Confined Water Molecules Research Articles

Molecular dynamics simulation of carbon peapod-like nanomaterials in desalination process

In this research, the performance of carbon peapod-like nanomaterials consisting 1 and 3 fullerenes into (13,13) armchair carbon nanotube (CNT) has been investigated in desalination process using molecular dynamics (MD) simulations with the applied pressure of 350 MPa. We have also examined different charge effects on the fullerene, nanotube, and graphene surfaces in this work. We have calculated different properties including water and ion flux, self-diffusion coefficient, radial distribution function (RDF), hydrogen bonding (HB) analysis, potential of mean force (PMF), and barrier energy of penetration. Our results showed that the highest and lowest water fluxes are for the pure CNT (2565 ns−1) and reverse charged fullerene peapod (270 ns−1) systems, respectively. The pure CNT also showed the highest ion flux (26 ns−1) than the other systems. Presence of fullerene into nanotube decreased the ion flux (6.5 ns−1) which represents acceptable performance of the peapods in salt rejection during desalination process. Using of N-doped fullerenes has small effect and increased the ion rejection more than the pure fullerenes. Our results also indicated that using of charged plates, CNTs, and peapods leads to complete salt rejection (zero ion flux). The pure CNT has the smallest barrier energy whereas the reverse charged CNT and the reverse charged plates have the biggest barrier energy required to water molecules to penetrate into the nanotube. Presence of fullerenes into the CNT also changes the orientations of the confined water molecules.

Structure, dynamics, and morphology of nanostructured water confined between parallel graphene surfaces and in carbon nanotubes by applying magnetic and electric fields.

Water molecules experience certain changes in their properties when they feel an external magnetic or electric field. These changes are significant in different applications, such as biological and biotechnological processes, nano-pumping, and water treatment. In this work, we have performed molecular dynamics (MD) simulations to investigate the different thermodynamics, structure, and dynamics of water molecules confined between two parallel surfaces and also confined in carbon nanotubes (CNTs). We have also applied different electric and magnetic fields in different directions to the confined molecules. In the graphene system, no polygonal shape was formed in either low or high electric fields, whereas rhombic and pentagonal shapes were formed in low and high magnetic fields. In the CNT system, applying electric fields in all three dimensions made the pentagonal shape disappear and the confined water molecules formed a ring shape when the electric field was applied in the axial direction. Applying the electric field perpendicular to the graphene surfaces increases the self-diffusion of the confined molecules, whereas applying the electric and magnetic fields along the CNT axis decreases the self-diffusion of the confined water molecules. In the graphene system, applying the electric field perpendicular to the graphene surfaces decreases the average number of hydrogen bonds (〈HB〉) whereas the magnetic field has little effect on the 〈HB〉. In the CNT system, applying Ex also leads to a smaller number of HBs. Also, applying the magnetic field along the x-direction (along the CNT direction) leads to a greater number of HBs than the other fields.

Self-assembled rhomboidal ammonia monolayer confined in two vertically stacked graphene oxide/graphene nanosheets.

Confined water molecules have attracted widespread research interest due to their versatile phase behaviors. Ammonia (NH3, isoelectronic with water) molecules are also expected to realize the delicate self-assembled hydrogen-bonded network like water in confinement. Here, the structures and phase behavior of NH3 monolayers confined in two structurally symmetrical graphene oxide (GO) or graphene (G) nanosheets are investigated using first-principles calculations and ab initio molecular dynamics simulations. A highly ordered new rhomboidal phase with all NH3 molecules adopting a Y-shaped configuration, in which one N-H bond is parallel to the confining planes and two other N-H bonds point to the top/bottom GO/G layers, respectively, was discovered at low temperature, resulting from the symmetrical confinement and subtle interlayer/intermolecular interactions. Remarkably, this new phase is so stable that a quite large strain is needed to destroy it. At room temperature, these NH3 monolayers behave like a liquid. These rhomboidal NH3 monolayers confined in GO/G nanosheets not only offer diverse hydrogen-bonded networks but also possess potential piezoelectricity for future device applications.

