Effect of Salt Concentration on Sodium Polyacrylate Diffusion in Convex Lens-induced Confinement (CLiC)

Membrane charge can influence how polyelectrolytes pass through pores in a separation process. In solution, polyelectrolyte dynamics have been studied extensively with dynamic light scattering. The behavior is heavily influenced by the ionic strength of the solution. Particularly at low ionic strengths, polyelectrolytes demonstrate two diffusion modes: a "fast" mode, attributed to counterion concentration fluctuations, and a "slow" mode believed to stem from the formation of aggregates caused by dipole–dipole interactions. To determine how this anomalous polyelectrolyte diffusion is affected by charged pores, we have examined the dynamics of fluorescently tagged, negatively charged sodium polyacrylate (NaPA) near similarly charged glass surfaces in convex lens-induced confinement. Differential dynamic microscopy (DDM) is used to determine the diffusion coefficient of NaPA in 150, 50, 10, and 1 mM ionic strength solutions. Our findings demonstrate a shift toward slower diffusion rates at salt concentrations below 50 mM, consistent with chain stretching. Interestingly, the "slow" and "fast" modes are not observed by using DDM in these geometries. However, in 1 mM ionic strength solutions, as the pore size reaches ∼0.30 μm, NaPA begins to visibly aggregate and even bind to the negatively charged surfaces. The free energy of the pore also shifts from repulsive to attractive at this salt concentration. These insights can further contribute to understanding polyelectrolyte solutions, both in bulk and near like-charged surfaces, due to changes in ionic strength.