AbstractThe pH dependent adsorption behavior of chitosan onto a cellulose model surface was studied by quartz crystal microbalance with dissipation (QCM-D). The molecular level interactions between adsorbed chitosan layers were studied by atom force microscopy (AFM) colloidal probe force measurements in the liquid phase. Adsorption of chitosan increased with pH below the solubility limit of the polymer. The adsorption behavior could not be accounted for solely on the basis of electrostatic interactions; thus a specific interaction between the polymers existed. Swelling and viscoelastic properties of the adsorbed chitosan layer were strongly influenced by pH. At high pH, the layer deswelled and became more elastic due to insolubility of the chitosan. The colloidal probe force measurements showed a rise of electrosteric repulsion after adsorption of chitosan at pH 5. Above the solubility limit of the chitosan, at pH 7, the pull-off force and its range clearly increased compared to lower pH values, indicating that the wet adhesion between chitosan-coated cellulose surfaces increased. The presented results are discussed in relation to the ability of chitosan to improve the initial wet strength of paper.