Liquid Water Interface Research Articles

Interfacial Water Is Separated from a Hydrophobic Silica Surface by a Gap of 1.2 nm.

The interaction of liquid water with hydrophobic surfaces is ubiquitous in life and technology. Yet, the molecular structure of interfacial liquid water on these surfaces is not known. By using a 3D atomic force microscope, we characterize with angstrom resolution the structure of interfacial liquid water on hydrophobic and hydrophilic silica surfaces. The combination of 3D AFM images and molecular dynamics simulations reveals that next to a hydrophobic silica surface, there is a 1.2 nm region characterized by a very low density of water. In contrast, the 3D AFM images obtained of a hydrophilic silica surface reveal the presence of hydration layers next to the surface. The gap observed on hydrophobic silica surfaces is filled with two-to-three layers of straight-chain alkanes. We developed a 2D Ising model that explains the formation of a continuous hydrocarbon layer on hydrophobic silica surfaces.

Distinct Site Motifs Activate O2 and H2 on Supported Au Nanoparticles in Liquid Water.

Au nanoparticles catalyze the activation and conversion of small molecules with rates and kinetic barriers that depend on the dimensions of the nanoparticle, composition of the support, and presence of catalytically culpable water molecules that solvate these interfaces. Here, molecular interpretations of steady-state rate measurements, kinetic isotope effects, and structural characterizations reveal how the interface of Au nanoparticles, liquid water, and metal oxide supports mediate the kinetically relevant activation of H2 and sequential reduction of O2-derived intermediates during the formation of H2O2 and H2O. Rates of H2 consumption are 10-100 fold greater on Au nanoparticles supported on metal oxides (e.g., titania) compared to more inert and hydrophobic materials (carbon, boron nitride). Similarly, Au nanoparticles on reducible and Lewis acidic supports (e.g., lanthana) bind dioxygen intermediates more strongly and present lower barriers (<22 kJ mol-1) for O-O bond dissociation than inert interfaces formed with silica (>70 kJ mol-1). Selectivities for H2O2 formation increase significantly as the diameters of the Au nanoparticles increase because differences in nanoparticle size change the relative fractions of exposed sites that exist at Au-support interfaces. In contrast, site-normalized rates and barriers for H2 activation depend weakly on the size of Au nanoparticles and the associated differences in active site motifs. These findings suggest that H2O aids the activation of H2 at sites present across all surface Au atoms when nanoparticles are solvated by water. However, molecular O2 preferentially binds and dissociates at Au-support interfaces, leading to greater structure sensitivity for barriers of O-O dissociation across different support identities and sizes of Au nanoparticles. These insights differ from prior knowledge from studies of gas-phase reactions of H2 and O2 upon Au nanoparticle catalysts within dilute vapor pressures of water (10-4 to 0.1 kPa H2O), in which catalysis occurs at the perimeter of the Au-support interface. In contrast, contacting Au catalysts with liquid water (55.5 M H2O) expands catalysis to all surface Au atoms and enables appreciable H2O2 formation.

Second-Harmonic Generation Provides Insight into the Screening Response of the Liquid Water Interface

Second-Harmonic Generation Provides Insight into the Screening Response of the Liquid Water Interface

Effect of monovalent and divalent salts on the interfacial and structural properties of vapor-liquid water interface in presence of sodium dodecyl sulfate

The impact of divalent cations on the interfacial and structural properties of the vapor–liquid water interface covered with an anionic surfactant such as sodium dodecyl sulfate (SDS) is of great relevance to several industrial applications. In the present work, all-atom molecular dynamics simulations are performed to investigate the interfacial and structural properties of the vapor-SDS-liquid water interface with divalent salts viz., CaCl2 and MgCl2. The surface tension rises with the rise in divalent salt concentration, whereas it remains almost constant with NaCl concentration. In the absence of divalent salts, the accumulation of Na+ ions near the SDS headgroups is observed. Conversely, in the systems with divalent salts, the divalent cations (Ca2+ and Mg2+) enriched beside the SDS monolayer cause the monovalent cations (Na+) to displace from the interface into the bulk. The structural analysis shows that Na+ and Ca2+ cations enter the first hydration shell of the sulfate group of SDS. On the other hand, Mg2+ usually coordinates with the sulfate group of SDS. The potential of mean force (PMF) shows that the escape free energy of divalent cations is higher than monovalent cations, and the interaction between the cations and sulfate head group of SDS follows the order: S-Ca2+ > S-Mg2+ > S-Na+.

Quantifying the Molecular Polarization Response of Liquid Water Interfaces at Heterogeneously Charged Surfaces.

