Effects of supporting and buffer electrolytes (NaCl, CH 3COOH and NH 4OH) on the diffusion of BSA in porous media

Abstract

In this paper, a Nernst-Planck diffusion model including intraparticle and film diffusions has been developed to study the diffusion of charged proteins in porous particles. Hindered diffusion of proteins in the porous media is allowed for in this model and a new concept of coupled mass transfer through a stagnant film is first introduced. Bovine serum albumin (BSA) was used in this study as the model protein. The Nernst-Planck diffusion process of BSA coupled with more mobile counterion will result in an increase of protein flux and a decrease of the counterion flux relative to their corresponding Fickian diffusion fluxes, while the supporting electrolytes (NaCl) and buffer electrolytes (CH 3COOH, NH 4OH) are found to significantly reduce the diffusion rate of BSA. Detailed simulations have found that the effect of Nernst-Planck diffusion could be ignored only when the ionic fraction of a protein [defined in eq. (350)] is less than 0.1. When the net protein charge is larger, more electrolytes are needed to neglect the effect of Nernst-Planck diffusion. Numerical simulations also showed that nonuniform distributions of pH and buffer ion concentrations exist during the coupled diffusion process of protein and its counterion as well as the supporting electrolytes. This observation has far reaching consequences in processes such as ion exchange, where the role of pH and salt concentration is important.