Appropriate conditions for polyelectrolyte titration to determine the charge of cellulosic fibers

Sammanfattning: The polyelectrolyte titration method has been developed overthe years in order to determine the surface charge ofcellulosic fibers. The conditions have been varied depending onthe author. This work has been aimed at resolving theappropriate conditions for measuring the charge, such aselectrolyte concentration and molecular mass of thepolyelectrolyte. The charge ratio of variously treated pulpswas also investigated.The polyelectrolyte titration technique is based on a 1:1adsorption stoichiometry between fiber and polyelectrolytecharges. Adsorption of polydiallyldimethylammonium chloride(poly-DADMAC) was first performed at various electrolyteconcentrations and then with various molecular masses. ESCA(Electron Spectroscopy for Chemical Analysis) was used toindependently validate the polyelectrolyte titrationmethod.Results showed that stoichiometry prevailsat lowelectrolyte concentrations. Increasing the electrolyteconcentration screens the fiber charges, initially enhancingthe adsorption and causing a deviation from stoichiometry.Further increases in electrolyte concentration eventuallydecrease the adsorption. Deviation from stoichiometry occurredat higher electrolyte concentrations for higher charge densitypulps. ESCA-measurements showed that high and low molecularmass cationic polyelectrolyte adsorb to the same extent on thefiber surface, confirming stoichiometry. There was a goodagreement between the two techniques, hence, thepolyelectrolyte titration technique is a good method to measuresurface charges. Comparing the charge ratio between differentkind of pulps and treatments, it was found that mechanicalpulps have a higher surface charge than chemical pulps. Thecharge ratio of chemical pulps was, however, practicallyunchanged when comparing different types of wood and bleachingsequences.KEYWORDS:Adsorption, electrolyte, polyelectrolyte,polyelectrolyte titration, charge stoichiometry, charge ratio,diffuse electric double layer, Debye length, poly-DADMAC,cellulosic fibers, ESCA.