Evaluation of the Average Molecular Weight of Polyelectrolytes via Pulsed Streaming Potential Measurements.

Many methods have been established for determining the molecular weight of polymers; however, significant challenges remain when the polymers are highly charged, largely due to the so-called "polyelectrolyte effect". In this work, the molecular weight of polyelectrolytes was evaluated based on their adsorption kinetics onto oppositely charged surfaces in a microchannel by pulsed streaming potential (SP) measurements. The initial rate constant (ki) was found to be dependent on the molecular weight of a polyelectrolyte under controlled experimental conditions. Assuming that adsorption at the initial stage is diffusion controlled, ki is approximately proportional to Mb, where M is the average molecular weight of the polyelectrolyte and b is an empirically determined constant for the system. By flushing the polyelectrolyte solution through a capillary with an oppositely charged surface, the initial adsorption change rate (dEr/dt) was obtained within 20 s, and ki was derived from measurements at five different concentrations in about 3 min. Cationic (poly(diallyldimethylammonium chloride) (PDDA) and poly-l-lysine hydrobromide (PLL)) and anionic (poly(acrylic acid) (PAA) and poly(styrenesulfonic acid sodium salt) (PSS)) polyelectrolytes with different molecular weights were used to determine ki values. The results confirmed that ki is proportional to Mb and that b is a fixed constant under controlled conditions, demonstrating the potential of pulsed SP measurement as a facile method for evaluating the molecular weight of polyelectrolytes.