Abnormal Properties of Low-Dimensional Confined Water

Soft confinement of water in aliphatic alcohols: MIR/NIR spectroscopic and DFT studies

Here, we present the first examination of the state of water under a soft confinement in eight aliphatic alcohols including cyclopentanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, 2-octanol and 3-octanol. Due to relatively large size of the aliphatic part, water has limited solubility in all studied alcohols. Water content in saturated solutions was determined by Karl Fischer titration and correlated with the spectroscopic data. This way, we determined the molar absorptivity of the ν2+ν3 combination mode. The effect of addition of water and temperature variation was monitored by ATR-IR and NIR spectroscopy. Analysis of the experimental results was guided by DFT calculations, which provided the structures, harmonic MIR spectra and binding energies of selected alcohol-water complexes. Our studies demonstrated that the state of water in alcohols is related to its solubility, which depends on structure of solvent molecules. The solubility of water in 1-alcohols decreases on increasing of the chain length, but for long chain alcohols this effect is less evident. More apparent solubility reduction appears in going from the primary to secondary alcohols. The effective shielding of the OH group in the linear alcohols is achieved when on both sides of the OH group are ethyl or longer substituents, while the shielding by methyl groups is less efficient. Water is much better soluble in the cyclic alcohols as compared with the linear ones due to better accessibility of the OH group. The soft confinement of water in aliphatic alcohols allows for flexible structural arrangements and interactions. Even at low water content, we did not observe free molecules of water. At these conditions, the molecules of water are singly or doubly bonded to the OH groups from the alcohol. Increasing solubility of water reduces the number of the free OH groups and leads to formation of water clusters. Obtained results allow concluding that in alcohols with sizable aliphatic part the molecules of water are confined in the vicinity of the OH groups.

Universality in the Structure and Dynamics of Water under Lipidic Mesophase Soft Nanoconfinement

Water under soft nanoconfinement features physical and chemical properties fundamentally different from bulk water; yet, the multitude and specificity of confining systems and geometries mask any of its potentially universal traits. Here, we advance in this quest by resorting to lipidic mesophases as an ideal nanoconfinement system, allowing inspecting the behavior of water under systematic changes in the topological and geometrical properties of the confining medium, without altering the chemical nature of the interfaces. By combining Terahertz absorption spectroscopy experiments and molecular dynamics simulations, we unveil the presence of universal laws governing the physics of nanoconfined water, recapitulating the data collected at varying levels of hydration and nanoconfinement topologies. This geometry-independent universality is evidenced by the existence of master curves characterizing both the structure and dynamics of simulated water as a function of the distance from the lipid-water interface. Based on our theoretical findings, we predict a parameter-free law describing the amount of interfacial water against the structural dimension of the system (i.e., the lattice parameter), which captures both the experimental and numerical results within the same curve, without any fitting. Our results offer insight into the fundamental physics of water under soft nanoconfinement and provide a practical tool for accurately estimating the amount of nonbulk water based on structural experimental data.

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.

Confined Water Dynamics in the Scaffolds of Polylactic Acid

Resorbable polylactic acid (PLA) ultrathin fibers have been applied as scaffolds for tissue engineering applications due to their micro- and nanoporous structure that favor cell adhesion, besides inducing cell proliferation and upregulating gene expression related to tissue regeneration. Incorporation of multiwalled carbon nanotubes into PLA fibers has been reported to increase the mechanical properties of the scaffold, making them even more suitable for tissue engineering applications. Ideally, scaffolds should be degraded simultaneously with tissue growth. Hydration and swelling are factors related to scaffold degradation. Hydration would negatively impact the mechanical properties since PLA shows hydrolytic degradation. Water absorption critically affects the catalysis and allowance of the hydrolysis reactions. Moreover, either mass transport and chemical reactions are influenced by confined water, which is an unexplored subject for PLA micro- and nanoporous fibers. Here, we probe and investigate confined water onto highly porous PLA microfibers containing few amounts of incorporated carbon nanotubes by Fourier transform infrared (FTIR) spectroscopy. A hydrostatic pressure was applied to the fibers to enhance the intermolecular interactions between water molecules and C=O groups from polyester bonds, which were evaluated over the wavenumber between 1600 and 2000 cm-1. The analysis of temperature dependence of FTIR spectra indicated the presence of confined water which is characterized by a non-Arrhenius to Arrhenius crossover at T0 = 190 K for 1716 and 1817 cm-1 carbonyl bands of PLA. These bands are sensitive to a hydrogen bond network of confined water. The relevance of our finding relies on the challenge detecting confined water in hydrophobic cavities as in the PLA one. To the best of our knowledge, we present the first report referring the presence of confined water in a hydrophobic scaffold as PLA for tissue engineering. Our findings can provide new opportunities to understand the role of confined water in tissue engineering applications. For instance, we argue that PLA degradation may be affected the most by confined water. PLA degradation involves hydrolytic and enzymatic degradation reactions, which can both be sensitive to changes in water properties.

