Session S25: Behavior of Liquids Confined on the Nanometer Scale III

Live

Developing structure-property relationships is essential for the improved design of membranes for water purification. By definition, macroscopic properties of water and ionic solutions are distinct in the confined pores of membranes. As a result, computational modeling can play an essential role in elucidating structure-property relationships. I will describe our efforts to build computational models to understand anomalous behavior of confined liquids in state-of-the-art nanoporous polyamide membranes. I will discuss how with a combination of first-principles and classical physics-based modeling, we have shown that shifts in local water dielectric constant alter local pKa of carboxyl sidechains in these materials and influence their ion selectivity[1]. I will then discuss how to identify how to improve design and understand relationships for ion-specific trends in selectivity we have built quantitative structure-property relationships using a data driven approach. We have combined a range of experimental and computational descriptors of ions and membrane materials to use as inputs to one-shot, direct feature selection machine learning (ML) models (i.e., LASSO and random forest) in order to build predictive ML models of the thermodynamic quantities observed experimentally to indicate preferential ion selectivity. This data-driven approach overcomes challenges of conventional physics-based modeling when the experimental system is sufficiently heterogeneous, making it difficult to model. Importantly, our ML models provide essential insight into the most important factors for advancing membrane materials design.

[1] Cody L. Ritt, Jay R. Werber, Mengyi Wang, Zhongyue Yang, Yumeng Zhao, Heather J. Kulik, and Menachem Elimelech "Ionization behavior of nanoporous polyamide membranes" Proc. Natl. Acad. Sci. in press.   

Live

New energy-efficient architectures inspired by the brain have been growing as an alternative to traditional von Neumann computing. Yet, existing hardware implementations use electrons as charge carriers, while neurons rely on transport of ions to carry out computations. We predict neuromorphic behaviour in a recently demonstrated two-dimensional electrolyte confined between graphite surfaces. We show that ions in the monolayer form tightly bound Bjerrum pairs that assemble into micelle-like clusters when an electric field is applied. Our model can be extended to the time-dependent case, where the slow dynamics of ionic assemblies induce memory effects in the system’s conductivity. These artificial ‘memristors’ can then be assembled to implement the Hodgkin-Huxley neuron model in a nanofluidic device capable of emitting voltage spikes trains. Our findings reveal a minimal, experimentally-accessible neuron architecture and pave the way for the development of ion-based computing and prototype ionic machines.

Reference:
P. Robin, N. Kavokine, L. Bocquet, “Artificial nanofluidic memristors and Hodgkin-Huxley dynamics in two-dimensional graphene slits”, submitted (2020).   

Live

I shall provide an overview of our research on atomic-scale channels fabricated by der Waals assembly of 2D crystals. These ultimately narrow structures can be viewed as if an individual atomic plane were extracted from a bulk crystal leaving behind a 2D empty space, essentially an angstrom-size slit connecting two edge dislocations. Water and ion transport has been studied using such 2D channels down to one atom in height, revealing interesting and sometimes completely unexpected behavior.   

Live

A molecular understanding of interfacial transport characteristics such as fluid slip and surface ionic conductivity is necessary for the design of pores exhibiting tailored functionality for applications in nanofluidics and membrane science (e.g., blue energy generation and desalination). The expression of surface interactions exhibits strong material-dependance, illustrated by the comparison of fluid slip measurements in graphitic materials (Gr)—such as carbon nanotubes and layered graphite channels—and hexagonal boron nitride (hBN); in this case, despite the nearly identical crystallographic structure of the two materials, hBN is consistently found to exhibit low fluid slip, while Gr is consistently found to exhibit nearly free slip. In this work, we examine interfacial transport characteristics in classical molecular dynamics simulations of layered Gr and hBN; our models correctly predict the equilibrium structure of the confined water and reproduce the pronounced difference in interfacial characteristics of the two materials. Our computational results allow us to elucidate the static and dynamic molecular contributions to the dramatically different characteristics of these materials, illuminating the fundamental mechanisms at play in confined interfacial transport processes.   

Live

The fluid flow in a macroscopic channel is typically determined assuming no-slip boundary conditions at the walls. Such an assumption no longer holds for nanoscale flow, and the finite flow slippage at the walls is a crucial determinant of a channel’s permeability. However, there is to date no predictive theory of the solid-liquid friction coefficient. Indeed, existing models rely on the idea of surface roughness, which is no longer relevant for atomically smooth surfaces that occur in nanofluidic devices. Particularly, one has to account for the presence of conduction electrons on the surface, which at the relevant length and time scales require the framework of many-body quantum mechanics.
We have developed a non-equilibrium field theory formalism that consistently describes the dynamics of a fluid coupled to surface conduction electrons. This description reveals a new contribution to solid-liquid friction, which results from electronic excitations in the solid, namely plasmons and electron-hole pairs. As such, we show that fluid flow at the nanoscale is impacted by intrinsically quantum properties of the confining material. Our results are of particular relevance to the water-carbon system, and provide an explanation for the peculiar slippage behavior of carbon nanotubes.   

