Monolayer Of Water Molecules Research Articles

First-principles study on the co-adsorption of water and oxygen molecules on chalcopyrite (112)-M surface

Chalcopyrite is a common copper-bearing mineral with antiferromagnetic properties. However, this property has rarely been considered in previous studies for detailed adsorption behaviors of molecules on chalcopyrite. Based on density functional theory (DFT), new adsorption pathways by H2O and O2 on the chalcopyrite metal terminated (112) surface ((112)-M) is found in this work. First, through simulating the adsorption of an isolated water molecule and monolayer water molecules, it is confirmed that H2O molecules tend to adsorb on the surface Fe atoms more than on the surface Cu atoms. Then, we studied various adsorption behaviors of the O2 molecule. It is found that the adsorption on the hollow FeFe site is the most stable case; however, O2 is undissociated. Two adsorption cases will happen when H2OO2 adsorb simultaneously on the surface. For the S site, the H2O molecule thoroughly dissociated and formed SO species, and the other case is H2O undissociated adsorbing at the Cu site. For the former case, it is interesting that H2O is dissociated before O2.

Fate of Water Molecules on (11-20) and (1-102) α-Alumina Surfaces: 2D Periodic Self-Consistent Charge-Density Functional Tight-Binding/Molecular Mechanics Molecular Dynamics Study

Numerous studies on the adsorption and dissociation of water molecules on α-alumina surfaces, both experimentally and theoretically, have been presented within decades. However, the dynamical properties of water at the solid–liquid interface are still a challenging problem. To explain the dynamics of the adsorption and dissociation of water molecules on α-alumina surfaces, mainly (11-20) and (1-102) faces, we carried out a two-dimensional periodic self-consistent charge density-functional tight-binding/molecular mechanics molecular dynamics study. In this study, we used the same approach previously applied to the (0001) α-alumina surface. The adsorption of single, dimer, and monolayer water molecules was employed to study the adsorption and dissociation of water molecules at (11-20) and (1-102) α-alumina surfaces. The single water system showed that no significant changes occurred on both surfaces, which indicates that no water dissociation was observed. Dimer water molecules on both surfaces were dissociated to form new OsHw groups following a 1-4 dissociation channel. Monolayer water molecules showed dissociation that occurred on both surfaces. Protonation analysis showed that both surfaces form stable OH groups without attempting to form water molecules. At the 100 ps simulation period, for the (11-20) α-alumina surfaces, up to 1.93 OH groups/nm2 were formed. The (1-102) α-alumina surface showed higher reactivity with a formation rate of up to 5.0 OH groups/nm2. By contrast, 2.1 OH groups/nm2 were formed on the (0001) α-alumina surface. The formation of OH groups on the (11-20) and (1-102) α-alumina surfaces occurred in the equilibration step and continued during the sampling step. In studying these three surfaces using a similar approach, the (1-102) α-alumina surface is the most reactive with water.

Water molecules bonded to the carboxylate groups at the inorganic-organic interface of an inorganic nanocrystal coated with alkanoate ligands.

High-quality colloidal nanocrystals are commonly synthesized in hydrocarbon solvents with alkanoates as the most common organic ligand. Water molecules with an approximately equal number of surface alkanoate ligands are identified at the inorganic–organic interface for all types of colloidal nanocrystals studied, and investigated quantitatively using CdSe nanocrystals as the model system. Carboxylate ligands are coordinated to the surface metal ions and the first monolayer of water molecules is found to bond to the carboxylate groups of alkanoate ligands through hydrogen bonds. Additional monolayer(s) of water molecules can further be adsorbed through hydrogen bonds to the first monolayer of water molecules. The nearly ideal environment for hydrogen bonding at the inorganic–organic interface of alkanoate-coated nanocrystals helps to rapidly and stably enrich the interface-bonded water molecules, most of which are difficult to remove through vacuum treatment, thermal annealing and chemical drying. The water-enriched structure of the inorganic–organic interface of high-quality colloidal nanocrystals must be taken into account in order to understand the synthesis, processing and properties of these novel materials.

