On the charge stoichiometry upon adsorption of a cationic polyelectrolyte on cellulosic materials

Abstract

Adsorption isotherms and adsorption stoichiometry of a low-molecular-weight ( M w = 5.9·10 3) cationic polyelectrolyte, 3.6-ionene (Polybrene), on cellulosic fibers were studied. The charge density of the fibers was varied by means of carboxymethylation. In order to study the effects of increased accessibility to the charges within the fiber cell wall, part of the fiber material was treated in a high-pressure homogenizer to produce a microfibrillated cellulose (MFC). It is shown that the adsorption isotherms are of the high-affinity type with a pronounced plateau level. The charge of the polymer adsorbed at this plateau level provide a relatively good measure of the charge (in meq g −1) on both the fibers and the MFC, except for the unsubstituted fiber sample. By measuring simultaneously the adsorption of polymer and the amount of released counterions it was also shown that there is an almost 1:1 stoichiometry between the charges on the polyelectrolyte and the charges on the fibers. However, the stoichiometry drops rapidly when the amount of adsorbed polymer charges exceed the number of charges on the cellulosic fibers. Finally, the deviation from a 1:1 relation between the total charge on the cellulosic material and the adsorbed polymer charges for the unsubstituted fibers is also shown to be caused by a slow adsorption process.