Determination of absolute molecular weight of sodium lignosulfonates (NaLS) by laser light scattering (LLS)

The problems related to the determination of the absolute molecular weight (Mw) of sodium lignosulfonate (NaLS) are the focus of the present work. Four narrow-dispersed NaLS fractions were collected by filtration, ultrafiltration, and gel permeation chromatography (GPC), and their Mw measured by laser light scattering (LLSMw) was compared with that obtained by GPC (GPCMw). The LLSMw values were much larger than GPCMw, and this Mw difference increased with increasing Mw of NaLS. This finding confirms the known polyelectrolyte and aggregation effects of NaLS. The deviation is also partly due to the conformation differences between the GPC standard (sodium polystyrene sulfonate) and the NaLS molecules. The effects of polyelectrolytes and NaLS aggregates could be inhibited by adding enough NaCl salt in the NaLS solution and filtering the NaLS/NaCl solution through a syringe filter. As a result, the slow mode in dynamic LLS measurement disappeared. By this approach, the absolute LLSMw values were only slightly larger than that determined by GPC due to the conformation differences between the GPC standard and the NaLS molecules.