Nano-confined Water Research Articles

The Interplay between Dynamics and Structure on the Dielectric Tensor of Nanoconfined Water: Surface Charge and Salinity Effect.

Under confinement, the water dielectric constant is a second-order tensor with an abnormally low out-of-plane element. In our work, we investigate the dielectric tensor of an aqueous NaCl solution confined by a quartz slit-pore. The static dielectric constant is determined from local polarization density fluctuations via molecular dynamics simulations. In a pioneering investigation, we evaluate not only the effect of salinity but also surface charge. The parallel dielectric constant decreases with salinity due to dielectric saturation. From a dynamic perspective, the relaxation of water dipoles is slower within the hydration shells of ions. An anisotropic arrangement on the quartz surface results in preferred orientations of interfacial water molecules. By embedding charge, the surface structure changes, and extra dipole fluctuations in one direction may develop anisotropy in the parallel dielectric constant at the interface. Both surface charge and salinity increase the perpendicular dielectric constant. Nevertheless, the surface charge effect is more pronounced and may even recover the bulk dielectric constant value. The electric field established by the charged surface may disturb the planar hydrogen bond network at the interface, increasing out-of-plane dipolar fluctuations. Our work advances the knowledge of confined dielectric behavior, shedding light on the key role that charged surfaces play.

Complexity of confined water vitrification and its glass transition temperature

The ability of vitrification when crossing the glass transition temperature (Tg) of confined and bulk water is crucial for myriad phenomena in diverse fields, ranging from the cryopreservation of organs and food to the development of cryoenzymatic reactions, frost damage to buildings, and atmospheric water. However, determining water's Tg remains a major challenge. Here, we elucidate the glass transition of water by analyzing the calorimetric behavior of nano-confined water across various pore topologies (diameters: 0.3 to 2.5 nm). Our approach involves subjecting confined water to annealing protocols to identify the temperature and time evolution of nonequilibrium glass kinetics. Furthermore, we complement this calorimetric approach with the dynamics of confined water, as seen by broadband dielectric spectroscopy and linear calorimetric measurements, including the fast scanning technique. This study demonstrated that confined water undergoes a glass transition in the temperature range of 170 to 200 K, depending on the confinement size and the interaction with the confinement walls. Moreover, we also show that the thermal event observed at ~136 K must be interpreted as an annealing prepeak, also referred to as the "shadow glass transition." Calorimetric measurements also allow the detection of a specific heat step above 200 K, which is insensitive to annealing and, thereby, interpreted as a true thermodynamic transition. Finally, by connecting our results to bulk water behavior, we offer a comprehensive understanding of confined water vitrification with potential implications for numerous applications.

Solar-driven abnormal evaporation of nanoconfined water.

Intrinsic water evaporation demands a high energy input, which limits the efficacy of conventional interfacial solar evaporators. Here, we propose a nanoconfinement strategy altering inherent properties of water for solar-driven water evaporation using a highly uniform composite of vertically aligned Janus carbon nanotubes (CNTs). The water evaporation from the CNT shows the unexpected diameter-dependent evaporation rate, increasing abnormally with decreasing nanochannel diameter. The evaporation rate of CNT10@AAO evaporator thermodynamically exceeds the theoretical limit (1.47 kg m-2 hour-1 under one sun). A hybrid experimental, theoretical, and molecular simulation approach provided fundamental evidence of different nanoconfined water properties. The decreased number of H-bonds and lower interaction energy barrier of water molecules within CNT and formed water clusters may be one of the reasons for the less evaporative energy activating rapid nanoconfined water vaporization.

Molecular dynamics study of the influence of electric fields on water penetration in metal confined nanochannels

Electric field regulation of water penetration in nanochannels is an important method to address membrane fouling issues. Investigating the mechanism of electric field effects on water penetration in metal-confined nanochannels is highly meaningful. This article uses molecular dynamics methods to study the characteristics of water penetration in copper nanochannels. The influence and mechanisms of different types of applied electric fields on the water penetration within the channels were analyzed. The results indicate that applying electric fields in the y and z directions can enhance water penetration efficiency within the channels. Furthermore, a sinusoidal electric field has a greater enhancing effect on water penetration compared to a uniform electric field. Particularly, with a sinusoidal electric field frequency of 50 GHz, favorable water penetration effects were observed, with maximum increases in transport rate and axial diffusion coefficient of 197.5 % and 249.7 % respectively, relative to the absence of an electric field. The sinusoidal electric field exhibited greater capability in accelerating water molecule movement, leading to a rise in system temperature, with a maximum temperature increase of 46.9 %. These findings provide microscopic evidence for the feasibility of electric field regulation of water penetration in nanochannels, offering a theoretical foundation for enhancing nanoconfined water penetration behavior within channels.

