Polymer migration induced by concentration gradients of salts or crowding agents

Abstract

Diffusion-based mass transfer plays an important role in many biological and industrial processes. Diffusion of a solute can be induced by the concentration gradient of another solute in solution. This transport mechanism, which is not well understood, is known as cross-diffusion. For a ternary solution composed of polyethylene glycol (PEG, 1) and salt (2), the system can be described using two cross-diffusion coefficients. The first cross-diffusion coefficient, which characterizes polymer migration induced by the presence of a salt gradient, is denoted as polymer diffusiophoresis (D12). The other cross-diffusion coefficient, which characterizes salt diffusion induced by the presence of a polymer gradient, is denoted as salt osmotic diffusion (D21). Related diffusion experiments were performed by Rayleigh interferometry and dynamic light scattering (DLS) at 25oC.^The first objective of this dissertation is to investigate the effect of polymer size (10, 20, 35, and 100 kg mol-1) on the two cross-diffusion mechanisms in the presence of aqueous KCl. We show that D21 remains approximately the same for all the investigated polymer sizes; however, D12 increases with PEG size. The second objective of this dissertation is to investigate D12 and D21 in ternary solutions containing PEG (20 kg mol-1) in the presence of different salt types. We divided the study into two different categories: salting-out (sulfate and chloride salts) and salting-in (thiocyanate salts) according to the Hofmeister series. We found that both D12 and D21 can be related to preferential hydration and hydration of the polymer. Furthermore, we developed a model that allows us to characterize salting-out strength from our D21 results. In the case of the thiocyanate salts, we developed another model to describe our D12 results.^This model shows that our experimental results can be explained by considering polymer-anion binding. The final objective of this dissertation is to investigate the effect of cross-diffusion in a ternary aqueous polymer system in which the additive is large (crowding agent) compared to PEG. In this system, we employed the use of PEG (2 kg mol-1) and the crowding agent is tyloxapol micelles. Our results were explained in terms of excluded volume interactions and solute hydration--Abstract.