Macromolecular characterisation of lignosulfonates and their interactions with chitosans

Abstract

An HPLC-based SEC-MALLS method was developed for absolute molecular weight and molecular weight distribution determination for lignosulfonates. An aqueous phosphate/DMSO/SDS mobile phase yielded satisfactory SEC results. Refractive index increments and second viral coefficient values were determined, and fluorescence filters were included to eliminate the effect of fluorescence.

The weight average molecular weight (Mw) of a lignosulfonate sample from spruce was determined to 64 000 g/mol and with relatively high polydispersity. Six fractions prepared from the same sample had weight average molecular weights ranging from 4 600 to 400 000 g/mol. The degree of sulfonation for the fractions decreased with increasing molecular weight (0.64 to 0.39). It is proposed that the sulfonate groups in lignosulfonates must be positioned both in the interior and exterior of the lignosulfonate particles.

The SEC-MALLS method offered the possibility to analyse and compare softwood and hardwood lignosulfonates. Hardwood lignosulfonates were found to have significantly lower weight average molecular weights (5 700-12 000 g/mol) than softwood lignosulfonates (36 000-64 000 g/mol). The Mark-Houwink-Sakurada exponent was determined to 0.36 for lignosulfonates. The intrinsic viscosity for lignosulfonates depended primarily on the molecular weight and not on the type of wood.

Lignosulfonates and chitosans, a polycation, form insoluble polyelectrolyte complexes when mixed at pH 4.5. The composition and charge stoichiometry of the complexes may vary with the lignosulfonate/chitosan mixing ratio, but nearly neutral complexes were formed when the mixing charge ratio (SO3- /NH3+) was close to 1:1. The formation of complex was irreversible and could not be reversed either by increasing the pH above chitosans pKa value or increasing the ionic strength or temperature. A chitosan sample soluble at all pH was introduced, and no complex formation with lignosulfonate could be observed when pH was above chitosans pKa value. This indicates that the interactions between lignosulfonates and chitosans are electrostatic, and that opposite charges are essential for complex formation.