Pull-off of a polyelectrolyte chain from an oppositely charged surface

The conformation at equilibrium of a single polyelectrolyte molecule adsorbed electrostatically at an ideal planar charged surface with one of its ends constrained at a distance z from the surface is investigated using different theoretical methods, and in particular a scaling theory; The force F(z) applied at the fixed end of the chain is calculated. Exact results are obtained for a Gaussian chain when all screening effects are negligible, and at high salt concentration, using a self-consistent field theory. When the salt concentration is large enough, the force profile F(z) reaches a plateau where the force is independent of chain length. When the electric field is screened by the counterions,a pseudoplateau is obtained, where the force increases logarithmically with z. The chain leaves the surface before it is fully extended, although the distance of detachment from the surface is proportional to the number of monomers N. The electrostatic interactions between the charged monomers of the polyelectrolyte chain can be taken into account using scaling arguments.