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Apr . 09, 2024 10:39 Back to list

New Insights into Membrane Science and Technology: Polymeric and Biofunctional Membranes

4.1 pH

Control of membrane properties by precise pH control is the most easily visualized concept. The definite pKa for each functional group incorporated into the membrane structure allows for a very precise break in the permeability of a polymer grafted membrane. For example, the pKa for the carboxylic acid groups of polyaspartic acid is around 3.9. This is on a logarithmic scale, and thus small changes in the pH result in massive changes in the fraction of charged and uncharged groups. As the number of charged groups increases, repulsion along the polymer backbone increases such that the attached polymers extend into the membrane pores. This effectively decreases the pore size of the membrane, thereby decreasing the permeability of the membrane. This is shown schematically in Figure 14. This phenomenon has been observed experimentally by a number of researchers, and a specific example is given in Figure 15 [12]. The presence of a step change in the permeability is critical as this imparts a switching capability to the membrane. It should be noted that changing the charge of the fixed groups would invert the behavior displayed in Figure 15.

Ethylene Diamine Tetraacetate Acid Tetrasodium Salt (EDTA-4Na)

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Figure 14. Environmental effects on immobilized polyelectrolytes in a membrane pore.

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