(168c) Methodological Assessment of Nanofiltration Membrane Performance for Bio-Analytical Applications
Recent advances in instrumentation and experimental techniques have allowed for investigation of the chemistry of materials at the molecular scale. The the nanofiltration membrane is an ideal investigative material towards an understanding of material morphology that propagates to macro-scale performance (such as steric rejection of particles by a membrane) and as well as surface force interactions (such as electrostatic hindrance of ions by a charged membrane). The challenge for NF membrane technology is to optimize solvent flux and solute rejection while minimizing fouling of the membrane. This mission, however, is rife with contradiction, since increasing membrane rejection usually requires reducing the pore size, which in turn normally leads to increased fouling and overall loss of performance. Hence our goal is to assess the effect of various surface properties on membrane performance. We assess the surface charge density as a function of pH by electrokinetic analysis, the surface roughness by AFM, the hydrophilicity by measuring the water contact angle by the sessile drop method, and the chemical structure of the active layer by ATR-IR. The results of these characterization probes are weighed against pure water flux, and organic solute rejection. The membrane materials chosen for the active layer was cellulose acetate active layer on a porous Celgard (polypropylene) support layer. The fabrication method employed was spin casting/wet inversion. We measured the flux of pure water and uncharged organic solutes in high pressure stirred cell. Results of the cellulose acetate membrane shows a specific water permeability value on the order of 10-8 m s-1 kPa-1, characteristic of nanofiltration membranes. The limiting rejection of uncharged organic solutes increased with increasing percentage of acetate, and this value was used to calculate the effective pore radius. The zeta potential was negative over the operating pH range for all membranes and became increasingly negative with increasing pH. Surface roughness values were also calculated from AFM images.