Dynamics of Polyelectrolytes in Convex Lens-induced Confinement (CLiC)

Understanding the transport of polyelectrolytes in confinement is crucial for various applications such as drug delivery, and separation processes. From dynamic light scattering experiments, dynamics of polyelectrolytes in solution as a function of ionic strength is complicated by long range electrostatic interaction and counterion association. Specifically in low ionic strength, polyelectrolytes exhibit “fast” and “slow” diffusive modes. The fast mode is due to fluctuations in the polyelectrolyte counterion concentration. The slow mode is thought to be due to aggregates forming because of dipole-dipole interactions. Here, we determined the diffusion of negatively charged, fluorescently labeled, sodium polyacrylate (NaPA) near negatively charged glass surfaces in Convex Lens-induced Confinement as a function of ionic strength using differential dynamic microscopy. We confirm the transition to slower dynamics in salt concentrations lower than 50 mM. The slower dynamics are consistent with the presence of the aggregates, confirmed by fluorescent microscopy. Interestingly, in 1 mM ionic strength solution, the negatively charged sodium polyacrylate adsorbs at negatively charged surfaces. To our knowledge, this is the first visual confirmation of the polyelectrolytes aggregating in low ionic strength solutions. We believe these findings can contribute to understanding polyelectrolyte solutions as a function of ionic strength in the bulk and close to similarly charged surfaces.