Membrane Surface Morphology and Membrane Performance

Abstract

The use of AFMto study surfaces provides information about pore size distribution, surface morphology and electrical properties, surface adhesion/membrane fouling behavior, and the correlation between membrane characteristics and process behavior. This information, in conjunction with mathematical models and performance data, allows for the development of a novel approach in the prediction of new desired membranes. We discussed in earlier chapters that nodules, nodule aggregates, pore sizes, pore size distributions, and roughness parameters can all be observed on the membrane surface by AFM and quantified using software. Thus, AFM has proved to be a very powerful tool to study membrane surface morphology. However, this does not necessarily satisfy the knowledge-seeking scientist. Although the above parameters are highly valuable for the purpose of membrane characterization, they are of little use, at least for separation membrane scientists and engineers, unless correlations can be found between those parameters andmembrane performance. Information onmembrane surface characterization will be complete only when the cause-and-effect relationships among membrane preparation, membrane morphology, and membrane performance are fully understood. Therefore, an attempt will be made in this chapter to find some relationships between the surface characterization parameters obtained by AFM and the membrane performance data. Most obviously, the pore size and the pore size distribution will have a direct influence on the selectivity and the permeation rate of NF, UF, and MF membranes, where pores are most visible. Furthermore, some interesting discussions can be found in the literature, although still controversial, on the relationship between the roughness parameter and the flux, with respect to RO and gas separation membranes, where the measurement of the pore size is not at all easy. The roughness parameters are also related to the fouling tendency of the RO/NF and UF membranes. It has to be emphasized that all these discussions are found in the papers published during the past decade since the emergence of AFM as a surface characterization tool. Some of the discussions have already provided important guidelines for the sophisticated design of separation membranes. Further progress is expected to take place during the next decade.