Hygroscopic property of inorganic salts in atmospheric aerosols measured with physisorption analyzer

Hygroscopicity is capable of significantly affecting atmospheric visibility, radiative forcing, and cloud condensation nuclei process. Herein, a physisorption analyzer was adopted to determine the hygroscopicity of inorganic salt particles in atmospheric aerosols (e.g., NaNO3, NaClO4, MgSO4, K3PO4, K2HPO4, and KH2PO4) at 298 K. By regulating the ambient relative humidity (RH), qualitative and quantitative information of the mentioned particles in humidifying and dehumidifying cycles was acquired. The deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) were measured, and supersaturation was observed and recorded during the dehumidification process. The water-to-salt molar ratios (WSR) of the mentioned inorganic salts depending on RH were determined, and the hygroscopic results of NaNO3 and NaClO4 particles were well consistent with the results achieved with other methods or by theoretical prediction. This study fitted WSR-to-RH equations suitable for NaNO3, NaClO4, MgSO4, K3PO4, and K2HPO4 particles. The adsorption heats for monolayer water molecules in the mentioned inorganic salt particles are calculated. The present study lays a solid foundation for gaining insights into the effects of RH on atmospheric aerosols.

Water Distribution and Clustering on the Lyophilized IgG1 Surface: Insight from Molecular Dynamics Simulations.

Water has a critical role in the stability of the higher-order structure of proteins. In addition, it is considered to be a major destabilization factor for the physical and chemical stability of freeze-dried proteins and peptides. Physical and chemical aspects of protein/water relationships are commonly studied with the use of water vapor sorption isotherms for amorphous lyophilized proteins, which, in turn, are commonly analyzed using the Brunauer-Emmett-Teller (BET) equation to obtain the parameters, Wm and CB. The parameter Wm is generally referred to as the "monolayer limit of adsorption" and has a narrow range of 6-8% for most proteins. In this study, the water distribution on an IgG1 surface is investigated by molecular dynamics (MD) simulations at different water contents. The monolayer of water molecules was found to have limited coverage of the protein surface, and the true monolayer coverage of the protein globule actually occurs at a hydration level above 30%. The distribution of water molecules on the IgG1 surface is also highly heterogeneous, and the heterogeneity is not considered in the BET theory. In this study, a mechanistic model has been developed to describe the water vapor sorption isotherm. This model is based on the analysis of the hydrogen bonding network extracted from the MD simulations. The model is consistent with the experimental Type-II isotherm, which is usually observed for proteins. The physical meaning of the BET monolayer was redefined as the onset of water cluster formation. A simple model to calculate the onset water level, Wm, is proposed based on the hydration of different amino acids, as determined from the MD simulations.

The hydration of periclase: Atomistic insights from quantum-chemical look

A first principles density functional theory study has been carried out to investigate the hydration kinetics of periclase (MgO) through the interaction between water and periclase (0 0 1) and (1 1 1) surfaces. A single water molecule associatively adsorbs on both surfaces. Dissociative adsorption of water molecule only occurs by the co-adsorption of water on the (0 0 1) surface. Excepting this co-adsorption on the MgO(0 0 1) surface, the adsorption of two or monolayer water molecules on both surfaces are associative in nature. The interaction of water monolayer with the MgO(1 1 1) surface leads to the formation of Mg(OH)2. The calculated energy barrier for the formation of Mg(OH)2 from MgO is 93.6 kJ/mol, which is higher than the experimental values (66–84 kJ/mol). The absence of defect in the model surface sets the upper limit to the energy barrier for the hydration reaction. The effect of surface defects is demonstrated through the adsorption of a single water molecule. A single water molecule adsorbs both associatively and dissociatively on the (1 1 1) defect surface. The energy barrier for the conversion of associative to dissociative adsorption is 36.7 kJ/mol. The important new insights from this study are: (1) only MgO(1 1 1) surface is involved in the hydration of periclase and (2) surface defects play very significant role in the experimental measurement of energy barrier for hydration reaction.

A Computational and Experimental Study on the Binding of Dithio Ligands to Sperrylite, Pentlandite, and Platinum

This benchmark study documents a combined computational and experimental investigation into the binding of three commercial dithio collector ligands used in industrial froth flotation processes to separate high-value minerals from lower-value materials. First-principles condensed-matter simulations showed that ethyl xanthate, N,N-diethyl dithiocarbamate, and diisobutyl dithiophospinate anions all bind least strongly to the [100] Miller index plane of the platinum-containing mineral sperrylite, followed by the [111] MI surface of the mixed nickel/iron sulfide mineral pentlandite, whereas all ligands showed the strongest binding affinity to the [111] MI surface model of pure platinum. Calculations also support experimental observations that neutral ethyl xanthogen disulfide formed upon oxidation of ethyl xanthate binds much more weakly than the monomer. A monolayer of water molecules would easily be displaced from all surfaces by any of the collector ligands. The hydroxide anion was found to have binding en...

