Consistent with the data in ‘Water in Crisis’, edited by Gleick [1
], the entire body of the world’s water resources is predominantly made up of oceans, with only a relatively small amount of fresh water making up the remaining 2.5%. Of this limited freshwater, only 31.3% is surface and ground water, which can potentially be used by humans, and the remaining 68.7% is found in the form of glaciers and ice caps (Figure 1
). Furthermore, many ground and surface water resources have been seriously contaminated due to inadequate sanitation, algal blooms, detergents, fertilizers, pesticides, chemicals, potentially toxic metals, salinity, and high sediment loads. These circumstances have effectively reduced the supply of freshwater for human use [2
Due to diminishing water resources, desalination has drawn more attention from scientists and governments and has begun playing an important role in some countries’ potable water supplies [6
]. However, the high cost involved in desalination, including infrastructure, energy, and the maintenance of facilities cannot be ignored and limits the use of desalination as a solution to potable water shortages. The energy consumption for various seawater desalination methods is shown in Table 1
. Although the energy consumption of some desalination methods has almost reached as low as 3 kWh/m3
, it is still much higher than local fresh water supplies that only use 0.2 kWh/m3
or less [7
]. A state-of-the-art reverse osmosis (RS) system may produce a thousand liters of drinking water by only using ~2.2 kWh of electrical energy; however, this value varies widely with geographical location, height, and distance from the source water [9
]. The total cost and increased environmental concerns have limited the widespread adoption of desalination technologies [5
]. An economical and environmentally sustainable method of water recycling is therefore essential. The application of membrane technology is widely used for water recycling in terms of cost-effectiveness and environmental friendliness.
The polymer membranes with nanometer size morphologies have great potential to be applied in various areas [13
]. Of particular interest here are the separation sieves using organic mesoporous membranes with a uniform pore size in the range of 1–5 nm and a high specific surface area for NF. NF membranes have opened up the possibility of performing highly selective small molecules and water purification based on the molecular size-exclusion of hydrated salt ions [17
]. A more permeable separation layer with uniform pore distribution will significantly maintain or improve salt rejection while increasing the flux compared to RO. The transport of water molecules through hydrophobic double-walled carbon nanotubes has demonstrated that the fluxes are over three orders of magnitude higher than those predictions using continuum hydrodynamic models [19
]. However, membranes with the necessary anisotropic transport properties due to oriented carbon nanotubes will be difficult and costly to prepare. The high performance of membranes is based on their diameter size (1–5 nm); however, various nanostructured and nanoreactive membranes are encouraged for use in water purification according to diverse pollutants [22
An ideal membrane for NF would require generating a thin film with physically continuous and vertically aligned nanopores in a narrow size distribution. However, currently developed NF membrane fabrication techniques such as tract etched, particle-assisted wet, and phase inversion precipitation are all suffering from kinetically controlling pores of random size and orientation and are therefore subject to a ubiquitous trade-off between permeability and selectivity, as shown in Figure 2
]. Highly permeable membranes lack selectivity and vice versa. The polymer membranes presenting the highest selectivity at a given permeability always lay near or on a line called the upper bound, and this behaviour (the slope), observed in all cases in both dense and porous membranes, absolutely depends on the parameters of the molecules/ion pair [27
]. This departs considerably far from the ideal and limits their use. Therefore, for the polymer NF membranes, the ability to precisely control the morphology, size, and anisotropy at a large scale is quite challenging but essential for this application.
A method for which the hexagonal arrangement of cylinders in LLCs provides a template presents a versatile strategy for the fabrication of NF membranes with uniform and adjustable pore size and large surface area and is promising in terms of overcoming the permeability-selectivity trade-off (attractive region in Figure 2
). The template for determining nanostructures is accessible by mixing amphiphiles and a hydrophilic medium like water at a designated ratio under conditions at or close to ambient temperature (Figure 3
The hydrophilic and hydrophobic areas within the assembly could be utilized by dissolving them with a polymerizable species, and therefore the nanostructure could be cured after a physicochemical process (Figure 4
). The continuity of cylinders (mesochannels) can also be controlled and extended to the bulk materials after reorientation through appropriate strategies. However, the two main difficulties associated with this method are the structure retention during polymerization from monomer to polymer phase and the reorientation of mesochannels in a facile way for high continuity and therefore high flux property. In this review paper, the current progress of the various methods applied for hexagonal LLCs templated-structure retention and the strategies used for reorientation are presented.