Confined Water Molecules in Binary Mixtures of Water and 2,6-Lutidine Near Lower Solution Critical Temperature

We study the concentration and temperature dependence of the reorientation dynamics of water molecules in binary mixtures of water and 2,6-lutidine below the lower solution critical temperature (LSCT) with femtosecond mid-infrared pump–probe spectroscopy. The measurements reveal the presence of water molecules interacting with both the hydrophobic groups of lutidine and forming a hydrogen bond with the nitrogen atom of lutidine. Both types of molecules show a strongly decreased rotational mobility in comparison to bulk water. From the temperature dependence of the slow water fraction, we conclude that the lutidine molecules form clusters that decrease in size when the temperature is decreased further below the LSCT.

Water accelerated self-healing of hydrophobic copolymers

Previous studies have shown that copolymer compositions can significantly impact self-healing properties. This was accomplished by enhancement of van der Waals (vdW) forces which facilitate self-healing in relatively narrow copolymer compositional range. In this work we report the acceleration of self-healing in alternating/random hydrophobic acrylic-based copolymers in the presence of confined water molecules. Under these conditions competing vdW interactions do not allow H2O-diester H-bonding, thus forcing nBA side groups to adapt L-shape conformations, generating stronger dipole-dipole interactions resulting in shorter inter-chain distances compared to ‘key-and-lock’ associations without water. The perturbation of vdW forces upon mechanical damage in the presence of controllable amount of confined water is energetically unfavorable leading the enhancement of self-healing efficiency of hydrophobic copolymers by a factor of three. The concept may be applicable to other self-healing mechanisms involving reversible covalent bonding, supramolecular chemistry, or polymers with phase-separated morphologies.

Pre-protection strategy performed in processable aqueous dispersions of TiS2 nanosheets to fabricate laminar films with improved electric properties

Processable aqueous dispersions of two-dimensional (2D) transition metal dichalcogenides (TMDs) nanosheets show extraordinary promise in assembling laminar films for a variety of applications but are usually limited by the oxidation of adsorbed water molecular on the edge and defect regions, which damage the stability of the colloidal solution and electric properties of the resulted films. In this work, we developed a pre-protection strategy by adding ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) as the protective agent into TiS2 nanosheets aqueous dispersions. The introduction of EDTA-2Na can effectively slow down the adsorption and decomposition of confined water molecules and enhance the oxidation resistance of TiS2 nanosheets in the colloidal solution. Compared with the untreated TiS2 nanosheets, the storage time of the pre-protected nanosheets in aqueous dispersions can be significantly improved, and the electrical conductivity of prepared laminar films as thermoelectric materials also increased from 2.43 × 103 S m−1 to 1.51 × 104 S m−1. Our exploration provides a new concept performed in aqueous solution for the protection of TMDs nanosheets, which can be adopted to fabricate TMDs films with stable electric properties by controlling the assembly process of colloidal chemistry

Electrochemically Desulfurized Molybdenum Disulfide (MoS₂) and Reduced Graphene Oxide Aerogel Composites as Efficient Electrocatalysts for Hydrogen Evolution.

Recent developments in graphene related materials including molybdenum disulfide (MoS₂) is gaining popularity as efficient and cost-effective nanoscale electrocatalyst essential for hydrogen production. These "clean" energy technologies require delicate control over geometric, morphological, chemical and electronic structure affecting physical and electrochemical catalytic properties. In this work, we prepared three-dimensional hierarchical mesoporous aerogels consisting of two-dimensional functionalized graphene and MoS₂ nanosheets of varying ratio of components under hydrothermal-solvothermal conditions (P <20 bar, T <200 °C). We systematically characterized these hybrid aerogels in terms of surface morphology, microstructure, understand heterointerfaces interaction through electron microscopy, X-ray diffraction, optical absorption and emission and Raman spectroscopy, besides electrochemical properties prior to and post electrochemical desulfurization that induces finely controlled sulfur vacancies. They feature enhanced electrical conductivity by means of eliminating contact resistance and meso-/nanoporous structure facilitating faster ion diffusion (mass transport). We demonstrate that controlled defects density, edges plane sites (nanowalls), mesoscale porosity and topological interconnectedness (monolithic aerogel sheets) invoked can accelerate electrocatalytic hydrogen production. For instance, low over potential with Tafel slope ~77 mV·dec-1 for 60 wt.% MoS₂, highcurrent density, and good stability was achieved with desulfurization. These results are compared with continuous multilayer MoS₂ films highlighting the multiple role of tunable structure and electronic properties. The adjacent S-vacancy defectsinduced increase in density of states, dissociation and confinement of water molecules at the pore edge and planar S-vacancy sites calculated using density functional theory helped in establishing improved heterogeneous electrocatalytic rate. This is supported with combined measurements of diffusion coefficient and heterogeneous electron transfer rate via surface-sensitive scanning electrochemical microscopy (SECM) technique.