Reorientation of Hydrogen Bonds Renders Unusual Enhancement in Thermal Transport of Water in Nanoconfined Environments

Liquid confined in a nanochannel or nanotube has exhibited a superfast transport phenomenon, providing an ideal heat and mass transfer platform to meet the increasingly stringent challenge of thermal management in developing high-power-density nanoelectronics and nanochips. However, understanding the thermal transport of confined liquid is currently lacking and is speculated to be fundamentally different from that of bulk counterparts due to the unprecedented thermodynamics of liquid in nanoconfined environments. Here, we report that the thermal conductivity of water confined in a silica nanotube is nearly 2-fold as that of bulk status. Further molecular dynamics simulations reveal that this unusual enhancement originates from the densification and reorientation of local hydrogen bonds close to the nanotubes. Thermal-confinement scaling law is established and quantitatively supported by comprehensive simulations with remarkable agreement. Our findings lay a theoretical foundation for designing nanofluidics-enabled cooling strategies and devices.

Tunable Surface Wettability via Terahertz Electrowave Controlled Vicinal Subnanoscale Water Layer

Achieving timely, reversible, and long-range remote tunability over surface wettability is highly demanded across diverse fields, including nanofluidic systems, drug delivery, and heterogeneous catalysis. Herein, using molecular dynamic simulations, we show, for the first time, a theoretical design of electrowetting to achieve remotely controllable surface wettability via using a terahertz wave. The key idea driving the design is the unique terahertz collective vibration identified in the vicinal subnanoscale water layer, which is absent in bulk water, enabling efficient energy transfer from the terahertz wave to the rotational motion of the vicinal subnanoscale water layer. Consequently, a frequency-specific alternating terahertz electric field near the critical strength can significantly affect the local hydrogen-bonding network of the contact water layer on the solid surface, thereby achieving tunable surface wettability.

Geocolloidal interactions and relaxation dynamics under nanoconfinement: Effects of salinity and particle concentration

HYPOTHESIS

Energy-related contaminants are frequently associated with geocolloids that translocate in underground fissures with dimensions comparable with geocolloids. To assess the transport and impact of energy-related contaminants in geological systems, fundamental understandings of interfacial behaviors of nanoparticles under confinement is imperative. We hypothesize that the dynamic properties of geocolloids, as well as their dependence on aqueous medium conditions would deviate from bulk behaviors under nanoconfinement.

EXPERIMENTS

Force profiles and rheological properties of 50 nm silica nanoparticles in aqueous media confined between mica surfaces as a function of surface separation, particle concentrations, and salinity were measured utilizing the surface forces apparatus.

FINDINGS

Force profiles revealed the critical surface separation for nonlinear rheological behaviors coincides with the onset of exponential repulsion between mica surfaces. When salts were absent, the normal forces and viscosity values of colloidal suspensions resembled pure water. In contrast, with salts, the force profiles and corresponding critical length scales were found to be highly sensitive to the particle concentration and the degree of confinement. A Newtonian to shear-thinning transition was captured with increasing degrees of confinement. Our results show that the interplay among confinement, particle, and ionic concentrations can alter the interparticle forces and rheological responses of true nanosized-colloidal suspensions and thus their transport behaviors under nanoconfinement for the first time.

Confined water-encapsulated activated carbon for capturing short-chain perfluoroalkyl and polyfluoroalkyl substances from drinking water

The global ecological crisis of perfluoroalkyl and polyfluoroalkyl substances (PFASs) in drinking water has gradually shifted from long-chain to short-chain PFASs; however, the widespread established PFAS adsorption technology cannot cope with the impact of such hydrophilic pollutants given the inherent defects of solid-liquid mass transfer. Herein, we describe a reagent-free and low-cost strategy to reduce the energy state of short-chain PFASs in hydrophobic nanopores by employing an in situ constructed confined water structure in activated carbon (AC). Through direct (driving force) and indirect (assisted slip) effects, the confined water introduced a dual-drive mode in the confined water-encapsulated activated carbon (CW-AC) and completely eliminated the mass transfer barrier (3.27 to 5.66 kcal/mol), which caused the CW-AC to exhibit the highest adsorption capacity for various short-chain PFASs (C-F number: 3-6) among parent AC and other adsorbents reported. Meanwhile, benefiting from the chain length- and functional group-dependent confined water-binding pattern, the affinity of the CW-AC surpassed the traditional hydrophobicity dominance and shifted toward hydrophilic short-chain PFASs that easily escaped treatment. Importantly, the ability of CW-AC functionality to directly transfer to existing adsorption devices was verified, which could treat 21,000 bed volumes of environment-related high-load (~350 ng/L short-chain PFAS each) real drinking water to below the World Health Organization's standard. Overall, our results provide a green and cost-effective in situ upgrade scheme for existing adsorption devices to address the short-chain PFAS crisis.

