On the Physical Chemistry of Kraft Lignin – Fundamentals and Applications

Abstract: This thesis describes the physical chemistry of softwood kraft lignin in aqueous alkaline solutions. Both fundamental and applicational aspects are discussed. Kraft lignin originates either from pulp fibres or from black liquor produced by kraft cooking. In wood, the lignin is an anionic macromolecular colloid of relatively high molecular mass that is gradually depolymerised under the kraft cook. From UV-spectrophotometric measurements, the dissociation of phenolic functional groups was shown to decrease by increasingly higher temperatures. By theoretical considerations, applying the Poisson-Boltzmann cell model, it was shown that high molecular weight macromolecules attribute higher apparent pKa values due to an increased electrostatic attraction of hydrogen ions. From 1H PFG NMR self-diffusion measurements, it was further seen that high molecular weight kraft lignin also expresses lower colloidal stability than low molecular weight fragments. Coagulation caused by kraft lignin self-aggregation could be qualitatively described by the DLVO-theory. It was also shown that the solution conditions at the end of conventional kraft cooking are dangerously close to the stability threshold of high molecular weight kraft lignin. In the kraft process this could be detrimental for the rate of delignification. The kinetics of kraft lignin self-aggregation, determined by light scattering, was shown to be universal and followed reaction-limited cluster-colloid aggregation (RLCA) or diffusion-limited cluster-colloid aggregation (DLCA), depending on the solution conditions. The structures of the formed aggregates, found to be cluster like and to attribute fractality, were shown by cryo-TEM analysis. Fractal dimensions in the range of df = 1.9 – 2.5, were determined for the aggregates. Anions belonging to the Hofmeister series were shown to affect the stability of kraft lignin solutions very differently. Anionic surfactant – kraft lignin interactions were also studied. Among the tested surfactants, the group of bile acid salts was found especially efficient in stabilising kraft lignin solutions also at elevated temperatures. Furthermore, even after that self-aggregation had occurred, the aggregates could be re-dissolved by additions of sodium taurodeoxycholate.