A Multi-Scale Study of Water Dynamics under Confinement, Exploiting Numerical Simulations in Relation to NMR Relaxometry, PGSE and NMR Micro-Imaging Experiments: An Application to the Clay/Water Interface.

Water mobility within the porous network of dense clay sediments was investigated over a broad dynamical range by using 2H nuclear magnetic resonance spectroscopy. Multi-quanta 2H NMR spectroscopy and relaxation measurements were first performed to identify the contributions of the various relaxation mechanisms monitoring the time evolution of the nuclear magnetisation of the confined heavy water. Secondly, multi-quanta spin-locking NMR relaxation measurements were then performed over a broad frequency domain, probing the mobility of the confined water molecules on a time-scale varying between microseconds and milliseconds. Thirdly, 1H NMR pulsed-gradient spin-echo attenuation experiments were performed to quantify water mobility on a time-scale limited by the NMR transverse relaxation time of the confined NMR probe, typically a few milliseconds. Fourthly, the long living quantum state of the magnetisation of quadrupolar nuclei was exploited to probe a two-time correlation function at a time-scale reaching one second. Finally, magnetic resonance imaging measurements allow probing the same dynamical process on time-scales varying between seconds and several hours. In that context, multi-scale modelling is required to interpret these NMR measurements and extract information on the influences of the structural properties of the porous network on the apparent mobility of the diffusing water molecules. That dual experimental and numerical approach appears generalizable to a large variety of porous networks, including zeolites, micelles and synthetic or biological membranes.

How different are the dynamics of nanoconfined water?

We unravel the combined effects of confinement and surface interactions by studying the position dependent, time-resolved dynamic response functions in nano-containers of different shapes. Spectroscopic signatures are additionally studied through solvation dynamics by placing ionic and dipolar probes at varying distances from the enclosing surface. We find that the confined water molecules exhibit exotic dynamical features and stark differences from that in the bulk liquid. We employ atomistic molecular dynamics simulation to obtain the solvation time correlation function, non-Gaussian parameter, and non-linear response function that reveal the existence of heterogeneous and non-exponential dynamics with a strong sensitivity to both the size and the shape of the enclosure. Importantly, the slower long-time decay constant exhibits a non-monotonic spatial dependence. The initial ultrafast component is reminiscent of the same in the bulk, but it is found to have a different origin in the present systems. We perform shell-wise analyses to understand the microscopic origin of these observations and the range of the propagation of the surface induced effects.

Entropy and Random Walk Trails Water Confinement and Non-Thermal Equilibrium in Photon-Induced Nanocavities.

Molecules near surfaces are regularly trapped in small cavitations. Molecular confinement, especially water confinement, shows intriguing and unexpected behavior including surface entropy adjustment; nevertheless, observations of entropic variation during molecular confinement are scarce. An experimental assessment of the correlation between surface strain and entropy during molecular confinement in tiny crevices is difficult because strain variances fall in the nanometer scale. In this work, entropic variations during water confinement in 2D nano/micro cavitations were observed. Experimental results and random walk simulations of water molecules inside different size nanocavitations show that the mean escaping time of molecular water from nanocavities largely deviates from the mean collision time of water molecules near surfaces, crafted by 157 nm vacuum ultraviolet laser light on polyacrylamide matrixes. The mean escape time distribution of a few molecules indicates a non-thermal equilibrium state inside the cavity. The time differentiation inside and outside nanocavities reveals an additional state of ordered arrangements between nanocavities and molecular water ensembles of fixed molecular length near the surface. The configured number of microstates correctly counts for the experimental surface entropy deviation during molecular water confinement. The methodology has the potential to identify confined water molecules in nanocavities with life science importance.