The retention of the hexagonal phase is the prerequisite for the fabrication of mesoporous membranes with porosity. The membranes with controllable nanostructure dimensionality such as pore size and wall thickness but favourable orientation are very important materials for further applications in the treatment of surface water, groundwater, and industrial wastewater in which different contaminants such as toxic metals, organic and inorganic compounds, bacteria, and viruses could be found. The dimensional elements of the hexagonal packing include pore size, water and head group thickness, and the average area available to each head group and could be calculated by using the volume fraction of amphiphiles and water molecules from SAXS/SANS measurement [79
]. To some extent, the pore size could be also adjustable by using some organic molecules [70
]. However, the inherent preference of the isotropic distribution of the mesochannels within the template severely limits the filtration efficiency even if the nanostructure is precisely controlled during the whole process. In addition, the mechanical property will be significantly improved for a monolith after aligning the cylinders through the film. Therefore, the continuity of nanopores through the whole film by reorientating the mesochannels will be another key obstacle for the actual application.
Polymer membranes with well-defined pores in the range of 1–5 nm have the potential to selectively distinguish and transport certain molecules or ions based on their shape, size, and chemical properties. Several methods have been developed to align the hexagonal mesochannels, with the long range being parallel or perpendicular to the substrate according to the actual requests. However, in contrast to the pores with their long axis parallel to the substrate, the materials with perpendicularly aligned channels in respect to the film surface are obviously more versatile for optoelectronic devices, separators with high flux, ultra-high-density recording media, dand novel controlled drug release systems [15
]. The strategies developed to align mesochannels templated from hexagonal LLCs are comprehensively discussed, as follows.
3.1. External Field
The application of commonly used external fields, including magnetic fields, electrical fields, and shear force, has shown a powerful ability to administrate the orientation of mesochannels. Magnetically induced anisotropy alignment has been achieved during the last few years by mainly using the high magnetic field of the NMR spectrometer permanent magnet [84
]. Deuterium NMR spectroscopy can be used to characterize the aggregate structure and to quantify the degree of alignment. The overall diamagnetic susceptibility of the aggregates is the key parameter that administrates the orientation with respect to the field direction. The overall orientation of the mesochannels could be easily controlled by incorporating the molecules with large positive/negative diamagnetic susceptibility and therefore adjusting the overall diamagnetic susceptibility of the aggregates [84
]. The field strength, on the other hand, is another key parameter to decide the degree of alignment. Due to the negative diamagnetic susceptibility of the alkyl chains in many surfactants and the low magnetic susceptibility of all surfactant molecules, the alignment normally requires a very high magnetic field (>3 Tesla) [86
]. Very recently, transparent membranes with vertically aligned 1 nm pores have been reported after reorientation under a magnetic field with a strength higher than 3 T (Figure 11
]. Additionally, the research found that introducing ferromagnetic nanoparticles into the hexagonal phase can make the nanostructures more easily aligned in a low magnetic field [50
]. However, the necessary condition is that the mesophase periodicity has to be larger than the diameter of the nanoparticles. The aggregation and phase separation of nanoparticles inside the system always confine the application of the magnetic field.
The electrical field is another promising tool to align the mesochannels in films. It was initially applied for the alignment of the lamellar mesophase [88
]. However, for non-lamellar mesophases, research has shown that the electric field can induce the phase transition between the hexagonal and cubic phase by the interaction between the electric field and the materials via different mechanisms [51
]. For dielectric materials, the dipole (polarization) effect can be used to orient liquid crystals, for which a high strength is normally needed (E
], while, in a system with charges, the electrokinetic effect will be sensitive to the applied strength and therefore the required strength is less [91
]. However, some inherent disadvantages like electrode contact issues and electrical breakdown concerns always limit the actual application, and therefore using anelectric field for reorienting hexagonal LLCs has not been reported on extensively.
Shear force, as another important external field to induce alignment, was first demonstrated by Hillhouse [92
]. Normally the mesochannels in the hexagonal phase, with their direction aligned along the flow direction (air flow or water flow), and the induced shear force can guide the reorientation of tubular domains according to the flow rate, flow directions, and the ambient conditions [46
]. Sometimes, the other external forces can be used together with the shear force to improve the reorientation effect (Figure 12
3.2. Confinement in a Small Space
It is possible to align the mesochannels when the hexagonal LLCs are formed in a small space as the structure of crystals with long axes will form along with a specified direction. Research has shown that, when CTAB was introduced into the matrices of polyacrylic acid (PAA) channels as a structural directing agent, the mesochannels of hexagonal aggregates were mostly parallel to the PAA channels under suitable conditions (Figure 13
]. In addition, some researchers also used this micro- or nano-space in situ with the external field or some other effects to align the mesochannels [97
]. However, this method is difficult to apply in membrane fabrication.
