Most commercially successful reverse osmosis and nanofiltration membranes for water purification are thin-film composites (TFC) whose “active layer” consists of an ultrathin (~20-200 nm) polyamide film, that has been traditionally thought to be a dense polymer film. We have recently confirmed (<i>J. Membr. Sci.</i> 2016, <i>497</i>, 365-376) using a selection of commercial membranes with a broad range of performance levels that polyamide active layers are not dense films, but sponge-like films with spheroidal voids that account for a significant volume fraction (15-32%) of the active layer. These voids fill up with water when immersed in it, have sizes of up to tens of nanometers, and do not form passageways connecting the feed and permeate sides. In this work, we present experimental evidence using transmission electron microscopy (TEM), elemental mapping, water sorption measurements, and spectroscopic ellipsometry analyses of active layers that demonstrate the presence of the voids. We also describe three independent approaches to estimate the volume fraction of voids in the active layers using TEM imaging, water sorption measurements, and spectroscopic ellipsometry analyses. Finally, we discuss the implications of the existence of the voids for the analysis of the water and solute transport properties of active layers using experimental data as well as analytical and atomistic modelling approaches.

Water-filled voids account for a significant volume fraction of the polyamide active layers of thin-film composite membranes and affect their water and solute transport properties

251st National Meeting (2016)

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