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 (<i>k</i>_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, <i>k</i>_i is approximately proportional to <i>M</i>^<i>b</i>, where <i>M</i> is the average molecular weight of the polyelectrolyte and <i>b</i> 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<i>E</i>_r/d<i>t</i>) was obtained within 20 s, and <i>k</i>_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 <i>k</i>_i values. The results confirmed that <i>k</i>_i is proportional to <i>M</i>^<i>b</i> and that <i>b</i> 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.