3.3. Other Methods
Apart from the ways mentioned above, some other methods were also developed to align the mesochannels in hexagonal LLCs and have shown potential value. The most commonly used method was the modification of the substrate surface. Some scientists reported that the mesochannels were prepared and aligned on mica, graphite, silica, quartz glass, and polymer films, which acted as substrates, by using a substrate-molecule interaction that definitely regulates the orientation of mesochannels [99
]. Interestingly, a versatile strategy to achieve the perpendicular alignment by the π-π interaction between organic template molecules and 2-dimensional π plane graphite or silicon wafer surfaces with different surface energies was reported [68
]. Meanwhile, the application of the electrochemically assisted alignment of the mesochannels in silica films was explored, in which various conducting supports and even electronic paper were explored to conduct the alignment of the mesochannels [102
]. In addition, a series of novel Gemini surfactants, which have a high charge density and two polar head groups, were also successfully used to prepare the mesoporous membranes with mesochannels normal to the membrane surface [45
]. Most recently, a simple method called the stöber-solution growth approach was explored to prepare highly ordered perpendicular mesochannels transformed from uniform mesoporous nanospheres after a series of interactions between components in stöber-solution under suitable conditions [44
4. Future Directions and Conclusions
4.1. Future Directions
The introduction of the silica network has been found to reinforce the hexagonal LLCs template and significantly boost the structure retention rate in combination with the CO2
critical point drying method from our previous work [29
]. Researchers have been trying to control over the phase curvature chemically or physically to increase the retention rate of nanostructures. However, an important issue overlooked by the researchers but that should be emphasised is the distribution of the monomer(s) within the hexagonal LLCs template, which could also be a critical mechanism that can control the phase retention and its filtration efficiency. Research has demonstrated the preferred distribution of the water molecules and the solubilised species within the lamellar phase [104
]. Meanwhile, the physical robustness of the polymerized materials highly depends on the thickness of hydrophilic area, where the thickness of pore wall comes after polymerization. Undoubtedly, the thicker water channel and the evenly distributed monomer(s) are preferable for the robustness of membranes. However, all this detailed information on the thickness of the hydrophilic area and the distribution of the water/monomer(s) within the hexagonal LLCs has not been explored.
A suitable thin yet mechanically robust film will significantly improve the permeability of NF processes. The inherent viscoelasticity of the hexagonal LLCs makes it troublesome to prepare a membrane template precursor with an exact thickness. The robustness, on the other hand, sets a lower threshold on the thickness of the film. Fortunately, a reversible phase transition as a function of temperature could be realized by incorporating small molecules into the template without significantly affecting the structure. Those small molecules enable the further process of the samples in a flow regime at as low as about 45 °C, depending on various systems, via molecular design, thereby enabling the thickness to be controlled after cooling to room temperature. The introduction of small molecules with a high number of reactive groups also helps to increase the cross-linking density and further benefits the mechanical property. Additionally, alcoholic molecules such as methanol or ethanol are also good options to disperse the samples under a flow state; after that, the desired membrane thickness could be obtained by evaporating the alcoholic molecules. However, the evaporation process should be carefully maneuvered to maintain the water contents in the template.
Further, to get a monolithic membrane template for the fabrication of mesoporous membrane and improve its mechanical properties, the facile and scalable methods for the reorientation of mesochanels are essential to put the fabrication of mesoporous membrane templated from hexagonal LLCs into actual practice. The most commonly used method for reorientation is mainly the strong magnetic field (>3 Tesla) [87
]. However, a more promising method probably should be the electric field, which presents a similar reaction mechanism, as an electric field is easily accessible in the laboratory and in actual practice. Although it has some disadvantages such as contact issues and breakdown concerns compared to the magnetic field, those concerns can be overcome by appropriate design. More importantly, the electric field could provide an excellent compatibility with thin film geometries and is inherently scalable from the lab to actual application.
Finally, due to an increasing emphasis on environmental concerns, the renewable/sustainable materials used as the structure-directing agent or the reactive monomer will gain more attention. Recent research for the NF membranes polymerized by using plant-derive fatty acid molecules templated into highly ordered columnar mesophase opened up a promising route for the sustainable development of NF membranes [106
]. Actually, the amphiphile, rich in living systems such as plants or animals, is environmentally friendly and could be considered a good potential candidate for membrane materials.
The template method from hexagonal LLCs provides a facile and scalable path for the fabrication of mesoporous membranes with a pore size in the range of 1–5 nm and large surface areas. The thermodynamically controlled phase behaviours and the separation from the decrease of entropy and the reduction in enthalpy during polymerization have been proven to be the main reason for structure distortion. Several considerations and strategies were given to compensate the thermodynamic changes during polymerization, and further processes followed for structure retention. The retention rate has been significantly improved for mesoporous membrane fabrication, accordingly.
The other key part discussed in the paper is the alignment of mesochannels for continuity through the whole film. Various techniques have been developed for the reorientation of mesochannels like external fields, electrochemical methods, special interaction, or novel designed structure-directing agents. However, those methods still need to be further developed to improve their effectiveness and scalability, although some of them have been applied to achieve a good transparent membrane product.