Impact of liquid-filled voids within the active layer on transport through thin-film composite membranes

Abstract

Transport through composite membranes is strongly influenced by the morphologies of both the porous support membrane and overlying selective thin-film (i.e., active layer). Recently, the occurrence of water-filled voids within the active layer has been suggested in the literature; however, their effects on transport are uncertain. Here, we theoretically consider, through numerical modeling, the effect that liquid filled-voids have on the transport of water and solutes through supported thin-film morphologies. Specifically, we evaluated the effect of volume void fraction, void size, and relative location of the voids within the active layer with respect to both surface roughness features and the pores of the support. Transmission electron microscopy image analysis was used to obtain evidence supporting the existence of voids in two commercial brackish and seawater reverse osmosis membranes; the volume fraction of the active layer the voids occupied was determined to be about 30% for both membranes. Our calculations show that, for films with a constant polymer volume, a rough film containing voids is more permeable than an equivalent, homogeneous flat film, due to the creation of shorter paths for diffusion, not due to increased surface area, though the latter is shown to correlate positively with permeability when no base-film exists under the void. Results further illustrate the importance of void position within the thin-film, indicating that even with a significant void fraction, the presence of an underlying polymer base-film negatively impacts the permeability. Voids created closer to the bottom of the active layer will increase membrane permeability. Conversely, results show that even at significant void fractions, voids located closer to the top of the active layer negatively impact the permeability. Variations in the proportion of the active layer overlaying the voids will impact the flux distribution along the membrane, and may be used to reduce flux ‘hotspots’, which may enhance localized concentration polarization and fouling propensity. A strategy for creating high permeability membranes with relatively even flux distributions may include a combination of a rough film with a reduced base thickness, and with thicker regions of the film aligned with the support pore locations. Understanding the role of the voids in determining the transport properties of the membranes provides motivation for controlling their formation.