Observing Departures from Expected Diffusion Behavior in Concentrated Polyelectrolyte Solutions Using Single Particle Tracking

Polyelectrolytes are of wide experimental interest due to their biological origins and the unique applications rooted in leveraging their charge-carrying groups. Effective design of polyelectrolyte systems for industrial uses requires understanding of how these polymers interact with neighboring chains. Polyelectrolyte diffusion is central to the success of these applications, often deviates from neutral polymer behavior, and allows us to gauge solution self-assembly. We used single particle tracking (SPT) to study the effect of ion and polymer concentration on the diffusion behavior of polyelectrolyte solutions to measure the relative effects of physical and charge-based interactions using polylysine (PL). These solutions show that PL diffuses as a single, Fickian population that has a strikingly strong concentration dependence on diffusivity. This is indicative of high degrees of interaction and ordering between neighboring chains. Changes in counterion concentrations and valency lead to large differences in the resulting MSD values of the polymers but the strong concentration dependence on diffusivity is retained. This work is further supplemented by complementary techniques to provide a molecular explanation of interpolymer interactions and measure diffusivity over a broad concentration range. Notably, the strong concentration dependence occurs in a concentration regime that is obtainable using fluorescence-based SPT but unanalyzable using other common experimental methods.