The hydration shells of proteins mediate interactions, such as small molecule binding, that are vital to their biological function or in some cases their dysfunction. However, even when the structure of a protein is known, the properties of its hydration environment cannot be easily predicted due to the complex interplay between protein surface heterogeneity and the collective structure of water's hydrogen bonding network. This manuscript presents a theoretical study of the influence of surface charge heterogeneity on the polarization response of the liquid water interface. We focus our attention on classical point charge models of water, where the polarization response is limited to molecular reorientation. We introduce a new computational method for analyzing simulation data that is capable of quantifying water's collective polarization response and determining the effective surface charge distribution of hydrated surfaces over atomistic length scales. To illustrate the utility of this method, we present the results of molecular dynamics simulations of liquid water in contact with a heterogeneous model surface and the CheY protein.

Algorithmic graph theory for post-processing molecular dynamics trajectories

This paper reviews some of the graph theory-based methods that were recently developed in our group for post-processing molecular dynamics trajectories. We show that the use of algorithmic graph theory not only provides a direct and fast methodology to identify conformers that are sampled over time, but it also allows to follow in time the interconversions between the conformers through graphs of transitions, also provides statistical characterisations, that would otherwise be hard to obtain. Examples for a gas phase peptide and for the more complex inhomogeneous charged air–liquid water interface are presented in order to demonstrate the power of topological 2D-graphs and their versatility and transferability.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials.

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Formation and Structure of Nanotubes in Imidazolium-Based Ionic Liquid Aqueous Solution.

Self-assembled structures have attracted much attention for their potential applications in biological and electrochemical studies. Understanding the aggregation mechanism is necessary for utilizing the structures and improving the properties. In this study, the tubular cluster aggregations formed by the 1-dodecyl-3-methylimidazolium salicylate ([C12mim][Sal]) have been studied by molecular dynamics simulations. The rod-like and funnel-shaped structures were observed during the simulations, and finally, the nanotube structure enclosed by a bilayer membrane was formed. For the first time, the point cloud fitting method was used to obtain the axis equation of the tubular cluster. Based on the equation, the structure of tubular clusters was analyzed in detail. The imidazolium ring and anions were distributed at the ionic liquid-water interface, while the dodecyl groups were buried in the nanotube membrane away from the water. Electrostatic interactions between cations and anions played a dominant role in stabilizing the structure of the nanotube. The tubular cluster size, membrane thickness, and permeability of water molecules through the membrane of the cluster were also calculated. Furthermore, the orientation analysis revealed that multitudinous aggregation structures could be formed by the long alkyl chain in aqueous solution, which might be beneficial for the strengthening and separating processes.

Structure and Reactivity of α-Al2O3(0001) Surfaces: How Do Al–I and Gibbsite-like Terminations Interconvert?

The α-Al2O3(0001) surface has been extensively studied because of its significance in both fundamental research and application. Prior work suggests that in ultra-high-vacuum (UHV), in the absence of water, the so-called Al–I termination is thermodynamically favored, while in ambient, in contact with liquid water, a Gibbsite-like layer is created. While the view of the α-Al2O3(0001)/H2O(l) interface appears relatively clear in theory, experimental characterization of this system has resulted in estimates of surface acidity, i.e., isoelectric points, that differ by 4 pH units and surface structure that in some reports has non-hydrogen-bonded surface aluminol (Al–OH) groups and in others does not. In this study, we employed vibrational sum frequency spectroscopy (VSFS) and density functional theory (DFT) simulation to study the surface phonon modes of the differently terminated α-Al2O3(0001) surfaces in both UHV and ambient. We find that, on either water dosing of the Al–I in UHV or heat-induced dehydroxylation of the Gibbsite-like in ambient, the surfaces do not interconvert. This observation offers a new explanation for disagreements in prior work on the α-Al2O3(0001)/liquid water interface─different preparation methods may create surfaces that do not interconvert─and shows that the surface phonon spectral response offers a novel probe of interfacial hydrogen bonding structure.

Liquid-Metal-Mediated Electrocatalyst Support Engineering toward Enhanced Water Oxidation Reaction.

Functional and robust catalyst supports are vital in the catalysis field, and the development of universal and efficient catalyst support is essential but challenging. Traditional catalyst fabrication methods include the carbonization of ordered templates and high−temperature dehydration. All these methods involve complicated meso−structural disordering and allow little control over morphology. To this end, a eutectic GaInSn alloy (EGaInSn) was proposed and employed as an intermediate to fabricate low−dimensional ordered catalyst support materials. Owing to the lower Gibbs free energy of Ga2O3 compared to certain types of metals (e.g., Al, Mn, Ce, etc.), we found that a skinny layer of metal oxides could be formed and exfoliated into a two−dimensional nanosheet at the interface of liquid metal (LM) and water. As such, EGaInSn was herein employed as a reaction matrix to synthesize a range of two−dimensional catalyst supports with large specific surface areas and structural stability. As a proof−of-concept, Al2O3 and MnO were fabricated with the assistance of LM and were used as catalyst supports for loading Ru, demonstrating enhanced structural stability and overall electrocatalytic performance in the oxygen evolution reaction. This work opens an avenue for the development of functional support materials mediated by LM, which would play a substantial role in electrocatalytic reactions and beyond.

