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The adsorption of polyelectrolytes on pulp fibers is complicated by the complex surface topography of the fibers, which are porous and fibrillated. The porosity and fibrillation determine to a large degree the internal and external surface area of the fibers accessible for polymer or polyelectrolyte adsorption. Other complications in practical applications are that the polyelectrolytes used are usually polydisperse and the ionic strength and pH are not always well controlled, all of which affect the adsorption. From a comparison of the adsorption behavior of polydisperse polyethylenimine (PEI) onto smooth glass fibers with than on pulp fibers under well‐controlled conditions, a consistent model for PEI adsorption onto pulp fibers can be deduced. At low salt concentrations, PEI molecules are quasi‐irreversibly adsorbed. The high molecular weight fraction can be modeled as rigid spheres that deposit on the accessible surface, maintaining their shape. Low molecular weight fractions cover the spaces between the large molecules and also penetrate into the pores. The increase of PEI adsorption with pH is ascribed to a decrease in size of a PEI molecule with increasing pH. This results in a larger number of molecules that can be accommodated on the external surface and a larger fraction penetrating deeper into the porous structure as well. In the presence of salt (around 0.1 M), a dynamic equilibrium between adsorption and desorption is established.

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