Dynamics and response of a humidity sensor based on a Love wave device incorporating a polymeric layer

In this study, we investigate the dynamics and response of a humidity sensor based on a polymer-coated Love wave device. A review is presented for the theoretical model of Love waves in a layered structure with a viscoelastic layer on a piezoelectric substrate. Numerical illustrations are executed for the mass velocity sensitivity and mass loss sensitivity of a polymer-coated device. The BET equation and its improved equation are introduced to describe the adsorption mechanism of gas on the detector surface. A method is introduced for calculating the surface area of the polymer layer, which is proved a porous material. An experiment is performed for a humidity sensor based on a Love wave device consisting of two 28μm-periodic interdigital transducers, a 0.47μm-thick PVA layer, and an ST-90°X quartz substrate. The operation frequency and insertion loss of the Love wave device are measured by using a network analyzer; the relative surface area of the PVA layer is calculated through comparing the fitted and the theoretical frequency shift caused by the monolayer of water molecules. The frequency shifts and insertion loss increments are shown as functions respect to the relative humidity; the theoretical curves agree well the experimental results.

Density functional theory modeling of multilayer "epitaxial" graphene oxide.

CONSPECTUS: Graphene oxide (GO) is a complex material of both fundamental and applied interest. Elucidating the structure of GO is crucial to achieve control over its properties and technological applications. GO is a nonstoichiometric and hygroscopic material with a lamellar structure, and its physical chemical properties depend critically on synthesis procedures and postsynthesis treatments. Numerous efforts are in place to both understand and exploit this versatile layered carbon material. This Account reports on recent density functional theory (DFT) studies of "epitaxial" graphene oxide (hereafter EGO), a type of GO obtained by oxidation of graphene films grown epitaxially on silicon carbide. Here, we rely on selected X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and X-ray diffraction (XRD) measurements of EGO, and we discuss in great detail how we utilized DFT-based techniques to project out from the experimental data basic atomistic information about the chemistry and structure of these films. This Account provides an example as to how DFT modeling can be used to elucidate complex materials such as GO from a limited set of experimental information. EGO exhibits a uniform layered structure, consisting of a stack of graphene planes hosting predominantly epoxide and hydroxyl groups, and water molecules intercalated between the oxidized carbon layers. Here, we first focus on XPS measurements of EGO, and we use DFT to generate realistic model structures, calculate core-level chemical shifts, and through the comparison with experiment, gain insight on the chemical composition and metastability characteristics of EGO. DFT calculations are then used to devise a simplistic but accurate simulation scheme to study thermodynamic and kinetic stability and to predict the intralayer structure of EGO films aged at room temperature. Our simulations show that aged EGO encompasses layers with nanosized oxidized domains presenting a high concentration of oxygen functionalities and local structural order, surrounded by regions of pristine graphene. Through the analysis of XRD and IR measurements, our DFT calculations finally show that in EGO, the oxidized domains of stacked layers overlap and locally confine about a monolayer of water molecules. The overall water content in EGO remains below 10%, and intralayer and interlayer spatial ditribution of oxygen species in EGO lead to a layered porous film with an interlayer spacing of about 10 Å. The basic insight gained from our DFT studies, from chemical composition to a nanoscale characterization of the film structure, will be used to fine-tune synthesis methods and EGO properties.

Effect of Adsorption Stress-Induced Change in Neutral Layer Position on Static Behavior of Microcantilever Gas Sensor

The static bending model of microcantilever with monolayer molecules has been established based on energy method, in which the change in neutral layer position caused by adsorption-induced stress is introduced. On this basis, we have analyzed the relationship between the bending curvature radius of a microcantilever with its thickness, Young’s modulus and molecule-molecule distance of adsorbed molecules when it is adsorbed with monolayer water molecules. Additionally, we have investigated the effect of change in neutral layer position on the static behavior of microcantilever sensors. The results show that 1)the bending curvature radius of microcantilever is the linear, quadratic and eight approximation function of its Young’s modulus, thickness and distance of adsorbed molecules, respectively; 2)the predicted error of bending curvature radius caused by the change in neutral layer position slightly increases with decreasing Young’s modulus and thickness, whereas the effect of distance between adsorbed molecules on the error is significant; 3)the change in neutral layer position can cause a significant effect on the sensitivity and surface strain of the microcantilever

