Anomalous cation diffusion in salt-doped confined bilayer ice

Ultrahigh Lithium Selective Transport in Two-Dimensional Confined Ice

Inspired by selective ion transport in biological membrane proteins, researchers developed artificial ion channels that sieve monovalent cations, catering to the increasing lithium demand. In this work, we engineered an ion transport channel based on the confined ice within two-dimensional (2D) capillaries and found that the permselectivity of monovalent cations depends on the anisotropy of the confined ice. Particularly, the 2D confined ice showed an anomalous lithium selective transport along the (002) direction in the vermiculite capillary, with the Li+/Na+ and Li+/K+ permselectivity reaching up to 556 ± 86 and 901 ± 172, respectively, superior to most ion-selective channels. However, the 2D confined ice along the (100) direction showed less Li+ permselectivity. Additionally, the anisotropy of 2D confined ice can be tuned by adjusting the interlayer spacing. By providing insights into the ion transport in the 2D confined ice, our work may inspire more design of monovalent ion-selective channels for efficient lithium separation.

Voltage-Mediated Water Dynamics Enables On-Demand Transport of Sugar Molecules in Two-Dimensional Channels

The constructing of artificial channels with gating functions is an important undertaking for gaining insight into biological process and achieving efficient bionic functions. Typically, controllable transport within such channels relies on either electrostatic or specific interactions between the transporting species and the channel. However, for molecules with weak interactions with the channel, achieving precise gating of the transport remains a significant challenge. In this regard, this study proposes a voltage gating membrane of two-dimensional channels that selectively transport of neutral molecules glucose with a dimension of 0.60 nm. The permeation of glucose is switched on/off by electrochemically manipulating the water dynamics in the nanochannel. Voltage driven-intercalation of ion into the two-dimensional channel causes water to stratify and move closer to the channel walls, thereby resulting in the channel center being emptier for glucose diffusion. Due to the sub-nanometer size dimension of the channel, selective permeation of glucose over sucrose is also achieved in this approach.

Ion Hydration under Nanoscale Confinement: Dimensionality and Scale Effects

How ions are hydrated in nanoconfined spaces is crucial for understanding many natural phenomena and practical applications, such as biological functionalities and energy conversion devices. In real systems, nanoconfinement shows structural diversity, but the influence of dimensionality and scale on ion hydration remains considerably unrevealed. Here, we study ion hydration under various confinements by systematic molecular dynamics simulations. In a given dimension, the structure and dynamics of water molecules in the first hydration shell are altered to a degree inversely correlated with the confinement scale, as long as there is no central bulk-like region. Further comparison of ion hydration among different dimensional systems shows that this scale effect becomes more pronounced in systems with lower dimensionality, due to a more significant water layering effect and lower probability for ions to stay away from confining surfaces. These findings provide a qualitatively new understanding of ion transport in biological channels and are instrumental for the design of functional nanofluidic devices.

Anomalous Phase Behaviors of Monolayer NaCl Aqueous Solutions Induced by Effective Coulombic Interactions within Angstrom-Scale Slits

Interests in subnanofluidic devices have called for molecular dynamics (MD) simulation studies of the thermodynamic behavior of monolayer salt solution within angstrom-scale slits. However, it still remains a grand challenge to accurately describe the Coulombic interactions by incorporating the effects of charge transfer and electronic dielectric screening. Herein, by using the electronic continuum model, where the effective ion charges are fine-tuned with a scaling factor of λ, we present simulation evidence that the effective Coulombic interactions among Na⁺/Cl^- ions can strongly affect the behavior of monolayer ionic aqueous solution. Our microsecond-scale MD simulations show that only the counterions with moderate effective charges (0.3 ≤ λ ≤ 0.8) can dissolve in monolayer water, whereas the high effective charges (λ ≥ 0.85) induce ions to assemble into monolayer nanocrystals, and ions with the low effective charges (λ ≤ 0.2) exhibit gas-like nanobubble. These findings could provide deeper insights into the physical chemistry behind subnanofluidic iontronic devices.

