Water scarcity is a reality in our world, and scenarios predicted by leading scientists in this area indicate that it will worsen in the next decades. However, new technologies based in low-cost seawater desalination can prevent the worst scenarios, providing fresh water for humanity. With this goal, membranes based in nanoporous materials have been suggested in recent years. One of the materials suggested is MoS$_2$, and classical Molecular Dynamics (MD) simulation is one of the most powerful tools to explore these nanomaterials. However, distinct Force Fields employed in MD simulations are parameterized based on distinct experimental quantities. In this paper, we compare two models of salt that were build based on distinct properties of water-salt mixtures. One model fits the hydration free energy and lattice properties, the second fits the crystal density and the density and the dielectric constant of water and salt mixtures. To compare the models, MD simulations for salty water flow through two nanopores sizes were used -- one pore big enough to accommodate hydrated ions, and one smaller in which the ion has to dehydrate to enter, and two rigid water models from the TIP4P family -- the TIP4P/2005 and TIP4P/$ε$. Our results indicate that the water permeability and salt rejection by the membrane are more influenced by the salt model than by the water model, especially for the narrow pore. In fact, completely distinct mechanisms were observed, and they are related to the characteristics employed in the ion model parameterization. The results show that not only the water model can influence the outcomes, but the ion model plays a crucial role.
Water desalination through nanopores has been shown to be a promising alternative to the currently water purification processes. In spite the results in this direction obtained by means of computational simulations were animating there are still pending issues to be resolved. For example, water desalination involves macro numbers (in size and time) but in such a scale it is literally impossible to attack this problem using all-atoms simulations. It is common to extrapolate results from nano to macro sizes in order to estimate quantities of interest, which must be taken with care. Here we prese…
Graphyne-4 sheet exhibits promising potential for nanoscale desalination to achieve both high water permeability and salt rejection rate. Extensive molecular dynamics simulations on pore-size effects suggest that graphyne-4, with 4 acetylene bonds between two adjacent phenyl rings, has the best performance with 100% salt rejection and an unprecedented water permeability, to our knowledge, of ~13L/cm2/day/MPa, about 10 times higher than the state-of-the-art nanoporous graphene reported previously (Nano Lett.s 2012, 12, 3602-3608). In addition, the membrane entails very low energy consumption fo…
Nanoscale materials are a promising desalination technology. While fast water flow in nanotubes is well understood, this is not the case for water permeability in single-layer membranes. The physical-chemical balance between nanopore size, shape, and charge might be the answer.
One of the most promising applications in nanoscience is the design of new materials to improve water permeability and selectivity of nanoporous membranes. Understanding the molecular architecture behind these fascinating structures and how it impacts the water flow is an intricate but necessary task. We studied here, the water flux through multi-layered nanoporous molybdenum disulfide (MLNMoS$_2$) membranes with different nanopore sizes and length. Molecular dynamics simulations show that the permeability do not increase with the inverse of the membrane thickness, violating the classical hydr…
Nanopores in solid state membranes are a tool able to probe nanofluidic phenomena or can act as a single molecular sensor. They also have diverse applications in filtration, desalination or osmotic power generation. Many of these applications involve chemical, or hydrostatic pressure differences, which act on both the supporting membrane and the ion transport through the pore. By using pressure differences between the sides of the membrane, and an alternating current approach to probe ion transport, we investigate two distinct physical phenomena: the elastic deformation of the membrane through…
Nanopore desalination technology hinges on high water-permeable membranes which, at the same time, block ions efficiently. In this study, we consider a recently synthesized [Science 363, 151-155 (2019)] phenine nanotube (PNT) for water desalination applications. Using both equilibrium and non-equilibrium molecular dynamics simulations, we show that the PNT membrane completely rejects salts, but permeates water at a rate which is an order-of-magnitude higher than that of all the membranes used for water filtration. We provide the microscopic mechanisms of salt rejection and fast water-transport…
Nanopores are both a tool to study single-molecule biophysics and nanoscale ion transport, but also a promising material for desalination or osmotic power generation. Understanding the physics underlying ion transport through nano-sized pores allows better design of porous membrane materials. Material surfaces can present hydrophobicity, a property which can make them prone to formation of surface nanobubbles. Nanobubbles can influence the electrical transport properties of such devices. We demonstrate an approach which uses hydraulic pressure to probe the electrical transport properties of so…
We revisit the classical problem of flow of electrolyte solutions through charged capillary nanopores or nanotubes as described by the capillary pore model (also called "space charge" theory). This theory assumes very long and thin pores and uses a one-dimensional flux-force formalism which relates fluxes (electrical current, salt flux, fluid velocity) and driving forces (difference in electric potential, salt concentration, pressure). We analyze the general case with overlapping electric double layers in the pore and a nonzero axial salt concentration gradient. The 3$\times$3 matrix relating …
Most thermodynamic modeling of hydrogels focused on predicting their final volumes in equilibrium with water, built on Flory's theories for the entropy of mixing and rubber elasticity, and Donnan's equilibrium conditions if polyelectrolyte polymer and mobile ions are involved. This work will focus on water and ions in and outside hydrogels, which are of interests in solar interfacial water evaporation for desalination and waste water treatment, atmospheric water harvesting, and forward osmosis. Via a reformulation of Flory's classical hydrogel thermodynamic model by considering a combined syst…
