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 (k_i) 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, k_i is approximately proportional to M^b, 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 (dE_r/dt) was obtained within 20 s, and k_i 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 k_i values. The results confirmed that k_i is proportional to M^b 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.