Abstract
Hexagonal lyotropic liquid crystals (HLLC) with uniform pore size in the range of 1~5 nm are highly sought after as promising active separation layers of thin-film composite (TFC) membranes, which have been confirmed to be efficient for water purification. The potential interaction between an amphiphile-based HLLC layer and the substrate surface, however, has not been fully explored. In this research, hydrophilic and hydrophobic microporous polyvinylidene fluoride (PVDF) substrates were chosen, respectively, to prepare TFC membranes with the active layers templated from HLLC, consisting of dodecyl trimethylammonium bromide, water, and a mixture of poly (ethylene glycol) diacrylate and 2-hydroxyethyl methacrylate. The pore size of the active layer was found to decrease by about 1.6 Å compared to that of the free-standing HLLC after polymerization, but no significant difference was observable by using either hydrophilic or hydrophobic substrates (26.9 Å vs. 27.1 Å). The water flux of the TFC membrane with the hydrophobic substrate, however, was higher than that with the hydrophilic one. A further investigation confirmed that the increase in water flux originated from a much higher porosity was due to the synergistic effect of the hydrophilic HLLC nanoporous material and the hydrophobic substrate.
Highlights
The drastic growth of the economy and population has led to a wide scarcity of water resources around the world [1]
The cross-linkable monomers were dissolved in the hydrophilic areas of the formed Hexagonal lyotropic liquid crystals (HLLC) phase, and the monomers were free-radically polymerized by UV curing to retain the template structures, followed by surfactant removal to achieve a nanoporous structure with cylindrical pores
The nanostructural preservation of the free-standing HLLC mixture during UV curing is important for the fabrication of Thin-film composite (TFC) membranes
Summary
The drastic growth of the economy and population has led to a wide scarcity of water resources around the world [1]. Membrane technologies have been playing important roles in obtaining fresh water from seawater and brackish water. Thin-film composite (TFC) membranes consisting of an active separation layer, a microporous support, and a polyester non-woven backing layer have been regarded as the state-of-the-art design due to their high permselectivity, chemical stability, and compaction resistance [1]. The middle microporous support layer enables the active separation layer to withstand highpressure compression. The active separation layer, on the other hand, mainly contributes to the nanofiltration performance. The pore size, porosity, and continuity of nanopores are, key parameters for the active layer to perform well [2], while its thickness needs to be minimized to decrease the resistance for a high permeability
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