Application of Microscopic Highly Hydrophilic Silica-Based Nanocomposites with High Surface Exposure in the Efficient Identification of Intact N-Glycopeptides

Glycosylation of proteins regulates the life activities of organisms, while abnormalities of glycosylation sites and glycan structures occur in various serious diseases such as cancer. A separation and enrichment procedure is necessary to realize the analysis of the glycoproteins/peptides by mass spectrometry, for which the surface hydrophilicity of the material is an important factor for the separation and enrichment performance. In the present work, under the premise of an obvious increase of the surface silicon exposure (79.6%), the amount of surface polar silanol is remarkably generated accompanying the introduction of the active amino groups on the surface of silica. The microscopic hydrophilicity, which is determined with water physical-adsorption measurements and can directly reflect the interaction of water molecules and the intrinsic surface of the material, maximally increases by 44%. This microscopically highly hydrophilic material shows excellent enrichment ability for glycopeptides, such as extremely low detection limits (0.01 fmol μL^-1), remarkable selectivity (1:8000), and size exclusion effects (1:8000). A total of 677 quantifiable intact N-glycopeptides were identified from the serum of patients with cervical cancer, and the glycosylation site and glycan structure were analyzed in depth, indicating that this novel material can show a broad practical application in cervical cancer diagnosis.

Toward a quasiphase transition in the single-file chain of water molecules: Simple lattice model

Recently, Ma et al. [Phys. Rev. Lett. 118, 027402 (2017)] have suggested that water molecules encapsulated in (6,5) single-wall carbon nanotube experience a temperature-induced quasiphase transition around 150 K interpreted as changes in the water dipoles orientation. We discuss further this temperature-driven quasiphase transition performing quantum chemical calculations and molecular dynamics simulations and, most importantly, suggesting a simple lattice model to reproduce the properties of the one-dimensional confined finite arrays of water molecules. The lattice model takes into account not only the short-range and long-range interactions but also the rotations in a narrow tube, and both ingredients provide an explanation for a temperature-driven orientational ordering of the water molecules, which persists within a relatively wide temperature range.

Abiotic microcompartments form when neighbouring droplets fuse: an electrochemiluminescence investigation

Many studies have shown chemistry proceeds differently in small volumes compared to bulk phases. However, few studies exist elucidating spontaneous means by which small volumes can form in Nature. Such studies are critical in understanding the formation of life in microcompartments. In this study, we track in real-time the coalescence of two or more water microdroplets adsorbed on an electrified surface in a 1,2-dichloroethane continuous phase by electrogenerated chemiluminescence (ECL) imaging, uncovering the spontaneous generation of multiple emulsions inside the resulting water droplets. During the fusion of adsorbed water droplets with each other on the electrode surface, volumes of organic and water phases are entrapped in between and detected respectively as ECL not-emitting and emitting regions. The diameter of those confined environments inside the water droplets can be less than a micrometer, as described by scanning electron microscopy data. This study adds a new mechanism for the generation of micro- and nano-emulsions and provides insight into confinement techniques under abiotic conditions as well as new potential strategies in microfluidic devices.

Molecular-Level Insights into Unique Behavior of Water Molecules Confined in the Heterojunction between One- and Two-Dimensional Nanochannels

With the increasing importance of nanoconfined water in various heterostructures, it is quite essential to clarify the influence of nanoconfinement on the unique properties of water molecules in the pivotal heterojunction. In this work, we reported a series of classical molecular dynamics (MD) simulations to explore nanoconfined water in the subnanometer-sized and nanometer-sized heterostructures by adjusting one-dimensional (1-D) carbon nanotubes with different diameters and two-dimensional (2-D) graphene sheets with different interlayer distances. Our simulation results demonstrated that water molecules in the 1-D/2-D heterojunction show an obvious structural rearrangement associated with the remarkable breaking and formation of hydrogen bonds (HBs), and such rearrangements in the subnanometer-sized systems are much more pronounced than those in the nanometer-sized ones. When water molecules in the 1-D/2-D heterojunctions migrate from 2-D to 1-D confinements, the ordered multi-layer structure in the 2-D confinement are completely destroyed and then transform into different circular HB networks near the nanotube orifice for better connecting to the single-file or helical HB network in the 1-D nanotubes. Furthermore, water molecules in the 1-D/2-D heterojunctions can form stronger HBs with those water molecules further away from the 1-D confinement, leading to an asymmetrical orientational distribution near the orifice. More importantly, our comparison results revealed that the 1-D confinement plays a more important role than the 2-D confinement in determining both the structures and dynamics of water molecules in the 1-D/2-D heterojunction.