Molecular Dynamics Study of Water Diffusivity in Graphene Nanochannels

Transport properties of nano-confined fluids such as diffusivity can exhibit utterly distinctive characteristics compared to the transport properties in the bulk due to the interactions between atoms in the solid walls and fluid atoms as well as the confinements. In this paper, the diffusivity of water confined in the graphene nanochannels is calculated by molecular dynamics simulations through the Einstein equation, and the results show that the diffusivity of nano-confined water is obviously anisotropic, i.e., the perpendicular (vertical to the graphene walls) diffusivity is obviously lower than the diffusivity in the parallel plane. By studying the Lagrangian dynamics of molecules in the confined region, we realize that the anisotropy can be attributed to the trapping of water molecules in the potential wells near the graphene walls, resulting in the inhibition of the molecular mobility in the perpendicular direction. Meanwhile, the proportion of confined water molecules decreases with increasing channel height and the contributions of the trapped water molecules on the inhibited mobility in the perpendicular direction are weakened. As a result, the diffusivity in all directions approaches the bulk values at high channel heights. The obtained results are helpful in revealing the mechanisms of water diffusion in nanospaces from the molecular level.

Dynamical control by water at a molecular level in protein dimer association and dissociation

Water, often termed as the "lubricant of life," is expected to play an active role in navigating protein dissociation-association reactions. In order to unearth the molecular details, we first compute the free-energy surface (FES) of insulin dimer dissociation employing metadynamics simulation, and then carry out analyses of insulin dimerization and dissociation using atomistic molecular-dynamics simulation in explicit water. We select two sets of initial configurations from 1) the dissociated state and 2) the transition state, and follow time evolution using several long trajectories (∼1-2 μs). During the process we not only monitor configuration of protein monomers, but also the properties of water. Although the equilibrium structural properties of water between the two monomers approach bulklike characteristics at a separation distance of ∼5 nm, the dynamics differ considerably. The complex association process is observed to be accompanied by several structural and dynamical changes of the system, such as large-scale correlated water density fluctuations, coupled conformational fluctuation of protein monomers, a dewettinglike transition with the change of intermonomeric distance RMM from ∼4 to ∼2 nm, orientation of monomers and hydrophobic hydration in the monomers. A quasistable, solvent-shared, protein monomer pair (SSPMP) forms at around 2 nm during association process which is a local free-energy minimum having ∼50-60% of native contacts. Simulations starting with arrangements sampled from the transition state (TS) of the dimer dissociation reveal that the final outcome depends on relative orientation of the backbone in the "hotspot" region.

Enhanced methane gas storage in the form of hydrates: role of the confined water molecules in silica powders.

Methane hydrates are promising materials for storage and transportation of natural gas; however, the slow kinetics and inefficient water to hydrate conversions impede its broad scale utilisation. The purpose of the present study is to demonstrate rapid (2–3 h) and efficient methane hydrate conversions by utilising the water molecules confined in the intra- and inter-granular space of silica powders. All the experiments were conducted with amorphous silica (10 g) powders of 2–30 μm; 10–20 nm grain size, to mimic the hydrate formations in fine sand and clay dominated environments under moderate methane pressure (7–8 MPa). Encasing of methane molecules in hydrate cages was confirmed by Raman spectroscopic (ex situ) and thermodynamic phase boundary measurements. The present studies reveal that the water to hydrate conversion is relatively slower in 10–20 nm grain size silica, although the nucleation event is rapid in both silicas. The process of hydrate conversion is vastly diffusion-controlled, and this was distinctly observed during the hydrate growth in nanosize silica.