Controllable structure phases and tunable freezing zones of water confined in multi-walled carbon nanotube capillaries

One-dimensional nanoconfined water exhibits rich structure phases. The freezing behavior of water confined in single-walled carbon nanotubes has been studied by both experimental measurements and theoretical simulations, however, it lacks of studying the freezing behavior of water confined in multi-walled carbon nanotubes (MWCNTs). Herein, we investigated the freezing behavior of water confined in MWCNTs using molecular dynamics simulations. Flat-square-walled ice nanotubes (FSW-INTs) were formed in the gaps with the inter-wall spacing of 6.66 Å, while, puckered-rhombic-walled ice nanotubes (PRW-INTs) were observed in the gaps with the inter-wall spacing of 8.61 Å. Then, the INT structures formed among the inter-wall gaps of MWCNTs can be controlled by tuning the inter-wall spacings of MWCNTs. Specifically, various groups of FSW-INTs and/or PRW-INTs were designed to form by using MWCNTs with the inter-wall spacing of 6.66 and/or 8.61 Å. In addition, it was found that water could be frozen in a simultaneous way or be frozen in a zone-by-zone form by tuning the inter-wall spacings of the MWCNTs due to the freezing temperatures of INTs are related with their diameters. The study will provide deep insights into the freezing behavior of water in MWCNTs.

Mixture Adsorption in Nanoporous Zeolite and at Its External Surface: In-Pore and Surface Selectivity.

Using toluene, ethylene, and water as gas compounds with different representative molecular interactions, we perform atom-scale simulations for their mixtures to investigate the selectivity of the core nanoporosity and external surface in a prototypical zeolite. As expected, the overall behavior suggests that increasing the pressure of a given component promotes the desorption of the coadsorbing species. However, for water-toluene mixtures, we identify that the pseudohydrogen bonding between water and toluene leads to beneficial coadsorption as toluene adsorption in the low-pressure range promotes water adsorption. Moreover, when the zeolite is completely filled with water, toluene adsorption does not occur due to steric repulsion, and ethylene shows oversolubility as the amount of ethylene per water molecule is significantly larger than in bulk water. The underlying oversolubility mechanism is found to be due to localized ethylene adsorption in the density minima arising from the layering of water in nanoconfinement. Despite these specific effects, the relatively weak coadsorption effects in the zeolite nanoporosity, which are found to be reasonably captured using the ideal adsorbed solution theory, arise from the fact that adsorption of these gases having different molecular sizes occurs in distinct pore regions (channel type, channel intersection). Finally, in contrast to confinement in the nanoporosity, mixture adsorption at the external surface does not show coadsorption effects as it mostly follows the Henry regime. These results show that selectivity is mostly governed by the confinement effects as the external surface leads to a selectivity loss.

Quantifying the effects of dissolved nitrogen and carbon dioxide on drying pressure of hydrophobic nanopores.

The effects of a dissolved gas on the behavior of liquid in cylindrical nanopores are investigated in the framework of Gibbsian composite system thermodynamics and classical nucleation theory. An equation is derived relating the phase equilibrium of a mixture of a subcritical solvent and a supercritical gas to the curvature of the liquid-vapor interface. Both the liquid and the vapor phases are treated nonideally, which is shown to be important for the accuracy of the predictions in the case of water with dissolved nitrogen or carbon dioxide. The behavior of water in nanoconfinement is found to be only affected when the gas amount is significantly more than the saturation concentration of these gases at atmospheric conditions. However, such concentrations can be easily reached at high pressures during intrusion if there is sufficient gas present in the system, especially considering gas oversolubility in confinement. By including an adjustable line tension term in the free energy equation (-44pJ/m for all points), the theory can make predictions in line with the few data points available from recent experimental work. However, we note that such a fitted value empirically accounts for multiple effects and should not be interpreted as the energy of the three-phase contact line. Compared to molecular dynamics simulations, our method is easy to implement, requires minimal computational resources, and is not limited to small pore sizes and/or short simulation times. It provides an efficient path for first-order estimation of the metastability limit of water-gas solutions in nanopores.

