Depletion layers in polymer solutions : influence of the chain persistence length

We propose a simple model for the concentration profile induced by a non-adsorbing solid wall in dilute solutions of semi-flexible chains. Our approach is based on the assumption that the restriction in the chain local curvature of the chain creates a non-zero surface concentration. The monomer concentration profile ΦSF(Z) is deduced from the mean-field model for flexible chains, ΦF(Z) = Φb tanh 2(Z/RG2), by introducing an extrapolation distance D proportional to the persistence length q. We show that ΦSF(Z) = Φ b tanh2[(Z + D)/RG2] where RG is the chain radius of gyration and Φb is the bulk polymer concentration. This analytical expression provides a very good fit with recent experimental data obtained by the evanescent wave induced fluorescence method (EWIF) in aqueous xanthan solutions.