Determination of the porosity to thickness ratio Ak/Δx for UF and MF membranes by diffusion experiments

The structural parameter Ak/Δx, which represents the ratio of membrane porosity to membrane thickness, was determined by means of diffusion experiments using aqueous solutions of neutral solutes (formamid, urea) and strong electrolytes (HCl, KCl …). The membranes under investigation were commercial ultrafiltration (UF) polyethersulfone (PES), polyvinylidenefluoride (PVDF) and acrylic membranes of various molecular weight cut-offs. The method used to measure Ak/Δx involves diffusion experiments at the pseudo-stationary state between two half cells. Experiments are non-time-consuming, and exploitation of results does not require any pore model. Results show that neutral solutes, as well as salts or diluted strong acid, can be used for the determination of Ak/Δx. Discrepancies between Ak/Δx values obtained by diffusion and water permeability measurements are explained by the pore tortuosity and the interconnection of the pores.