Temporal nanofluid environments induce prebiotic condensation in water

Water is a problem in understanding chemical evolution towards life’s origins on Earth. Although all known life is being based on water key prebiotic reactions are inhibited by it. The prebiotic plausibility of current strategies to circumvent this paradox is questionable regarding the principle that evolution builds on existing pathways. Here, we report a straightforward way to overcome the water paradox in line with evolutionary conservatism. By utilising a molecular deposition method as a physicochemical probe, we uncovered a synergy between biomolecule assembly and temporal nanofluid conditions that emerge within transient nanoconfinements of water between suspended particles. Results from fluorometry, quantitative PCR, melting curve analysis, gel electrophoresis and computational modelling reveal that such conditions induce nonenzymatic polymerisation of nucleotides and promote basic cooperation between nucleotides and amino acids for RNA formation. Aqueous particle suspensions are a geochemical ubiquitous and thus prebiotic highly plausible setting. Harnessing nanofluid conditions in this setting for prebiotic syntheses is consistent with evolutionary conservatism, as living cells also work with temporal nanoconfined water for biosynthesis. Our findings add key insights required to understand the transition from geochemistry to biochemistry and open up systematic pathways to water-based green chemistry approaches in materials science and nanotechnology.

Effect of Internal Pressure on Incipient Ferroelectricity of Nanoconfined Water Molecules Observed in Hydrothermally Grown Beryl Crystals

Quasistatic dielectric permittivity of D2O type I molecules (electric dipole moment perpendicular to the crystallographic c‐axis) within hydrothermally grown beryl crystals characterized by different internal pressure and content of D2O type II molecules (dipole moment parallel to the c‐axis) is measured at temperatures 4–300 K. All crystals are found to display quantum paraelectric behavior of the D2O‐I molecular subsystem permittivity, that is, permittivity growth while cooling from room temperature followed by saturation below 15–40 K. Processing the data with the Barrett expression shows that excess internal pressure and excess content of D2O‐II molecules lead to an increase in quantum temperature T1 and a decrease in the Curie constant C, with the Curie temperature TC remaining unchanged. The discovered strengthening of quantum effects (growth of T1) within an ensemble of dipole–dipole‐coupled D2O‐I molecules is associated with an enhanced azimuthal tunneling of these molecules within the hexagonal localizing potential. The data indicate the possibility of using crystal growth conditions to “tune” the strength of quantum effects in the network of polar water molecules, which provides a workbench for further studies of exotic phases of a lattice of coupled “point” electric dipoles.

The first-principles phase diagram of monolayer nanoconfined water.

Water in nanoscale cavities is ubiquitous and of central importance to everyday phenomena in geology and biology. However, the properties of nanoscale water can be substantially different fromthose of bulk water, as shown, for example, by the anomalously low dielectric constant of water in nanochannels1, near frictionless water flow2 or the possible existence of a square ice phase3. Such properties suggest that nanoconfined water could be engineered for technological applications in nanofluidics4, electrolyte materials5 and water desalination6. Unfortunately, challenges in experimentally characterizing water at the nanoscale and the high cost of first-principles simulations have prevented the molecular-level understanding required to control the behaviour of water. Here we combine a range of computational approaches to enable a first-principles-level investigation of a single layer of water within a graphene-like channel. We find that monolayer water exhibits surprisinglyrich and diverse phase behaviour that is highly sensitive to temperature and the van der Waals pressure acting within the nanochannel. In addition to multiple molecular phases with melting temperatures varying non-monotonically by more than 400kelvins with pressure, we predict a hexatic phase, which is an intermediate between a solid and a liquid, and a superionic phase with a high electrical conductivity exceeding that of battery materials. Notably, this suggests that nanoconfinement could be a promising route towards superionic behaviour under easily accessible conditions.

