We report the assembly of an asymmetric membrane consisting of octadecyltrimethoxysilane (ODTMS) Langmuir film covalently attached to a sol-gel based sublayer, supported by a mesoporous membrane. The ODTMS Langmuir film was compressed at the interface of a subphase containing tetramethoxysilane (TMOS), formamide and polyethylenimine. Exposure of the Langmuir film to NH3 vapours increased the pH at the interface (pHinterface) and catalysed the condensation of TMOS. The assembly of the layers was of a self-healing nature, due to the π-pressure set-up of the Langmuir film and the faster diffusion of the NH3 through the defects. The resulting asymmetric film was transferred onto polysulfone ultrafiltration membranes to form a thin film composite (TFC) structure composed of a Langmuir organic skin, the sol-gel blend sublayer and the support. The asymmetry of the film was confirmed by various methods and a gradual transition from mesoporous support to dense ceramic-polymeric film was seen. A preliminary study demonstrated that the modification of the polysulfone membrane improved its selectivity from ultra to nanofiltration range. The obtained fluxes were significantly higher as compared to commercial membranes of similar selectivity. This generic approach is applicable for assembling large-area selective TFC membranes with ultrathin skin layers and high fluxes of effluents.
Bibliographical noteFunding Information:
The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University is acknowledged. M. Hitrik acknowledges the support of the Israel Water Authority. M. Hitrik is indebted to the Wertheimer family for their support. This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. Appendix A
The morphology of the cross-section of the resultant TFC membrane, comprising of ODTMS/silica(PEI) film supported by PES, was studied by XHR-SEM, STEM and X-ray mapping methods. A membrane coated with film was embedded into epoxy resin and sliced using microtome to 80?nm slices. Fig. 5 shows the SEM images of the obtained specimens. The morphological (Fig. 5A?C) and chemical (EDX, Fig. 5D) differences of the film layers are observed. Separate atomic maps are shown in Figs. S4A?E. The ODTMS/silica(PEI) layer is darker in STEM (transmittance) images (Fig. 5A) and brighter in SEM images (Fig. 5B?C) as compared with the PES and epoxy, indicating a dense layer. The PES layer is substantially more porous. The thickness of the coating ODTMS/silica(PEI) layer is ca. 4?5??m after 2?h exposure to NH3 and the layer is attached tightly to the polymer support. The porous structure could not be observed in the PES cross-section by 5?kV SEM, due to the penetration of the epoxy resin into PES pores, therefore a higher voltage image (20?kV) was taken for better contrast (Fig. S4F). High voltage damages of the epoxy layer and the porous structure of PES are well observed, exposing the pore-asymmetry between the silica sub-layer and the polymer support. The EDX mapping demonstrates that ODTMS/silica(PEI) film is denser and comprises mostly Si and O (red and white colours in Fig. 5D), while the PES-membrane is porous and carbon-rich (blue in Fig. 5D), as expected. Moreover, the red color fades with depth, alluding to the gradual change of Si density through the layer. The continuous gradient rather than abrupt morphological change endows stability to the film that comprises of different porosities. Internal pressure gradients are smoother when pore size gradients are smaller. Cross-sectional view shows that the ODTMS/silica(PEI) layer is dense, with no cracks, film detachment or other defects. Thus, the small nanometric damages obtained on the top-view images, taken with SEM (Fig. 4 ? the inserts) can be a result of drying of the membranes, or influence of high vacuum necessary for SEM, or as a result of non-uniform sputtering of the membranes with metallic NPs coating, needed to avoid the charging of the samples during the SEM imaging.The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University is acknowledged. M. Hitrik acknowledges the support of the Israel Water Authority. M. Hitrik is indebted to the Wertheimer family for their support. This research is supported by the National Research Foundation, Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme.
© 2019 Elsevier B.V.
- Air-water interface
- Asymmetric membrane
- Monomolecular skin