Various types of channel proteins, broadly named porins, present in the cell membrane of gram‐negative bacteria have specific functionalities depending on their selectivity toward different nutrients or toward water. The high selectivity of porins has led to their incorporation into synthetic systems, in a field called biomimetics. An example is the addition of water channel proteins, or aquaporins, to polymeric separations membranes in order to enhance their performance in terms of selectivity and permeability. The concept of incorporating aquaporins into synthetic membranes has been studied for the last 10 years; however, there are still limitations such as costs, alignment into the membrane assembly, and scalability of membrane fabrication. Therefore, in recent years, there has been an increase in the study of synthesizing molecules with similar structure–function relationships of porins. These artificial channels attempt to mimic the biological porins, while being synthesized using simpler chemistry, being solvent compatible, and requiring less space on the membrane surface which helps to incorporate more channels into the membrane assembly. In summary, the future of biomimetic and bioinspired membranes depends on the design strategies, level of nature imitation, and the performance of these systems on a commercial scale. © 2019 American Institute of Chemical Engineers Environ Prog, 38:e13215, 2019
Selectivity and polarization in water channel membranes: lessons learned from polymeric membranes and CNTs
Water channels are employed by nature to move pure water across cell membranes while selectively rejecting salts. At present, synthetic channels successfully mimic water permeation, yet even the best channels, such as carbon nanotubes (CNTs) and graphene oxide stacks, still fall short of the selectivity target. The present paper analyzes factors that may help to enhance and control salt rejection based on the lessons learned from conventional membranes and CNTs. First, it highlights the importance of raising the ion self-energy (dielectric mechanism), which suggests that having the channels both narrow and surrounded by a low-dielectric environment is key to high selectivity. In contrast, pore charge alone is insufficient, yet it may help to enhance and tune ion rejection, provided that non-mean-field effects enhanced in low-dielectric pores, such as ion association and sorption, especially of H + and OH − ions, are properly understood and addressed in the channel design. Second, the role of concentration polarization (CP) is analyzed, which shows that the CP level is apparently low in isolated channels or microscopically small membranes. However, the geometry of the diffusion field should change and CP should increase drastically in macroscopic membranes incorporating densely spaced channel arrays. If not properly addressed in membrane design, the increased CP level in scaled-up channel-based membranes may significantly compromise the observed selectivity and require that target of selectivity be re-set to an even more challenging value. These points may help guide the future development of high-performance artificial water channels and their scale-up towards utilization in next-generation water purification membranes.
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- Award ID(s):
- 1710211
- NSF-PAR ID:
- 10100413
- Date Published:
- Journal Name:
- Faraday Discussions
- Volume:
- 209
- ISSN:
- 1359-6640
- Page Range / eLocation ID:
- 371 to 388
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Desalination by membrane distillation (MD) using low‐grade or waste heat provides a potential route for sustainable water supply. Nonwetting, porous membranes that provide a selective pathway for water vapor over nonvolatile salt are at the core of MD desalination. Conventional water‐repelling MD membranes (i.e., hydrophobic and superhydrophobic membranes) fail to ensure long‐term desalination performance due to pore wetting and surface fouling. To address these challenges, a defect‐free carbon nanotube forest (CNTF) is engineered in situ on a porous electrospun silica fiber substrate. The engineered CNTF forms an ultrarough and porous interface structure, allowing outstanding wetting resistance against water in air and oil underwater. As a result of this antiwetting property, the composite CNTF membrane displays a stable water vapor flux and a near complete salt rejection (>99.9%) in the desalination of highly saline water containing low surface tension contaminants. The antimicrobial property of the composite CNTF membrane imparted by the unique forest‐like architecture and the oxidative effect of carbon nanotubes (CNTs) are further demonstrated. The results exemplify an effective strategy for engineering CNT architecture to elucidate the structure–property–performance relationship of the nanocomposite membranes and to guide the design of robust thermal desalination membranes.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8FD00054A
