TY - JOUR
T1 - Selectivity and polarization in water channel membranes
T2 - Lessons learned from polymeric membranes and CNTs
AU - Freger, Viatcheslav
N1 - Funding Information:
The nancial support by the Israel Science Foundation (grant #1152/11) and by a joint grant 2016627 from the United States-Israel Binational Science Foundation (BSF, Jerusalem, Israel), and the United States National Science Foundation (NSF) is acknowledged. The author thanks Dr Aleksandr Noy (LLNL and UC Merced) and Prof. Meni Wanunu (Northeastern U) for many illuminating discussions and for sharing data prior to publication.
Funding Information:
The financial support by the Israel Science Foundation (grant #1152/11) and by a joint grant 2016627 from the United States-Israel Binational Science Foundation (BSF, Jerusalem, Israel), and the United States National Science Foundation (NSF) is acknowledged. The author thanks Dr Aleksandr Noy (LLNL and UC Merced) and Prof. Meni Wanunu (Northeastern U) for many illuminating discussions and for sharing data prior to publication.
Publisher Copyright:
© 2018 The Royal Society of Chemistry.
PY - 2018/1/1
Y1 - 2018/1/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=85054135388&partnerID=8YFLogxK
U2 - 10.1039/c8fd00054a
DO - 10.1039/c8fd00054a
M3 - Article
C2 - 29972390
AN - SCOPUS:85054135388
SN - 1364-5498
VL - 209
SP - 371
EP - 388
JO - Faraday Discussions
JF - Faraday Discussions
ER -