Effect of nano-confinement on the structure and properties of water clusters: An ab initio study

An ab initio investigation on water clusters confined to armchair carbon nanotubes (CNT) with varying diameters has been performed using the density functional theory-based calculations. Different parameters have been investigated including structure, hydrogen bonding pattern and vibrational spectra of water-CNT complexes. Our results reveal that one-dimensional water chain parallel to CNT axis is formed in narrow nanotubes CNT(4,4) and CNT(5,5), whereas in CNT(6,6), zigzag structure is observed. An increase in the CNT diameter results in more symmetric structures similar to the gas phase. The vibrational analysis shows a redshift in stretching frequency of the hydrogen bond assisted O–H in CNT(6,6) due to the reduction in O—O separation whereas a significant blue shift in stretching frequency mode is observed in highly confined CNT(4,4) and CNT(5,5). It implies that the hydrogen bond strength between water molecules is strongest in CNT(6,6). It is also observed that water cluster tends to be near CNT wall due to H···π interaction between water molecule and the π-electron cloud of CNT. An inverse relation between the electronic charge transfer (from CNT to water) and the diameter is also established. This study demonstrates that the degree of confinement is extremely important in deciding the properties of confined water molecules. The structure and hydrogen bonding properties of the water clusters under nano-confined regime is investigated and the results reveal that the intramolecular charge separation for each water molecule increases under confinement and irrespective of the number of water molecules, the highest tube-water interaction energy is achieved in CNT(5,5) (diameter~7 A). The important finding is that the degree of confinement (diameter of carbon nanotube) is extremely important in deciding the properties of confined water molecules.

Fluorocarbon-Based Ionomers with Single Acid and Multiacid Side Chains at Nanothin Interfaces

Unraveling ion conduction limitations in nanothin ionomer films is crucial for designing efficient ionomer–catalyst interfaces and improving redox efficiency in electrochemical devices. This work took a multifaceted approach to understand local proton conduction environments in sub-μm thick films of three fluorocarbon-based ionomers, Nafion, 3M PFSA and 3M PFIA with IEC ∼ 0.91, 1.21, and 1.61 mequiv/g, respectively. After incorporating fluorescent photoacid probe pyranine (HPTS) into films, the extent of proton conduction (Id/Ip), local proton concentration, pH, and ionic domain size (did) were predicted by monitoring the ratio of fluorescence intensity of deprotonated (Id) to that of the protonated (Ip) state of HPTS. Id/Ip decreased with film thickness and followed the trend: 3M PFIA > 3M PFSA > Nafion. A higher water uptake did not necessarily lead to higher Id/Ip indicating that other factors than water uptake control proton conduction under confinement. Size of the ionic domains (did), measured independently using in-plane reflection small-angle X-ray scattering and fluorescence spectroscopy, followed the same trend as Id/Ip. As the RH and film thickness decreased, did became smaller. The close match of did obtained from both techniques supported the reliability of information confered by fluorescence spectroscopy about key controlling parameters of the local proton conduction environment. The highest Id/Ip of 3M PFIA films was attributed to its flexible, multiacidic side chain that helped to form larger ionic domains with better phase segregation. Conversely, smaller, extremely acidic and poorly phase segregated ionic domains with highly confined water molecules led to lower Id/Ip in Nafion films, despite high water uptake.

Moisture-induced crossover in the thermodynamic and mechanical response of hydrophilic biopolymer

The use of natural sustainable resources such as wood in green industrial processes is currently limited by our poor understanding of the impact of moisture on their thermodynamic and mechanical behaviors. Here, a molecular dynamics approach is used to investigate the physical response of a typical hydrophilic biopolymer in softwood hemicellulose—xylan—when subjected to moisture adsorption. A unique moisture-induced crossover is found in the thermodynamic and mechanical properties of this prototypical biopolymer with many quantities such as the heat of adsorption, heat capacity, thermal expansion and elastic moduli exhibiting a marked evolution change for a moisture content about 30 wt%. By investigating the microscopic structure of the confined water molecules and the polymer–water interfacial area, the molecular mechanism responsible for this crossover is shown to correspond to the formation of a double-layer adsorbed film along the amorphous polymeric chains. In addition to this moisture-induced crossover, many properties of the hydrated biopolymer are found to obey simple material models.