A hydrogen bond-modulated soft nanoscale water channel for ion transport through liquid-liquid interfaces

Ion transport through interfaces is of ubiquitous importance in many fields such as electrochemistry, emulsion stabilization, phase transfer catalysis, liquid-liquid extraction and enhanced oil recovery. However, the knowledge of interfacial structures that significantly affect ion transport through liquid-liquid interfaces is still lacking due to the difficulty of observing nanoscale interfaces. We studied here the evolution of interfacial structures during ion transport through the decane-water interface under different ionic concentrations and external forces using molecular dynamics simulations. The roles of hydrogen bonds in ion transport through interfaces are revealed. We identified a soft nanoscale channel during ion transport through liquid-liquid interfaces and the decane phase under specific external force. The stability of the water channel and the ion transport velocity both increase with ionic concentration due to the layered ordering structures of the water near the channel surface. We observed that the stability and connectivity of the water channel in the decane phase are remarkably improved both by the high increase of the number of hydrogen bonds in the water channel with increasing ionic concentration, and by the conformational change in water molecules near the water channel surface. Our discovery of a soft nanoscale water channel by molecular simulations implies that there is a potential stable passage for ion transport through liquid-liquid interfaces.

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

It is well known that NaCl salt crystals can easily dissolve in dilute aqueous solutions at room temperature. Herein, we reported the first computational evidence of a novel salt nucleation behavior at room temperature, i.e., the spontaneous formation of two-dimensional (2D) alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement. Microsecond-scale classical molecular dynamics (MD) simulations showed that NaCl or LiCl, initially fully dissolved in confined water, can spontaneously nucleate into 2D monolayer nanostructures with either ordered or disordered morphologies. Notably, the NaCl nanostructures exhibited a 2D crystalline square-unit pattern, whereas the LiCl nanostructures adopted non-crystalline 2D hexagonal ring and/or zigzag chain patterns. These structural patterns appeared to be quite generic, regardless of the water and ion models used in the MD simulations. The generic patterns formed by 2D monolayer NaCl and LiCl nanostructures were also confirmed by ab initio MD simulations. The formation of 2D salt structures in dilute aqueous solution at room temperature is counterintuitive. Free energy calculations indicated that the unexpected spontaneous salt nucleation behavior can be attributed to the nanoscale confinement and strongly compressed hydration shells of ions.

Aqueous solutions under nanoscale confinement exhibit interesting physicochemical properties. This work reports evidence on the spontaneous formation of two-dimensional alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement by computer simulations.

Continuous Water Filling in a Graphene Nanochannel: A Molecular Dynamics Study

Low dimensional materials especially carbon materials hold high promise in the fields of water purification, mineral separation, energy harvesting/conversion, and so on. The fluidic devices fabricated by direct synthesis, lithography, or self-assembly of low dimensional materials provide opportunities for exploring the novel properties and applications of nanoconfined transport. Here, continuous filling of water and acetone molecules into a graphene nanochannel is investigated. A stairlike nonlinear dependence of the number of filling water molecules on interlayer distance d is found when d < 1 nm due to the existence of out-plane layered and in-plane ordered monolayer structure, while near-linear dependence is found for acetone because of the freely rotating configurations along with varying d during the filling process. The entropy, potential energy, and free energy of the confined system during the continuous filling are analyzed to understand the structural evolution of water. The energy-costs are discussed depending on the structure evolution of water during the filling, which is crucial to understanding the swelling and capillary condensation widely existing in the angstrom/nanometer-scale separation membranes.

Phase-dependent friction of nanoconfined water meniscus

A water meniscus naturally forms under ambient conditions at the point of contact between a nanoscale tip and an atomically flat substrate. Here, we study the effect of the phase state of this nanoscale meniscus-consisting of coexisting monolayer, bilayer and trilayer phase domains-on the frictional behavior during tip sliding by means of molecular dynamics simulations. While the meniscus experiences a domain-by-domain liquid-to-solid phase transition induced by lateral compression, we observe an evident transition in measured friction curves from continuous sliding to stick-slip and meanwhile a gradual increase in friction forces. Moreover, the stick-slip friction can be modulated by varying lattice orientation of the monolayer ice domain in the meniscus, choosing the sliding direction or applying in-plane strains to the substrate. Our results shed light on the rational design of high-performance micro- and nano-electromechanical systems relying on hydration lubrication.

