Adsorption of polyethylenimine onto cellulose fibers

Abstract

The adsorption of a low and a high molecular weight polyethylenimine (PEI) onto pulp fibers has been investigated. These two cases represent two extreme modes of behavior: low molecular weight PEI can penetrate into the porous wall of the fibers, while for high molecular weight PEI adsorption in pores is negligible. From both equilibrium isotherms and the kinetics of adsorption it can be concluded that PEI molecules adsorbed on the surface maintain their identity as individual molecules and do not form a uniform adsorption layer. The final coverage depends on the size of a PEI molecule in solution, which, besides the molecular weight, is determined by the ionic strength, pH and PEI concentration. The kinetics of high molecular weight PEI adsorption closely follow Smoluchowski and Langmuir kinetics, similar to the deposition of colloidal particles. Adsorption on the wall of the lumen (hollow space inside the fiber) is determined by diffusion through pit apertures and cut ends of the fibers. The kinetics of low molecular weight PEI are mainly governed by the rate at which molecules can diffuse into the pores of the fiber wall. After a few days, a pseudo-equilibrium is established in which the rate of adsorption equals the rate of desorption. The rate of PEI rearrangement is much slower than the rate of desorption. The implications of these findings to papermaking are discussed.