Live

Using inelastic neutron scattering (INS) we studied dynamics of bassanite (CaSO4*0.5H2O), a structure of which has channels formed by CaO8 and CaO9 polyhedra with water molecules residing in the channels and occupying two different positions. The INS spectra showed that at low temperature (T=5 K) intramolecular O-H stretching modes of water are at high energy, around 445 meV (compared to 410 meV in ice-Ih), and the intermolecular librational band is at low energies, 35-90 meV (65-125meV in ice-Ih), indicating weak hydrogen bonds acting on water molecules. At lower energies we observed a peak at about 1 meV, which shows the behaviour of tunneling mode: its intensity decreases with temperature increase and shows nonmagnetic momentum transfer dependence. In the talk we will discuss the observed INS results and attempt to explain the tunneling of water molecule, which was not observed before in other INS experiments involved confined water molecules in the presence of hydrogen bonds.   

Live

Water is not only a substance No.1, but also a media of electromagnetic waves propagation on different scales from the molecular to the global [1]. We report anomalous high dc conductivity of interfacial water, five orders of magnitude higher than that of the bulk water, which was detected in nano-pores of diamond from 5 µm to 5 nm in diameter [2]. We also report on the experimental detection of short-lived ionic species in bulk water [3], which dramatically changes our understanding of the water dynamics at ultrashort timescale. We show that short-living ions, with concentrations of 2%, coexist with long-living pH-active ions.
References:
[1] V. G. Artemov, Phys. Chem. Chem. Phys., 21, 8067 (2019)
[2] V. G. Artemov, E. Uykur, P. O. Kapralov, A. Kiselev, K. Stevenson, H. Ouerdane, M. Dressel, J. Phys. Chem. Lett., 11, 3623-3628 (2020)
[3] V. G. Artemov, E. Uykur, S. Roh, A. Pronin, H. Ouerdane, and M. Dressel, Sci. Rep., 10, 11320 (2020)   

Live

Electrically charged liquid-solid interfaces play an important role in many electrochemical phenomena encountered in biology, energy, and the environment. The ability to probe such systems with single-ion resolution is important to a basic understanding of their behaviors and the development of new ionic technologies. Using cryo-electron microscopy, we directly visualize individual counterions and reveal their discrete interfacial layering. Comparison with simulations suggests the strong effects of finite ionic size and electrostatic interactions. Besides, we reveal correlated ionic structures under extreme confinement, with the channel widths approaching the ion diameter (~1 nm). Our experiment opens up a new arena to study liquid-solid interfaces at the single-ion level.   

Live

Ionic transport in nano- to sub-nano-scale pores is highly dependent on translocation barriers and potential wells, which are primarily the result of ion dehydration and electrostatic interactions. For pores in atomically thin membranes, such as graphene, other factors come into play due to several commensurate length scales, such as the effective membrane thickness, radii of the first and the second hydration layers, pore radius, and Debye length. In particular, for 2D biomimetic pores, there are regimes where transport is highly sensitive to the pore size due to the interplay of dehydration and interaction with pore charge. Picometer changes in the size, e.g., due to a minute strain, can lead to a large change in conductance. Outside of these regimes, the small pore size itself gives a large resistance even in a near barrierless free energy landscape. The permeability, though, can still be large and ions will translocate rapidly after they arrive within the capture radius of the pore. This, in turn, leads to bulk diffusion and drift effects dominating the conductance. Measurement of this effect will give an estimate of the magnitude of kinetically-limiting features and experimentally constrain the local electromechanical conditions.   

Live

Porous silicon provides a scaffold structure to study the confinement related effects of soft matter. We investigate the electro-sorption of electrolyte anions and the electrochemical behaviour of nanoporous silicon in acidic electrolytes. The silicon-electrolyte interface acts as a capacitor which allows the accumulation of electrolyte anions in a chemical double layer by an applied voltage, whose characteristics can be measured by cyclic voltammetry. The surface stresses that are caused to the monolithic porous silicon membrane by such an accumulation lead to a macroscopic strain which can be measured in-situ with a dilatometer.
Furthermore, we investigate the properties of a functional filling of the electrically conductive polymer polypyrrole (PPy) into nanoporus silicon. We successfully demonstrate the filling of PPy into pores of substantial depth by electro-polymerisation. In particular, PPy allows for actoric applications for the hybrid system. Polypyrrole swells when it is exposed to a voltage in an electrolyte, since ions from the electrolyte are inserted into the polymer structure and expelled subsequently. The confinement effect of the pores on the swelling can be studied with dilatometry measurements.   

Live

Nanochannels based on carbon materials have been intensively studied in the last decade because of their promising application in nanofiltration and the novel physical phenomena arising between graphitic surfaces at the nanoscale. More recently, atomically-smooth graphitic channels with sub-nm heights have been fabricated and used to investigate the physics of ions and water molecules in slits comparable to the smallest ion sizes.
In this work, we investigated ionic flow in atomically-smooth graphitic channels with height ranging from 7Å to 35nm. We show that the mobilities of ions in such confinements do not scale with hydration shell in a simple fashion, and we further explored the role of the surface charge, physical confinement and chemical interactions. Engineering ionic mobilities within the graphitic channels, we could induce strong current driven by the salinity gradient. Such osmotic power generators – driven by mobility engineering, rather than surface charge – are more resilient on the variation of chemical environment and show orders of magnitude increase in osmotic power density compared to commercial membranes.