Interfacial thermal resistance between nanoconfined water and silicon: Impact of temperature and silicon phase

Molecular dynamics simulations are used to investigate the interfacial thermal resistance (Kapitza resistance) between crystalline or amorphous silicon and nanoconfined water at nanoscale. The simulations are performed under various conditions such as: different silicon phases (crystalline or amorphous), various water slab thicknesses, average system temperature and temperature difference between the thermostats. The results indicate that the Kapitza resistance is larger between crystalline silicon slabs and water (≈1.2 10−8 K m2 W−1) than between amorphous silicon slabs and water (≈0.7 10−8 K m2 W−1), which can be interpreted as a density effect using the acoustic mismatch model. We have not observed significant size effects related to the water slab thickness on the Kapitza resistance nor on the thermal conductivity of the nanoconfined water. Furthermore, the interfacial thermal resistance is linearly impacted by temperature unless the temperature difference between the thermostats is larger than 50 K. The presented results provide new insights in nano heat transfer in presence of a solid/liquid interface.

A Deep Neural Network Potential for Water Confined in Graphene Nanocapillaries

Water under nanoconfinement exhibits structural and dynamical properties remarkably different from those of bulk water, and empirical potential-based molecular dynamics has played an important role in furthering our understanding of the behavior of confined water. However, existing potentials for water were commonly parametrized based on the properties of the bulk water and may be unreliable for describing nanoconfined water. Here, we develop a machine learning potential for water confined in graphene nanocapillaries using deep neural networks trained on quantum mechanical density-functional theory (DFT) calculations. This deep neural network potential offers near-DFT accuracy in terms of potential energy and atomic forces but at a computational cost much lower than DFT-based ab initio molecular dynamics methods. In particular, this potential reproduces well the DFT reference for a wide range of properties, including O–H bond length distribution, density distribution, radial distribution functions, hydrogen bonding, etc. The developed deep neural network potential opens the door to simulations of nanoconfined water with large system sizes and time scales at near-DFT accuracy.

Correlation lengths in nanoconfined water and transport properties.

We report the existence of disparate static and dynamic correlation lengths that could describe the influence of confinement on nanoconfined water (NCW). Various aspects of viscous properties, such as anisotropy and viscoelasticity, of NCW are studied by varying the separation distance "d" between two confining hydrophobic plates. The transverse component of the mean square stress exhibits slow spatial decay (measured from the surface) beyond ∼1.8nm, which was not reported before. The static correlation length obtained from fitting the exponential decay of the transverse mean-square stress with d is 0.75nm, while the decay time of the stress-stress time correlation function gives a dynamic correlation length of only 0.35nm. The shortness of the dynamic correlation length seems to arise from the low sensitivity of orientational relaxation to confinement. In the frequency-dependent viscosity, we observe a new peak at about 50cm-1 that is not present in the bulk. This new peak is prominent even at 3nm separations. The peak is absent in the bulk, although it is close to the intermolecular -O-O-O- bending mode well known in liquid water. We further explore the relationship between diffusion and viscosity in NCW by varying d.

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below T = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(T). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')-1 down to T ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO3, KTaO3, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between T = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled point electric dipoles.

High Energy and Power Density Peptidoglycan Muscles through Super-Viscous Nanoconfined Water.

Water‐responsive (WR) materials that reversibly deform in response to humidity changes show great potential for developing muscle‐like actuators for miniature and biomimetic robotics. Here, it is presented that Bacillus (B.) subtilis’ peptidoglycan (PG) exhibits WR actuation energy and power densities reaching 72.6 MJ m−3 and 9.1 MW m−3, respectively, orders of magnitude higher than those of frequently used actuators, such as piezoelectric actuators and dielectric elastomers. PG can deform as much as 27.2% within 110 ms, and its actuation pressure reaches ≈354.6 MPa. Surprisingly, PG exhibits an energy conversion efficiency of ≈66.8%, which can be attributed to its super‐viscous nanoconfined water that efficiently translates the movement of water molecules to PG's mechanical deformation. Using PG, WR composites that can be integrated into a range of engineering structures are developed, including a robotic gripper and linear actuators, which illustrate the possibilities of using PG as building blocks for high‐efficiency WR actuators.