The coming of age of water channels for separation membranes: from biological to biomimetic to synthetic

Water channels are one of the key pillars driving the development of next-generation desalination and water treatment membranes. Over the past two decades, the rise of nanotechnology has brought together an abundance of multifunctional nanochannels that are poised to reinvent separation membranes with performances exceeding those of state-of-the-art polymeric membranes within the water-energy nexus. Today, these water nanochannels can be broadly categorized into biological, biomimetic and synthetic, owing to their different natures, physicochemical properties and methods for membrane nanoarchitectonics. Furthermore, against the backdrop of different separation mechanisms, different types of nanochannel exhibit unique merits and limitations, which determine their usability and suitability for different membrane designs. Herein, this review outlines the progress of a comprehensive amount of nanochannels, which include aquaporins, pillar[5]arenes, I-quartets, different types of nanotubes and their porins, graphene-based materials, metal- and covalent-organic frameworks, porous organic cages, MoS₂, and MXenes, offering a comparative glimpse into where their potential lies. First, we map out the background by looking into the evolution of nanochannels over the years, before discussing their latest developments by focusing on the key physicochemical and intrinsic transport properties of these channels from the chemistry standpoint. Next, we put into perspective the fabrication methods that can nanoarchitecture water channels into high-performance nanochannel-enabled membranes, focusing especially on the distinct differences of each type of nanochannel and how they can be leveraged to unlock the as-promised high water transport potential in current mainstream membrane designs. Lastly, we critically evaluate recent findings to provide a holistic qualitative assessment of the nanochannels with respect to the attributes that are most strongly valued in membrane engineering, before discussing upcoming challenges to share our perspectives with researchers for pathing future directions in this coming of age of water channels.

Universal dynamical onset in water at distinct material interfaces

Interfacial water remains liquid and mobile much below 0 °C, imparting flexibility to the encapsulated materials to ensure their diverse functions at subzero temperatures. However, a united picture that can describe the dynamical differences of interfacial water on different materials and its role in imparting system-specific flexibility to distinct materials is lacking. By combining neutron spectroscopy and isotope labeling, we explored the dynamics of water and the underlying substrates independently below 0 °C across a broad range of materials. Surprisingly, while the function-related anharmonic dynamical onset in the materials exhibits diverse activation temperatures, the surface water presents a universal onset at a common temperature. Further analysis of the neutron experiment and simulation results revealed that the universal onset of water results from an intrinsic surface-independent relaxation: switching of hydrogen bonds between neighboring water molecules with a common energy barrier of ∼35 kJ mol⁻¹.

We demonstrated that the dynamical onset of interfacial water is an intrinsic property of water itself, resulting from a surface independent relaxation process in water with an approximately universal energy barrier of ∼35 kJ mol⁻¹.

Weakly Anisotropic Dielectric Properties of Water Droplets at the Nanoscale

The slab-confined water at the nanoscale exhibits anomalous dielectric properties compared to bulk water, for example, significantly low dielectric constant. In this work, we study the dielectric properties of nanoscale water droplets at room temperature using molecular dynamics simulations. We find that the nanoscale water droplets feature weakly anisotropic dielectric constant: the radial component of dielectric constants is distinctly smaller than the tangential component although they both decrease with reducing droplet size in a similar way. Such dielectric behavior is closely related to the orientational preference of water molecules near the convex surface. The molecular dipole prefers to slightly orientate toward the interior of droplets in contrast to the out-of-plane preference for free-standing water films and slab-confined water, which suppresses the fluctuation of dipole moments in the radial direction. Meanwhile, it facilitates the formation of the open hydrogen-bond network in the surface layer and ultimately leads to the relatively weak suppression of tangential fluctuations. The differential suppression is responsible for the anisotropic dielectric constant of water droplets. This anisotropic characteristic is also found in dielectric relaxation: both the radial and the tangential relaxation are consistently slowed down upon approaching surface but the latter is universally slower.