Intermolecular electrostatic interactions and their effect on flux and protein deposition during protein filtration

Although membrane filtration is used extensively to process protein solutions containing a variety of electrolytes, there is currently little fundamental understanding of the effect of the solution environment (and in particular, the solution pH) on the filtrate flux in these systems. We have obtained data for the flux and sieving coefficients during the batch (stirred cell) filtration of solutions of bovine serum albumin, immunoglobulins, hemoglobin, ribonuclease A, and lysozyme through 0.16-micron microfiltration membranes at different pH values. The flux declined significantly for all five proteins due to the formation of a protein deposit on the upper surface of the membrane. The quasi-steady ultrafiltrate fluxes at the individual protein isoelectric pH's were essentially identical, despite the large differences in molecular weight and physicochemical characteristics of these proteins. The flux increased at pH's away from the isoelectric point, with the data well-correlated with the protein surface charge density. These results were explained in terms of a simple physical model in which the protein deposit continues to grow, and thus the flux continues to decline, until the drag force on the proteins associated with the filtrate flow is no longer able to overcome the intermolecular repulsive interactions between the proteins in the bulk solution and those in the protein deposit on the surface of the membrane.