Optimal nanocone geometry for water flow

AbstractThe pursuit, toward transport efficiency, is significantly necessary for energy conversion, water filtration. However, structure design, aiming at further enhancing nanoconfined water flow, is still lacking. With the motivation to bridge the knowledge gap, a simple yet practical model regarding the nanocone structure design is established. This research demonstrates that nanocone, with desirable opening angle and length, possesses the capacity to achieve the optimal flow behavior. Flow resistance occurring inside nanocones, and that at cone entrance, exit, are considered. Optimal nanocone geometry can be determined based on the minimization of total resistance. Results show that (a) suitable opening angle spans from 10° to 30° over a wide range of nanocone geometry; (b) evident decline tendency of the suitable opening angle toward the increasing surface wettability is captured; and (c) water transport capacity inside optimal nanocone is 4–50 times that within cylindrical nanopores. This article forms a theoretical framework for nanocone design.

Tuning the Dielectric Response of Water in Nanoconfinement through Surface Wettability.

The tunable polarity of water can be exploited in emerging technologies including catalysis, gas storage, and green chemistry. Recent experimental and theoretical studies have shown that water can be rendered into an effectively apolar solvent under nanoconfinement. We furthermore demonstrate, through molecular simulations, that the static dielectric constant of water can be modified by changing the wettability of the confining material. We find the out-of-plane dielectric response to be highly sensitive to the level of confinement and can be reduced up to 40× , in accordance with experimental data. By altering the surface wettability from superhydrophilic to superhydrophobic, we observe a 36% increase for the out-of-plane and a 31% decrease for the in-plane dielectric constants. Our findings demonstrate the feasibility of tunable water polarity, a phenomenon with great potential for scientific and technological impact.

Impedance of Thermal Conduction from Nanoconfined Water in Carbon Nanotube Single-Digit Nanopores

There has been recent interest in understanding the transport of nanoconfined fluids through single-digit nanopores (SDNs) or those smaller than 10 nm in diameters, where confinement alters the flu...

Extremely High Proton Mobility in Nanoscopic Titania Hydrates

Using protons as the ionic charge carriers in energy conversion and storage devices is an attractive prospect, but designing electrode/electrolyte materials with fast proton mobility at room temperature is still a challenge. Here we report on two novel titania hydrates with high proton mobility: one is one-dimensional H2Ti5O11·H2O material with ~2 µm length and ~150 nm width; the other is zero-dimensional TiO2·0.4H2O material with average dimension ~5 nm. To gain a deeper understanding of the correlation between materials local structure and fast proton transport mechanisms, extended X-ray absorption fine structure, high resolution transmission electron microscopy, and nuclear magnetic resonance pulsed field gradient diffusion measurements were conducted. The H-bonded species are identified with nano-confined water located between the TiO6 octahedra layered structures in H2Ti5O11·H2O and on the surface of TiO6 octahedra in TiO2·0.4H2O. The proton self-diffusion coefficients at room temperature are about 4×10−9 m2 s−1 for both H2Ti5O11·H2O and TiO2·0.4H2O, which exceed that in liquid water at ambient temperature by nearly a factor of two. These values decrease to about 1×10−9 m2 s−1 at −25 oC in both materials. These unusually high diffusion coefficients imply a Grotthuss transport mechanism and suggest attractive possible application in proton batteries or resistive switching. Further, this materials protocol can create new opportunities in designing other proton-based electrodes and electrolytes with nano-confined water as the fast proton carrier in solids.

Heterogeneous Microscopic Dynamics of Intruded Water in a Superhydrophobic Nanoconfinement: Neutron Scattering and Molecular Modeling.

With their strong confining porosity and versatile surface chemistry, zeolitic imidazolate frameworks-including the prototypical ZIF-8-display exceptional properties for various applications. In particular, the forced intrusion of water at high pressure (∼25 MPa) into ZIF-8 nanopores is of interest for energy storage. Such a system reveals also ideal to study experimentally water dynamics and thermodynamics in an ultrahydrophobic confinement. Here, we report on neutron scattering experiments to probe the molecular dynamics of water within ZIF-8 nanopores under high pressure up to 38 MPa. In addition to an overall confinement-induced slowing down, we provide evidence for strong dynamical heterogeneities with different underlying molecular dynamics. Using complementary molecular simulations, these heterogeneities are found to correspond to different microscopic mechanisms inherent to vicinal molecules located in strongly adsorbing sites (ligands) and other molecules nanoconfined in the cavity center. These findings unveil a complex microscopic dynamics, which results from the combination of surface residence times and exchanges between the cavity surface and center.