The porous system of a reverse osmosis membrane − operando studies with small-angle neutron scattering

This manuscript deals with operando small-angle neutron scattering (SANS) on a reverse osmosis (RO) thin film composite (TFC) membrane, which was exposed to salt-free water at a transmembrane pressure of 25 bar and a transverse flow rate of 18 L/h. Permeate flux and electric conductivity were detected in parallel to SANS. The relevant objects of investigation are macro- and nanopores in the range of μm and nm, which were investigated with two SANS diffractometers covering a scattering vector Q between 10-4 and 0.3 Å−1. Salt-free water isotopes of H2O and D2O and their mixtures were used for the purpose of contrast matching, thereby determining the size of pores in the membrane layers. Macropores of the order of μm dimension were found in the nonwoven polypropylen (PP) layer showing a decline in size between 2 and 2.5 % over the experimental time of 7 to 17 h. The macropores of the polysulfone layer (PSU) have a size of about 0.4 μm. Nanopores with an average total diameter of (23.7 ± 0.5) Å of volume fraction of the order of 2 % vol are found exclusively in the PSU layer. The pore morphology was compared with the results of local methods such as SEM and TEM tomography, showing the complementarity of these methods and their necessity for a more complete picture of membrane morphology. A correlation between permeate flux and SANS parameter is discussed.