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 (d E r /d t ) 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.