Unique water H-bonding types on metal surfaces: from the bonding nature to cooperativity rules

Understanding the nature of H-bonding interactions is essential to modern sciences, such as biology, chemistry, and physics. Using density functional theory calculations, herein, we have identified two unique H-bonding types existing in a single sheet of a mixed water–hydroxyl phase on close-packed metal surfaces, in sharp contrast to conventional H-bonds in liquid water and water ices. Interestingly, the shallow H-bonds show reduced electrostatic and Pauli repulsion interactions, with an electrostatic polar character resulted from complete σ resonances, whereas the deep H-bonds exhibit enhanced electrostatic and Pauli repulsion interactions, with an electrostatic dipolar feature originated from hybrid orbital interactions. A trade-off-like cooperativity law of the two types of H-bonds was discovered, that is, strengthening in the internal bonds (dO–H) leads to weakening in the external bonds (dO:H) or vice versa. However, the shallow H-bonds exhibit a non-linear cooperativity, whereas the deep H-bonds show a linear cooperativity. We also identified an oxygen backbone cooperativity rule that strengthening the adsorbate–metal interactions has a net effect in analogy to reducing the O–O repulsion within the adlayer. Furthermore, we have discovered several universality classes in geometrical, vibrational, and electronic spaces for the two H-bonding types. Although shared by electronic universality classes, the two contrasting H-bonding types are featured by divergent trends with significant overlapping, where competitive variations in the electrostatic and Pauli repulsion strengths are basic rules for the cooperative H-bonding types. The knowledge of the unconventional H-bonding types expands our current understanding of H-bonding interactions in liquid water and water ices and points to the importance of H-bonding manipulation at electronic levels. These findings not only shed new light on probing the fundamental nature of H-bonds in general but also have insightful implications for resolving the cooperative H-bonding nature of interfacial water, liquid water, water ices, and aqueous solutions.

Molecular simulation study of the adhesion work for water droplets on water monolayer at room temperature* *Project supported by the National Natural Science Foundation of China (Grant Nos. 12022508 and 12074394) and Sichuan Science and Technology Program (Grant No. 2017YJ0174).

The wetting phenomenon of water droplets coexisting with the ordered water monolayer termed an unexpected phenomenon of “water that does not wet a water monolayer” at room temperature has been found on several solid surfaces. Although the hydrogen bond saturation inside the monolayer can qualitatively describe this phenomenon, whether the Young–Dupré equation still holds under this unconventional wetting framework is still not answered. In this work, we have calculated the contact angle values of the droplets as well as the work of adhesion between the droplets and the monolayer based on an extended phantom-wall method. The results show that similar to the conventional solid–liquid interface, classical Young–Dupré equation is also applicable for the interface of liquid water and ordered water monolayer.

Ionic Liquids as Extractants for Nanoplastics.

Plastic waste in the ocean and on land in the form of nanoplastics is endangering food and drinking water supplies, raising the need for new strategies for the removal of plastic nanoparticles from complex media. In the present contribution we suggest considering ionic liquids as extractants, since they show several advantageous properties that may facilitate the design of efficient separation processes. Through varying the anion and the side chain at the cation, the interactions between the extractant and the polymer can be strengthened and tuned, and thereby the disintegration of the particle into separate polymer chains can be controlled. Oxidized moieties can also be efficiently solvated, given the amphiphilic nature of the considered ionic liquids, allowing also realistic particles to be extracted into these solvents. The phase transfer was found to be thermodynamically and kinetically possible, which is supported by the complicated structure of the ionic liquid‐water interface through the rearrangement of the interfacial ions, and the formation of a micelle around the plastic already at the edge of the aqueous phase.

Water Purification by Activation Plasma Technology

In modern water treatment technologies Water needs to be processed to a level that can be reused in a meaningful way. Plasma production or plasma injection in liquid water provides an opportunity to inject advanced Oxidation processes in water for purification or chemical processing. Such technology can revolutionize Drinking water treatment, as well as current practices of chemical processing by removing physical catalysts. An overview of current water treatment is presented here where Technology, its limitations and the future, may include Plasmabased advanced oxidation method. In plasma Introduction to liquid water produces and attacks a host of reactive species finally mineralize the contaminants in the solution. This interaction takes place at the boundary layer or at the interaction zone is concentrated at the plasma - liquid water interface. There are limitations of traditional plasma injection procedures which include limited throughput capacity, electrode shear and reduced Process Volume. Here the technical limitations of plasma based water reactor will also be discussed.

