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• W3009867865 abstract "In this issue of Matter, Malmir and colleagues use sophisticated methods to give an atomic-scale view of saltwater permeation through a graphitic pore with high salt rejection. Na+ and Cl− traverse the pore at different rates, leading to an electrostatic potential across the membrane. In this issue of Matter, Malmir and colleagues use sophisticated methods to give an atomic-scale view of saltwater permeation through a graphitic pore with high salt rejection. Na+ and Cl− traverse the pore at different rates, leading to an electrostatic potential across the membrane. Although reverse osmosis membranes for water desalination are established and considered a mature technology, the molecular control accessible with these synthetic membranes pales in comparison to biological ion-conducting trans-membrane proteins. Although the amazing properties of K+-channel proteins were known experimentally, the particular mechanisms that allow these properties were not understood until atomic-scale structures were obtained (in work that led to a Nobel Prize for Roderick MacKinnon).1Doyle D.A. Morais Cabral J. Pfuetzner R.A. Kuo A. Gulbis J.M. Cohen S.L. Chait B.T. MacKinnon R. The structure of the potassium channel: molecular basis of K+ conduction and selectivity.Science. 1998; 280: 69-77Crossref PubMed Scopus (5736) Google Scholar If it is possible to understand the atomic-scale details of water and salt permeation through synthetic membranes, it may ultimately be possible to leverage this knowledge to broaden the conditions under which these membranes are useful. In this issue of Matter, Malmir et al. have taken a step in this direction by introducing methods that model permeation through an ultra-thin membrane in impressive detail.2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar Analyzing permeation of saltwater through a membrane using atomistic molecular simulations seems superficially simple, but the very limited timescales imposed by molecular dynamics (MD)—on the order of nanoseconds to microseconds—place severe constraints on the number of molecular events that can be observed. This restricts the use of these methods in simulating stochastic dynamic events, such as ion permeation. The challenge is particularly acute for a membrane with a high level of salt rejection, because ions in the solution pass through the membrane orders of magnitude slower than water molecules. Malmir et al.2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar tackled this timescale challenge in two ways. First, they modeled a membrane of just three layers of graphite made porous by including a single sub-nm pore through the layers. Crudely, the flux through a membrane of thickness L scales as 1/L, so the choice of an ultra-thin membrane maximizes this flux. By applying hydrostatic pressure in their MD simulations, Malmir et al.2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar were able to directly observe water permeation from a 1.5 M NaCl solution. Second, they applied a sophisticated rare-event sampling technique (specifically, a method called jumpy forward-flux sampling) that gives accurate information on the permeation rates of Na+ and Cl− ions. This approach gives direct information on the relation permeation rates and mechanisms of H2O, Na+, and Cl− through a graphite nanopore. Malmir et al.’s2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar simulations show water fluxes that are in good agreement with predictions from the macroscopic Hagen-Poiseuille law. This is in contrast to earlier work that showed much higher water fluxes through membranes based on carbon nanotubes.3Holt J.K. Park H.G. Wang Y. Stadermann M. Artyukhin A.B. Grigoropoulos C.P. Noy A. Bakajin O. Fast mass transport through sub-2-nanometer carbon nanotubes.Science. 2006; 312: 1034-1037Crossref PubMed Scopus (2491) Google Scholar Strong deviations from macroscopic predictions in carbon nanotubes occur because of the atomistic smoothness of the walls of these structures.4Chen H. Johnson J.K. Sholl D.S. Transport diffusion of gases is rapid in flexible carbon nanotubes.J. Phys. Chem. B. 2006; 110: 1971-1975Crossref PubMed Scopus (150) Google Scholar The hydrogen-terminated pore simulated by Malmir et al.2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar does not share this smoothness. The estimated salt rejection of the simulated membrane is >99.99%, meaning that >10,000 water molecules pass through for every ion. A more striking outcome from Malmir et al.’s2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar simulation is that Na+ and Cl− traverse the membrane at very different rates. At a qualitative level, some differences in rates is to be expected because the size of the solvated ions, and hence their ability to enter and move through a sub-nm pore, differ. Malmir et al.’s2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar simulations, however, are able to quantify this effect for the first time. They find that Na+ permeates much more slowly than Cl−, with a mean passage time of ~900 μs for Na+. This timescale is orders of magnitude larger than any reasonable MD simulation, illustrating the necessity of using advanced sampling methods. The net permeation of Cl− relative to Na+ through the membrane in Malmir et al.’s2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar simulations implies the buildup of an electrostatic potential across the membrane, an effect known as reversal potential. Under the specific conditions simulated, a reversal potential of around −100 mV is found. Malmir et al.'s2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar results point to an intrinsic challenge with their MD methods, which probe a transient situation. In experimental settings, one is typically interested in situations more closely mimicking steady-state operation in which the reversal potential and related effects such as concentration polarization are strongly impacted by flow geometries and similar effects that lie outside the direct scope of a molecular simulation. Challenges still remain in using molecular simulations to describe realistic synthetic membranes. Although membranes just a few atoms in thickness have attracted research attention, fabrication of practical membranes of this type remains extremely challenging. Moreover, it is not clear that the property that makes these membranes so attractive for simulation, their extremely high flux, is the key attribute that needs to be met in many applied settings.5Werber J.R. Osuji C.O. Elimelech M. Materials for next-generation desalination and water purification membranes.Nat. Rev. Mater. 2016; 1: 16018Crossref Scopus (1615) Google Scholar Comparisons of simulated ultra-thin and “bulk-like” gas separation membranes have shown that the former are dominated by so-called surface resistances that become far less important in the latter.6Newsome D.A. Sholl D.S. Influences of interfacial resistances on gas transport through carbon nanotube membranes.Nano Lett. 2006; 6: 2150-2153Crossref Scopus (66) Google Scholar,7Dutta R.C. Bhatia S.K. Interfacial barriers to gas transport in zeolites: distinguishing internal and external resistances.Phys. Chem. Chem. Phys. 2018; 20: 26386-26395Crossref Google Scholar When this is the case, it can be challenging to extrapolate insight from ultra-thin membranes to more experimentally realistic structures. Simulations are best suited to membranes that are high ordered,6Newsome D.A. Sholl D.S. Influences of interfacial resistances on gas transport through carbon nanotube membranes.Nano Lett. 2006; 6: 2150-2153Crossref Scopus (66) Google Scholar,7Dutta R.C. Bhatia S.K. Interfacial barriers to gas transport in zeolites: distinguishing internal and external resistances.Phys. Chem. Chem. Phys. 2018; 20: 26386-26395Crossref Google Scholar as in the single pore simulated by Malmir et al.,2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar but more realistic polymeric membranes are disordered and amorphous. Efforts have been made to simulate membranes of this kind, but these face even greater challenges associated with timescales and sampling.8Liyana-Arachchi T.P. Sturnfield J.F. Colina C.M. Ultrathin molecular-layer-by-layer polyamide membranes: insights from atomistic simulations.J. Phys. Chem. B. 2016; 120: 9484-9494Crossref Scopus (14) Google Scholar It will be fascinating to see what can be learned as the clever sampling methods used by Malmir et al.2Malmir H. Epsztein R. Elimelech M. Haji-Akbari A. Induced charge anisotropy: a hidden variable affecting ion transport through membranes.Matter. 2020; 2: 735-750Abstract Full Text Full Text PDF Scopus (15) Google Scholar are extended to more complex realizations of ion-selective membranes. This work was supported by UNCAGE-ME, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0012577. Induced Charge Anisotropy: A Hidden Variable Affecting Ion Transport through MembranesMalmir et al.MatterJanuary 15, 2020In BriefConventional molecular simulations are generally incapable of capturing the kinetics of solute transport through ultra-selective membranes. Here, we use a recently developed path-sampling technique called jumpy forward-flux sampling to accurately and efficiently compute long passage times for sodium and chloride ions traversing a nanoporous graphitic membrane. We also demonstrate that an ion's passage is not only impeded by its partial dehydration but also by a negative restraining force due to the charge anisotropy that it induces at its rear. Full-Text PDF Open Archive" @default.
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• W3009867865 date "2020-03-01" @default.
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• W3009867865 title "Watching Water, Sodium, and Chloride Passing through a Graphitic Pore" @default.
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