(22c) [Invited Talk] Polymer Dynamics Near Complex Surfaces

[INVITED TALK] Understanding the transport and thermodynamics of polymers near surfaces is helpful for many separation processes like water purification, drug delivery, and oil recovery. Specifically for water purification, typical membrane foulants are polyelectrolytes, which have been modeled using polyacrylic acid. Uncovering how these polyelectrolytes interact in confinement can reduce the fouling of organic membranes and will lead to better separation processes overall. We have determined the diffusion coefficient, D, of negatively ionizable sodium polyacrylate (NaPA) and neutral dextran in Convex Lens-induced Confinement (CLiC) using differential dynamic microscopy (DDM). In this setup, the confinement ranges continuously from 0.085 – 21.7 µm. It was found that dextran diffusion slows in higher confinement, which is consistent with hydrodynamic interactions with the surface. When the surface of the glass is modified by a neutral surfactant layer, hydrodynamic interactions with the glass surface no longer contribute, and the dextran diffusion is constant for all confinement heights. We also find that the diffusion of NaPA heavily depends on the ionic strength and interaction with the glass surfaces. In high salt concentrations, NaPA is excluded from the smaller gap channels. Surprisingly, as the salt concentration is reduced, the polymer stretches and starts to aggregate in the highest confinement. The free energy of the geometry also goes from repulsive to attractive, and NaPA is observed adsorbed to the similarly charged glass surface. Understanding these complex behaviors at interfaces will inform both future theoretical work and engineering design of interfaces and polymer separations.