Facilitated diffusion in the dissolution of carboxylic polymers

Carrier-mediated transport plays an important role in the dissolution of carboxylic polymers in aqueous solutions. Experiments with a rotating disk apparatus showed that the rate of polymer dissolution increased significantly with the addition of proton-carriers over the pH range of 6 to 13. The facilitated diffusion phenomenon in the dissolution of carboxylic polymers differs from that in membrane and biological systems in that the transport of polymer chains is not directly facilitated by any carriers. Proton-carriers facilitate the diffusion of hydrogen ions away from the polymer interface. As the concentration of hydrogen ions at the polymer interface decreases, the polymer solubility at the interface increases significantly, leading to a substantial increase in the polymer concentration driving force and, hence, the diffusion rate. A homogeneous chemico-diffusion model that elucidates the effects of the solution pH, the concentration and acidity of carriers, and the polymer acidity on the facilitated diffusion was developed. Good agreement between experimental and theoretical results was achieved. There are optimum values of the carrier's pK(a), and of the solution pH which give a maximum facilitation effect. As the diffusion rate of the polymer is increased by the carrier, the overall polymer dissolution process changes from diffusion-limited to disentanglement-limited. (C) 2005 American Institute of Chemical Engineers.