Relationship between Structural Behavior and Transport Performance of the Biomimetic Membranes.
The present study leads to the discovery of an unexplored mechanistic strategy allowing homogeneous AWC incorporation starting from their in situ colloidal self-assembled superstructures leading to the identification of sponge-like particles present within the hybrid PA-AWC materials. During IP, the MPD is interacting with soft self-assembled HC6 colloidal nanoparticles via H-bonding. Due to their amphiphilic properties, the AWC aggregates may also contribute to enhance the diffusion of the MPD into the organic phase to react with the TMC as revealed by previous studies (25). The formation of the hybrid layers may be further generated via the interaction of the amphiphilic AWC/MPD nanoparticles with nascent PA oligomers via H-bonding. These interactions depend strongly on the HC6/MPD ratio and under favorable conditions, they effectively promote the PA formation and facilitate the gentle incorporation of AWC aggregates, whereby preventing the formation of defects, which are commonly observed when solid state nanoparticles are directly incorporated within the PA (1, 2). Other than the seamless in situ adaptive incorporation of distributed AWC/PA-sponge like nanostructures into the PA layer, a morphological result observed in this study suggests the homogeneous formation of unique highly porous AWC/PA structures with reduced water transport resistance at the interface of the selective layer with the PSf support. Such membranes provided the best performances in terms of water transport and selectivity, and the porous structure did not affect the mechanical resistance or the membrane properties under RO filtration, unlike previous results reported for PA membranes prepared with additives (26, 28).
It can be concluded that the improved transport performances stem from a combination of higher porosity of the overall PA-based layer, the absence of defects, and the occurring fast transport through the HC6 nanostructures, whose excellent supramolecular adaptive properties confirm their ability to selectively translocate water while rejecting small ions even when incorporated into hybrid PA-AWC membranes. A significant loss in the perm-selectivity was instead observed for the membranes fabricated with too low MPD concentration or too high HC6 loadings. For example, membranes fabricated with insufficient MPD exhibited a dense, more symmetric, and extremely thin selective layer. These results suggest that the presence of an excess of MPD seems to be imperatively needed for synthetizing defect-free bioinspired layers (29).
We know from literature data that the use of a spray-coated carbon nanotubes layer on the polyethersulfone support before the IP, provides an interface that enables the generation of a highly permeable and selective PA layer with a large effective surface area for water transport (30). Some similarities may explain the formation mechanism related to this study, whereby the incorporation of HC6 in the PA layer leads to combined effects, such as the formation of a gutter layer or the generation of higher surface roughness and/or a leaf-like morphology that might be related to the amphiphilic properties of the HC6 (in contrast to the use of additives, such as sodium dodecyl sulfate). Nonetheless, if we look at the experimental results in the upper bound graph (Fig. 5C), our membranes show to be highly permeable and selective, lying at the limit that exists between BWRO and SWRO regions. High recovery and fouling experiments (SI Appendix, Figs. S8 and S9) confirm the high-quality features of our biomimetic membranes. Therefore, in the range/area that delimitates the BWRO membranes, our membranes are highly selective, more selective than the other membranes prepared on laboratory scale, including thin-film nanocomposites obtained using nanofillers (i.e., zeolites, carbon nanotubes, graphene oxide), additives (i.e., proton acceptors, surfactants, acids), or optimized by using salts/hydrophilic additives, different solvent or cosolvent, or by varying other experimental conditions (i.e., pH, T, or concentrations in the casting solutions) during the membrane fabrication (11).
