High Performance Ultrafiltration Membranes. Pore Geometry and Charge Effects

Ultrafiltration is a well-established process for protein concentration, buffer exchange, and purification. The key parameters describing the performance of these ultrafiltration processes are the filtrate flux, which is directly related to the membrane permeability, and the selectivity, which is determined by the degree of protein retention. Most efforts to improve the tradeoff between the permeability and selectivity of ultrafiltration membranes have focused on trying to narrow the pore size distribution, but this strategy has met with relatively little success. Experimental data and model calculations are presented that clearly demonstrate membrane performance can be significantly improved by controlling the membrane surface charge and/or the membrane pore geometry. In particular, electrically charged ultrafiltration membranes provide very high retention of like-charged protein by exploiting electrostatic (partitioning) phenomena. The use of novel silicon membranes with slit-shaped pores can provide nearly a threefold increase in permeability at a given selectivity due to the different hydrodynamic interactions associated with the change in pore geometry. However, these benefits can be largely obscured due to concentration polarization effects. Model calculations are presented that provide additional insights into the key factors governing the performance of ultrafiltration membranes and processes.