝓁0, such as DNA or fully charged synthetic polyelectrolytes, for which the condition τ2𝓁B𝓁0 > 1 (with 𝓁B = e2/4πεkBT denoting the Bjerrum length) is satisfied. In this case the effective bending rigidity (which includes chain stiffening due to electrostatic monomer−monomer repulsion) is always larger than the screening length, and we obtain very thin adsorption layers, where the polymers essentially lie flat on the surface and can be considered as a two-dimensional layer. We distinguish different adsorbed phases, including a two-dimensional lamellar phase where the polymers run parallel and do not overlap and a disordered phase where the polymers form a random network with multiple crossings. By explicitly including lateral correlations within the adsorbed layer, we find a regime of strong charge reversal, where the adsorbed phase exhibits a much higher surface charge density than the substrate and thus reverses the charge of the substrate by a factor much larger than unity. This charge-reversed regime is characterized by a typical distance between polymers larger than the screening length. We explicitly take image charge effects due to a dielectric jump at the substrate surface and the impenetrability of the substrate for salt ions into account. These effects lead to an increase of the electrostatic persistence length close to the substrate and inhibit adsorption for strongly charged polymers and low-dielectric substrates. At the end, we present some scaling arguments for the formation of charge-oscillating multilayers.