Of particular interest is the potential ability of such PA films to present directional pathways for water transport. Herein, microscopy studies revealed that such hybrid PA-AWC materials are composed by AWC crystalline nanoparticles, randomly dispersed in the PA matrix (Fig. 5B). For 1.2 to 1.8% (wt/wt) MPD, these particles are homogeneously and densely distributed within all the thickness of the PA layer, while for lower concentration of 0.2 to 0.8% (wt/wt) MPD they are less dense and are situated the middle part of the PA layer. For the sponge-like nanoparticles (17) of 20 to 40 nm the permeability of water can be theoretically estimated up to PAWC = 131 L·m−2⋅h−1⋅bar−1 (SI Appendix). Independently of what is possible by microscopy or not, a high density of channels percolating from one side to the other of the membranes would be ideal, but we are not there yet. It is not about alignment, which is clearly proved to occur along nanometric distances of crystals of AWC that are showing order. It is about percolation and high density of channels that for the moment is achieved at this nanometric scale. Although these nanoparticles do not merge to cross the micrometric PA films (Fig. 5A), they are randomly distributed within the PA layer (PPA = 1 LMH/bar). The AWC crystals locally contribute to enhance translocation of water, as PAWC >> PPA (Fig. 5B). We do not intend to prove anything in a definite or exactly mathematical way here; this is only an intuitive way to explain our results on AWC-PA membrane permeabilities that are experimental and unquestionable. The hybrid PA-AWC materials are reminiscent of previous hybrid ion-channels siloxanes, providing high ionic conduction through their nanometric self-organization of binding sites in hybrid materials (31⇓–33).
The assumption is that, besides 1) the tunable formation of colloidal nanoparticles within the PA, 2) the formation of inner porosity/voids (as a consequence of the effect of the HC6 “interlayer”), and 3) the gutter layer-like structure, there is the diffusion of hydrophilic and selective sponge-like AWCs-PA (which we surmise are also in the form of I-quartets). Thus, we assume that this performance is achieved thanks to the combined effect of all of these aspects. It is also true that the AWCs are intrinsically “nanofillers,” but by comparison, our membranes are more selective and permeable, possibly due to the compatibility of the HC6. Obviously, a high density of channels is desired to promote high permeance. Alignment is not essential, but the percolation of AWC particles is important and should be optimized to have the water transport mainly taking place through the channels. Homogeneous distribution with particle percolation should be preferred. High density of particles without aggregation should be obtained. Achieving these conditions was a main goal of the optimization in this work.
In this work, bioinspired membranes for low-salinity BWRO and TWRO water desalination were fabricated by incorporating I-quartet AWC in the classical PA layer. Specifically, the performance of hybrid AWC-PA membranes was tuned by studying the effect of the optimal AWC loads and that of MPD monomer concentration during IP, thus optimizing the selective layer. It may be concluded that self-aggregated AWC colloidal nanoparticles were incorporated by means of their supramolecular interactions with MPD monomer and their presence altered the IP process, and thus the final layer properties. These dynamic self-assembly processes amount to adaptive colloidal entities with the nascent PA oligomers.
This study illustrates a complete interplay of supramolecular aggregation and IP processes 1) related to supramolecular aggregation, once the AWC load is increasing, the selective PA-AWC layers became more porous, and AWC nanoaggregates were homogeneously distributed within the hybrid PA, resulting in high-performance membranes. However, overly high concentrations of AWC nanoaggregates led to the formation of defects, resulting in the poorest performances. 2) The MPD monomer concentration had also an important effect during membrane fabrication and seamless biomimetic layers were synthetized by adjusting this parameter reaching the optimal HC6/MPD ratio between 0.7 and 1.9 (wt/wt). In general, a minimum concentration of MPD [∼0.8% (wt/wt)] was required to obtain high selectivity. However, even this concentration should not be too high in order to maintain high water fluxes. In particular, the membranes fabricated with 1.2% (wt/wt) MPD and 1.5% (wt/wt) HC6 provided a ∼360% increase in water permeance at equivalent water-to-salt flux ratio with respect to the membrane fabricated using 3.4% (wt/wt) MPD.
The best membranes displayed excellent productivity of 110 L⋅m−2⋅h−1 under 15.5-bar applied pressure and 35 L⋅m−2⋅h−1 under 6-bar applied pressure, while maintaining high selectivity properties, outperforming current BWRO and TWRO commercial membranes. Ultimately, we demonstrated that bioinspired membranes incorporating I-quartet AWCs own the potential for improving existing low-pressure RO applications, and their composition can be adjusted to tune their superstructures and performances to target different applications.