New insights into the fundamental nature of lignocellulosic fiber surface charge

Abstract

The surface and total charge density of industrially manufactured hardwood and softwood grade kraft lignocellulosic fibers can mainly be attributed to carboxylic acid functionalities as determined from topochemical adsorption of a cationic polymer (methyl glycol chitosan) and the use of conductometric titration. These charge density measurement methods have led to a powerful and accurate photometric method for the quantitative analysis of surface charge. The method is based on directly monitoring the iodide ion, the counterion of the chitosan polymer, which is released upon electroneutralization between a chitosan cationic site and an anionic site at the fiber surface, thereby directly providing the concentration of surface charge. The stoichiometry for the chitosan adsorption onto the fiber surfaces is unity despite pulp types and adsorption quantity of cationic polymer. Given the high charge density of the chitosan molecule and the determined physical charge density limitations for its distribution onto the surface of pulps, a new and heretofore unused concept referred to as the subsurface was posited to adequately describe the surface charge determination after the adsorption of cationic polymer on the fibers. More specifically, within the subsurface of the suspended fibers in solution, carboxylates (COO −) were neutralized by the cationic sites of chitosan. The COO − sites in the subsurface can electrically attract positive sites of the cationic polymer, resulting in the complete release of counterions. To account for the charge density measurements, the charge measurement of the fibers includes not only nominal topochemical surface charges but also charges below this surface (subsurface).