Identifying facile and accurate methods to measure the thickness of the active layers of thin-film composite membranes – A comparison of seven characterization techniques

Abstract

Despite the important role played by active layer thickness in the performance of thin-film composite (TFC) membranes, and in the understanding of the intrinsic transport properties (i.e., permeability, and water and solute partition and diffusion coefficients) of active layers, there is no study in the peer-reviewed literature evaluating whether existing measurement techniques provide consistent results among each other. Thus, we compared active layer thickness results obtained for each of six commercial TFC membranes with seven measurement techniques, including four techniques commonly used in the literature (scanning electron microscopy – SEM, transmission electron microscopy – TEM, atomic force microscopy – AFM, Rutherford backscattering spectrometry – RBS) and three non-commonly used techniques (quartz crystal microbalance – QCM, profilometry and ellipsometry). The six membranes tested covered performance levels ranging from nanofiltration to seawater reverse osmosis. Our results showed that AFM, RBS, QCM, profilometry and ellipsometry produced consistent results among each other, and thus likely provide the most accurate thickness results. SEM and TEM produced thickness results that were greater than those obtained with the five non-electron microscopy techniques, thus suggesting that SEM and TEM should only be used for rough estimates of active layer thickness. On the basis of nine different factors used to evaluate the advantages and disadvantages of the measurement techniques, ellipsometry was found to be the most advantageous technique for measuring active layer thickness. Results for active layer thickness and mass were used to obtain experimental estimates of the density of polyamide active layers, which for uncoated polyamide layers was found to be 1.26±0.21gcm−3.