The uses of non-steady-state membrane characterisation techniques for the study of transport properties of active layers of nanofiltration membranes: theory with experimental examples

Nanofiltration (NF) is a relatively new membrane process suitable for the separation of solutes of close molecular weight. The rejection mechanisms of nanofiltration membranes have not been reliably identified, yet. The electrostatic repulsion of coions by fixed membrane charge (Donnan exclusion) is considered one of most probable rejection mechanisms. However, practically no direct information on the electrochemical and/or electrokinetic properties of NF membranes is available. The interpretation of conventional electrochemical and/or electrokinetic measurements with NF membranes is complicated by their multilayer structure. Under linear conditions only average membrane transport properties can be obtained from steady-state measurements. Information on the properties of constituent layers can be obtained from non-steady-state and/or non-linear measurements, alone. In this paper the opportunities offered by non-steady-state techniques are explored. Volume flows and changes in solute concentration cannot usually be observed at sufficiently short times when the system is still far away from steady state. Therefore the only suitable response is electrical. It is not immediate due to the concentrational polarisation of boundaries between membrane constituent layers. The theory of transient membrane potential, transient filtration potential and low-frequency electrical impedance is briefly presented. The suitability of various non-steady state techniques for the determination of transport and accumulation properties of constituent layers of NF membranes is discussed and compared with available experimental data on transient membrane potential. It is concluded that each of techniques has its advantages and drawbacks. Therefore, the application of several techniques to the same system is desirable for the minimisation of systematic errors.