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Diffusional contributions and electrostatic exclusion effects on transport through ultrafiltration membranes

1. Introduction Ultrafiltration is used for protein concentration and buffer exchange in the downstream processing of essentially all recombinant protein products [1]. Although ultrafiltration has traditionally been viewed as a convective pressure-driven process, with the separation determined on the basis of size, solute transport through ultrafiltration membranes is also strongly influenced by solute diffusion and electrostatic interactions [2, 3]. Electrostatic interactions can be exploited for high-resolution protein separations, taking advantage of the unique surface charge characteristics of individual proteins. Diffusional transport in ultrafiltration not only influences the separation characteristics, it also provides a directional dependence to solute transport through typical asymmetric ultrafiltration membranes [4].