Multiscale modeling of protein adsorption in ultrafiltration process

The aim of the present paper was the formulation of an improved multi-scale model aimed at rigorously describing membrane fouling in UF process. With reference to a target protein, namely the Bovine Serum Albumin (BSA), the actual interactions existing between BSA and membrane surface as well as among each of the proteins constituting the cake deposited on the membrane were accurately computed by an ab-initio procedure. The local charges on either the BSA contact surface or the polymer fragments representing the considered membrane surface were calculated through a first-principle-based approach and taking into account the solution pH . The so-obtained local charges were exploited to achieve a detailed knowledge about BSA and polymer behavior at nanoscopic scale and to get some fundamental parameters that allowed a smooth transition to a larger scale. In particular, it was demonstrated, by means of some optimization algorithms aimed at calculating the minimum of potential energy characterizing different layers of proteins, that BSA molecules did actually accumulate on membrane surface in a pattern, which could be reliably described by a cubic centered lattice. It is worth noting that such a result was obtained by applying rigorous Quantum and Molecular Mechanics (QM/MM) calculation and had not to be considered as a modeling hypothesis, as it was done in already-available articles. . The detailed knowledge acquired at both sub-nanoscopic and nanoscopic scale allowed also estimating the BSA surface potential, which was exploited to formulate, at a microscopic scale, a balance of the forces acting on each BSA molecule, assumed as a spherical colloidal particle. An overall additional resistance, as due to both the first protein layer deposited on the membrane surface and to BSA layers constituting the cake, was then computed, thus allowing the final transition to a macroscopic scale, where an unsteady-state mass transfer model was formulated to describe the behavior of a typical dead-end UF process. The calculated values of both BSA surface potential were compared with some experimental measurements of BSA aqueous solutions Z-potential, showing a remarkable agreement between theoretical calculations and measured values.