Graphene nanopore based sensor devices are exhibiting the great potential for the detection of DNA. To understand the fundamental aspects of DNA translocating through a graphene nanopore, in this work, molecular dynamics (MD) simulations and potential of mean force (PMF) calculations were carried out to investigate the layer impact of small graphene nanopore (2 nm-3 nm) to DNA translocation. It was observed that the ionic conductance was sensitive to graphene layer of open-nanopores, the probability for DNA translocation through graphene nanopore was related with the thickness of graphene nano…
Reproducing the exquisite ion selectivity displayed by biological ion channels in artificial nanopore systems has proven to be one of the most challenging tasks undertaken by the nanopore community, yet a successful achievement of this goal offers immense technological potential. Here we show a strategy to design solid-state nanopores that selectively transport potassium ions and show negligible conductance for sodium ions. The nanopores contain walls decorated with 4'-aminobenzo-18-crown-6 ether and ssDNA molecules located at one pore entrance. The ionic selectivity stems from facilitated tra…
Using molecular dynamics simulations, we show that, when subject to a periodic external electric field, a nanopore in ionic solution acts as a capacitor with memory (memcapacitor) at various frequencies and strengths of the electric field. Most importantly, the hysteresis loop of this memcapacitor shows both negative and diverging capacitance as a function of the voltage. The origin of this effect stems from the slow polarizability of the ionic solution due to the finite mobility of ions in water. We develop a microscopic quantitative model which captures the main features we observe in the si…
The development of energy and infrastructure efficient water purification systems are among the most critical engineering challenges facing our society. Water purification is often a multi-step process involving filtration, desalination, and disinfection of a feedstream. Shock electrodialysis (shock ED) is a newly developed technique for water desalination, leveraging the formation of ion concentration polarization (ICP) zones and deionization shock waves in microscale pores near to an ion selective element. While shock ED has been demonstrated as an effective water desalination tool, we here …
Solubility of poorly soluble species in bulk and nanopores from grand-canonical Wang-Landau sampling
Institute for Computational Physics, University of Stuttgart（斯图加特大学计算物理研究所）；SC SimTech, Stuttgart Center for Simulation Science, University of Stuttgart（斯图加特大学斯图加特模拟科学中心）；Institute for Physics of Functional Materials, Hamburg University of Technology（汉堡工业大学功能材料物理研究所）
Ion transport in extremely narrow nanochannels has gained increasing interest in recent years due to its unique physical properties, and the technological advances that allow us to study them. It is tempting to approach this regime with the tools and knowledge developed for wider microfluidic devices and use continuum models like the Poisson-Nernst-Planck equation. However, it turns out that some of the most basic principles we take for granted in a large system, such as electroneutrality, can breakdown under extreme confinement. We show that in a truly one-dimensional system, interacting with…
Using molecular dynamic simulations we show that single-layers of molybdenum disulfide (MoS$_2$) and graphene can effectively reject ions and allow high water permeability. Solutions of water and three cations with different valence (Na$^+$, Zn$^{2+}$ and Fe$^{3+}$) were investigated in the presence of the two types of membranes and the results indicate a high dependence of the ion rejection on the cation charge. The associative characteristic of ferric chloride leads to a high rate of ion rejection by both nanopores, while the monovalent sodium chloride induces lower rejection rates. Particul…
When channels were scaled down to the size of hydrated ions, ionic Coulomb blockade was discovered. However, the experimental CB phenomenon was rarely reported since Feng et.al., discovered in MoS2 nanopore. By using latent-track membranes with diameter of 0.6 nm, we found the channels are nearly non-conductive in small voltage due to the blockade of cations bound at surface, however turns to be conductive as rising of voltage due to releasing of bound ions, which differs from the mechanisms in MoS2 nanopore. By Kramers' escape framework, we rationalized an analytical equation to fit experimen…
We present coarse-grained simulation results for enhanced ion diffusion in a charged nanopore grafted with ionomer sidechains. The pore surface is hydrophobic and its diameter is varied from 2.0 nm to 3.7 nm. The sidechains have from 2 to 16 monomers (united atom units) and contain sulfonate terminal groups. Our simulation results indicate a strong dependence of the ion diffusion along the pore axis on the pore parameters. In the case of short sidechains and large pores the ions mostly occupy the pore wall area, where their distribution is strongly disturbed by their host sulfonates. In the ca…
Ion transport through nanopores permeates through many areas of science and technology, from cell behavior to sensing and separation to catalysis and batteries. Two-dimensional materials, such as graphene, molybdenum disulfide (MoS$_2$), and hexagonal boron nitride (hBN), are recent additions to these fields. Low-dimensional materials present new opportunities to develop filtration, sensing, and power technologies, encompassing ion exclusion membranes, DNA sequencing, single molecule detection, osmotic power generation, and beyond. Moreover, the physics of ionic transport through pores and con…
DNA translocation through nanopores is one of the most promising strategies for the next-generation sequencing technologies. Most part of experimental and numerical works has focused on polymer translocation biased by electrophoresis, where a pulling force acts on the polymer within the nanopore. An alternative strategy however is emerging, which uses optical or magnetic tweezers. In this case, the pulling force is exerted directly at one end of the polymer, which strongly modifies the translocation process. In this paper, we report numerical simulations of both linear and structured (mimickin…
We have developed a molecular mean-field theory -- fourth-order Poisson-Nernst-Planck-Bikerman theory -- for modeling ionic and water flows in biological ion channels by treating ions and water molecules of any volume and shape with interstitial voids, polarization of water, and ion-ion and ion-water correlations. The theory can also be used to study thermodynamic and electrokinetic properties of electrolyte solutions in batteries, fuel cells, nanopores, porous media including cement, geothermal brines, the oceanic system, etc. The theory can compute electric and steric energies from all atoms…