Nano-/Micro-confined Water in Graphene Hydrogel as Superadsorbents for Water Purification

Confined water has been proven to be of great importance due to its pervasiveness and contribution to life and many fields of scientific research. However, the control and characterization of confined water are a challenge. Herein, a confined space is constructed by flexibly changing the pH of a graphene oxide dispersion under the self-assembly process of a graphene hydrogel (GH), and the confined space is adjusted with variation from 10.04 to 3.52 nm. Confined water content in GH increases when the pore diameter of the confined space decreases; the corresponding adsorption capacity increases from 243.04 to 442.91 mg g^-1. Moreover, attenuated total reflectance Fourier transform infrared spectroscopy and Raman spectroscopy are utilized to analyze the hydrogen bonding structure qualitatively and quantitatively, and correlation analysis reveals that the improvement in the adsorption capacity is caused by incomplete hydrogen bonding in the confined water. Further, confined water is assembled into four typical porous commercial adsorbents, and a remarkable enhancement of the adsorption capacity is achieved. This research demonstrates the application potential for the extraordinary properties of confined water and has implications for the development of highly effective confined water-modified adsorbents.

Graphynes for Water Desalination and Gas Separation

Selective transport of mass through membranes, so-called separation, is fundamental to many industrial applications, e.g., water desalination and gas separation. Graphynes, graphene analogs yet containing intrinsic uniformly distributed pores, are excellent candidates for highly permeable and selective membranes owing to their extreme thinness and high porosity. Graphynes exhibit computationally determined separation performance far beyond experimentally measured values of commercial state-of-the-art polyamide membranes; they also offer advantages over other atomically thin membranes like porous graphene in terms of controllability in pore geometry. Here, recent progress in proof-of-concept computational research into various graphynes for water desalination and gas separation is discussed, and their theoretically predicted outstanding permeability and selectivity are highlighted. Challenges associated with the future development of graphyne-based membranes are further analyzed, concentrating on controlled synthesis of graphyne, maintenance of high structural stability to withstand loading pressures, as well asthe demand for accurate computational characterization of separation performance. Finally, possible directions are discussed to align future efforts in order to push graphynes and other 2D material membranes toward practical separation applications.

Electrolytes under Inhomogeneous Nanoconfinement: Water Structuring-Mediated Local Ion Accumulation

The behaviors of aqueous electrolytes confined in nanoscale spaces impact a broad range of biological processes and industrial applications. Our current microscopic understanding of confined electrolytes relies primarily on ideal model systems featuring homogeneous nanoconfinement. Here, we investigate the structure and dynamics of various electrolytes subject to inhomogeneous nanoconfinement, i.e., confined in two-dimensional nanochannels with gradually varying local channel heights, by means of molecular dynamics simulations. Our results reveal unexpected local ion accumulation in the inhomogeneous space occurring at boundaries between coexisting structured water phases, including trilayer, four-layer, and bulk-like waters. This contrasts markedly with the intuition that hydrated ions are more favorable to weakly confined regimes due to a steric exclusion effect. We further show that the location and intensity of the water structuring-mediated ion accumulation are sensitive to the nanochannel's geometry and surface wettability. The revealed anomalous ion behaviors in inhomogeneous nanoconfinement should help to improve our understanding of the microscopic mechanism underlying the operation of biological ion channels and to develop functional nanofluidic devices.

Renormalization of Ionic Solvation Shells in Nanochannels

Recently, experimental studies on selective ion transport across nanoporous membranes or through single nanochannels have unveiled interesting behaviors of dissolved ions under nanoconfinement. However, the exploration was limited by the resolution of experimental characterization. In this work, we present an atomistic simulation-based study, showing how the nanoconfinement and surface functionalization of graphene and graphene oxide nanochannels renormalize the solvation of ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-). We find that the spatial distribution of dissolved ions demonstrates a layered order in nanochannels. The 1st hydration shell structures of cations are well defined in channels with width beyond ∼1.0 nm, although the rotational degree of freedom is constrained, while the 2nd hydration shells could be destructed. In the graphene oxide nanochannels, oxygen-containing functional groups can participate in the hydration shells of univalent ions but not for the divalent ions, and the valence-dependent reduction in the ionic diffusivity offers good selectivity between the divalent and univalent ions with the interlayer spacing of ∼1.0 nm, which is absent in the graphene nanochannels. With these findings, we conclude that the assessment of permeability and selectivity of ions has to take the renormalized nature of ionic solvation shells into account in the design of nanoporous membranes or nanofluidic devices for energy and environmental applications.