Aggregation of Lignin Derivatives under Alkaline Conditions. Kinetics and Aggregate Structure

The kinetics of kraft lignin (KL) aggregation at alkaline conditions was studied by quasi-elastic light scattering (QELS) and turbidity measurements. Stability ratios (W) for KL were obtained at 70 °C and various concentrations of sodium chloride. By analyzing the early-time evolution data of aggregate growth obtained from QELS, fractal dimensions of flocs formed in both reaction-limited cluster−cluster aggregation regimes and diffusion-limited cluster−cluster aggregation regimes were determined. Correlations between the fractal dimension and the W-ratio were found in accordance to recent studies of a system containing monodisperse polystyrene colloids. By cryogenic transmission electron microscopy, the fractality of KL aggregate structures in the system was also shown. It was seen from stability studies of KL solutions that the effects of specific co- and counterions follow the Hofmeister series. From the outcome of the investigation, different modes of aggregation occurring in a KL system are proposed. Starting from the macromolecular state of KL and evolving through larger aggregates, the KL clusters finally exhibit a supramolecular structure similar to what earlier has been proposed for native softwood lignin.