Characterisation of nanofiltration membranes for predictive purposes - Use of salts, uncharged solutes and atomic force microscopy

An asymmetric nanofiltration membrane (Hoechst, PES5) has been characterised by three different techniques: modelling of the rejection of simple salts, modelling of the rejection of uncharged solutes and atomic force microscopy. Interpretation of experimental data for the rejection of three salts having common co-ion (LiCl, NaCl, KCl) with model calculations allows a characterisation of the membrane in terms of three parameters: an effective pore radius (r(p)), the ratio of effective thickness over porosity (Δχ/A(κ)) and an effective charge density (χ). Interpretation of experimental data with model calculations for uncharged solutes (Vitamin B₁₂, raffinose, sucrose, glucose, glycerin) allows a characterisation in terms of r(p) and Δχ/(κ). Atomic force microscopy (AFM) allows direct determination of surface pore radius r(p)(s), and surface porosity A(κ)(s). The AFM images provide direct confirmation of the presence of discrete surface pores in such membranes. Further comparison of the characterisation obtained with salts and that obtained with uncharged solutes shows that it is better to describe the transport through such membranes as occurring through discrete pores rather than using an homogenous description of the membrane structure. It is also shown that the complexity of a 'space-charge' description of the electric field distribution in the nanometre dimension pores of such membranes is not warranted. Direct experimental evidence of the charging mechanism of the membranes is provided. Overall characterisation parameters suitable for predictive purposes are suggested.