Entropic Origin of the Attenuated Width of the Ice–Water Interface

The solid–liquid interface of water is ∼50% narrower (or thinner) than that of argon. With the help of molecular dynamics simulations, we compare two water models, namely, TIP4P/ice and mW, with Le...

Super-Slippery Degraded Black Phosphorus/Silicon Dioxide Interface.

The interfaces between two-dimensional (2D) materials and the silicon dioxide (SiO2)/silicon (Si) substrate, generally considered as a solid-solid mechanical contact, have been especially emphasized for the structure design and the property optimization in microsystems and nanoengineering. The basic understanding of the interfacial structure and dynamics for 2D material-based systems still remains one of the inevitable challenges ahead. Here, an interfacial mobile water layer is indicated to insert into the interface of the degraded black phosphorus (BP) flake and the SiO2/Si substrate owing to the induced hydroxyl groups during the ambient degradation. A super-slippery degraded BP/SiO2 interface was observed with the interfacial shear stress (ISS) experimentally evaluated as low as 0.029 ± 0.004 MPa, being comparable to the ISS values of incommensurate rigid crystalline contacts. In-depth investigation of the interfacial structure through nuclear magnetic resonance spectroscopy and in situ X-ray photoelectron spectroscopy depth profiling revealed that the interfacial liquid water was responsible for the super-slippery BP/SiO2 interface with extremely low shear stress. This finding clarifies the strong interactions between degraded BP and water molecules, which supports the potential wider applications of the few-layer BP nanomaterial in biological lubrication.

Enhanced conductivity of water at the electrified air-water interface: a DFT-MD characterization.

DFT-based molecular dynamics simulations of the electrified air-liquid water interface are presented, where a homogeneous field is applied parallel to the surface plane. We unveil the field intensity for the onset of proton transfer and molecular dissociation; the protonic current/proton conductivity is measured as a function of the field intensity/voltage. The air-water interface is shown to exhibit a proton conductivity twice the one in the liquid water for field intensities below 0.40 V Å-1. We show that this difference arises from the very specific organization of water in the binding interfacial layer (BIL, i.e. the air-water interface region) into a 2D-HBond-network that is maintained and enforced at the electrified interface. Beyond fields of 0.40 V Å-1, water in the BIL and in the bulk liquid are aligned in the same way by the rather intense fields, hence leading to the same proton conductivity in both BIL and bulk water.

Transpiration Driven Electrokinetic Power Generator.

Transpiration is the process by which water is carried in plants from the roots to the leaves where evaporation takes place. Here, we report a transpiration driven electrokinetic power generator (TEPG) that exploits capillary flow of water in an asymmetrically wetted cotton fabric coated with carbon black. Accumulation of protons induced by the electrical double layer formed at the solid (carbon black)/liquid (water) interface gives rise to potential difference between the wet and dry sides. The conductive carbon black coating channels electrical current driven by the pseudostreaming mechanism. A TEPG of 90 mm × 30 mm × 0.12 mm yields a maximum voltage of 0.53 V, maximum current of 3.91 μA, and maximum energy density of 1.14 mWh cm-3, depending on the loading of the carbon black. Multiple TEPGs generate enough power to light up a light-emitting diode (20 mA × 2.2 V) or charge a 1 F supercapacitor.

Corrigendum: Unexpected rotational distribution of ground electronic state OH(X 2Π) generated by plasma at liquid water interface (2018 Plasma Sources Sci. Technol. 27 085001)

Corrigendum: Unexpected rotational distribution of ground electronic state OH(X 2Π) generated by plasma at liquid water interface (2018 Plasma Sources Sci. Technol. 27 085001)

Characterizing the bifurcating configuration of hydrogen bonding network in interfacial liquid water and its adhesion on solid surfaces.

The interfacial structures of liquid water molecules adjacent to a solid surface contribute significantly to the interfacial properties of aqueous solutions, and are of prime importance in a wide spectrum of applications. In this work, we use molecular dynamics (MD) simulations to explore the interfacial structures, mainly in term of hydrogen bonding network, of a liquid water film interacting intimately with solid surfaces, which are composed of [100] face centered cubic (FCC) lattices. We disclose the formation of a bifurcating configuration of hydrogen bonds in interfacial liquid water and ascribe its occurrence to the collective effects of water density depletion, hydrogen bonds and local polarization. Such bifurcating configuration of interfacial water molecules consists of repetitive layer by layer water sheets with intra-layer hydrogen bonding network being formed in each layer, and inter-layer defects, i.e., hydrogen bonds formed between two neighboring layers of interfacial water. A lower bound of 2.475 for the average number of hydrogen bonds per interfacial water molecule is expected. Our MD study on the interfacial configuration of water on solid surfaces reveals a quadratic dependence of adhesion on the solid–liquid affinity, bridging the gap between the macroscopic interfacial property Wadh and the microscopic parameter εSL of the depth of the Lennard-Jones solid–liquid potential.