Extending the Range of Controlling Protein Adsorption via Subsurface Architecture.

Recently, it has been shown that water, confined in a plasma polymer subsurface chemical gradient, nanometers below the surface, significantly reduced the amount of adsorbed protein bovine serum albumin (BSA). Relating to this effect, we proposed the hypothesis that oriented water molecules within the subsurface gradient generate a long-range dipolar field, which interacts with dipolar proteins such as BSA near the surface region. This study extends the above used in situ multistep plasma deposition process to introduce plasma oxidation modifications of the subsurface architecture with the aim to further control the effect on protein adsorption. Neutron reflectivity measurements reveal that the oxidation time increases the amount of matrix-confined water. There is, however, an optimal oxidation time to obtain minimal protein adsorption, which suggests that a minimal distance between confined water molecules plays an important role. Altogether we can extend the range of controlling the adsorbed protein mass by the introduction of this additional plasma oxidation step.

Mechanism of Water Dynamics in Hyaluronic Dermal Fillers Revealed by Nuclear Magnetic Resonance Relaxometry.

1H spin−lattice nuclear magnetic resonance relaxation experiments were performed for five kinds of dermal fillers based on hyaluronic acid. The relaxation data were collected over a broad frequency range between 4 kHz and 40 MHz, at body temperature. Thanks to the frequency range encompassing four orders of magnitude, the dynamics of water confined in the polymeric matrix was revealed. It is demonstrated that translation diffusion of the confined water molecules exhibits a two‐dimensional character and the diffusion process is slower than diffusion in bulk water by 3–4 orders of magnitude. As far as rotational dynamics of the confined water is concerned, it is shown that in all cases there is a water pool characterized by a rotational correlation time of about 4×10−9 s. In some of the dermal fillers a fraction of the confined water (about 10 %) forms a pool that exhibits considerably slower (by an order of magnitude) rotational dynamics. In addition, the water binding capacity of the dermal fillers was quantitatively compared.

Molecular level deformation mechanism of ettringite

Ettringite is an naturally occurring mineral as well as is formed as an intermediate product during cement curing. A comprehensive molecular deformation study of the material has been presented in this study. The molecular deformation of the material for quasistatic uniaxial tension and compression (along different directions) along with triaxial tension and compression demonstrate that the SO4 molecules lose periodicity much faster compared to the [Ca3[Al(OH)6]⋅12H2O]3+ columnar structure thereby accounting for the drop in the stress-strain plots. The role of confined water molecules within the molecule has also been investigated in details in this study.

High Polymer Mass Densities at the Mouths of Carbon Nanotubes (CNTs) Control the Diffusion of Small Molecules through CNT-Based Polymer Nanocomposite Membranes.

Detailed molecular dynamics (MD) simulations of model single-walled carbon nanotube (CNT) membranes based on atactic poly(methyl methacrylate) (aPMMA) indicate that PMMA chains significantly penetrate nanotubes through their faces. They predict very high-density values of the polymer in the interfacial area around the CNT mouths that can exceed by 50% the density of the bulk polymer at the same thermodynamic conditions. This dramatically decreases the diffusivity of relatively small penetrants (in our study, water molecules) in the nanocomposite membrane, because of the exceedingly long times needed by these small molecules to diffuse through such a dense interfacial layer before accessing the interior of the nanotubes where they can travel really fast. According to our simulations, the escape time of a confined water molecule from the blocked mouths of a CNT can exceed by several orders of magnitude the time needed by the same molecule to move through the CNT pore. Our work indicates the importance of completely avoiding (or at least minimizing) penetration of polymer chains into the CNT pores through the mouths of the tubes in enabling the efficient transport of small- to moderate-size molecules in model CNT-based polymer membranes, since this provides the highest resistance to their mobility through the membrane.