Water formation through O2 + D pathway on cold silicate and amorphous water ice surfaces of interstellar interest

The formation of the first monolayer of water molecules on bare dust grains is of primary importance to understand the growth of the icy mantles that cover dust in the interstellar medium. In this work, we explore experimentally the formation of water molecules from O(2) + D reaction on bare silicate surfaces that simulates the grains present in the diffuse interstellar clouds at visual extinctions (A(V) < 3 mag). For comparison, we also study the formation of water molecules on surfaces covered with amorphous water ice representing the dense clouds (A(V) ≥ 3 mag). Our studies focus on the formation of water molecules in the sub-monolayer and monolayer regimes using reflection absorption infrared spectroscopy and temperature-programmed desorption techniques. We provide the fractions of the products, such as D(2)O and D(2)O(2) molecules formed on three astrophysically relevant surfaces held at 10 K (amorphous olivine-type silicate, porous amorphous water ice, and nonporous amorphous water ice). Our results showed that the formation of D(2)O molecules occurs with an efficiency of about 55%-60% on nonporous amorphous water ice and about 18% on bare silicate grains surfaces. We explain the low efficiency of D(2)O water formation on the silicate surfaces by the desorption upon formation of certain products once the reaction occurs between O(2) and D atoms on the surface. A kinetic model taking into account the chemical desorption of newly formed water supports our conclusions.

Modified Model for Static Behavior of Microcantilever Gas Sensor Adsorbed with Monolayer Molecules

The modified static bending model of microcantilever with monolayer molecules has been established based on energy method, in which the change in neutral layer position caused by adsorption-induced stress has been considered. On this basis, we have analyzed the relationship between the bending curvature radius of a microcantilever with its thickness, Young’s modulus and molecule-molecule distance of adsorbed molecules when it is adsorbed with monolayer water molecules. Additionally, we have investigated the effect of change in neutral layer position on the static behavior of microcantilever sensors and have found that: 1) the bending curvature radius of microcantilever is affected by its Young’s modulus, thickness and distance of adsorbed molecules respectively; 2)the predicted error of bending curvature radius caused by the change in neutral layer position slightly increases with decreasing Young’s modulus and thickness, whereas the effect of distance between adsorbed molecules on the error is significant.

Hypergolic fuel detection using individual single walled carbon nanotube networks

Accurate and reliable detection of hypergolic fuels such as hydrazine (N2H4) and its derivatives is vital to missile defense, aviation, homeland security, and the chemical industry. More importantly these sensors need to be capable of operation at low temperatures (below room temperature) as most of the widely used chemical sensors operate at high temperatures (above 300 °C). In this research a simple and highly sensitive single walled carbon nanotube (SWNT) network sensor was developed for real time monitoring of hydrazine leaks to concentrations at parts per million levels. Upon exposure to hydrazine vapor, the resistance of the air exposed nanotubes (p-type) is observed to increase rapidly while that of the vacuum-degassed nanotubes (n-type) is observed to decrease. It was found that the resistance of the sample can be recovered through vacuum pumping and exposure to ultraviolet light. The experimental results support the electrochemical charge transfer mechanism between the oxygen redox couple of the ambient and the Fermi level of the SWNT. Theoretical results of the hydrazine-SWNT interaction are compared with the experimental observations. It was found that a monolayer of water molecules on the SWNT is necessary to induce strong interactions between hydrazine and the SWNT by way of introducing new occupied states near the bottom of the conduction band of the SWNT.

Hydration of Cellulose/Silica Hybrids Assessed by Sorption Isotherms

The hydration of cellulose/silica hybrids (CSH) containing 13, 35, and 46% (w/w) silica synthesized in situ by a mild sol-gel aqueous process was studied employing sorption isotherms and surface energy measurements. Water sorption is governed by two simultaneous equilibria at the silica-vapor and cellulose-vapor interfaces due to the presence of cellulose regions covered and uncovered with silica as confirmed by X-ray scattering analysis. The important contribution of a water impermeable cellulose-silica interface to the surface properties of CSH is highlighted. CSH exhibit type II isotherms in the temperature range 15-40 degrees C, more appropriately described by the Guggenheim-Anderson-de Boer (GAB) model than by the Brunauer-Emmett-Teller (BET) model. Specific surface areas calculated using both models (S(BET) and S(GAB)) increase linearly with the amount of silica influencing the higher moisture content of CSH for the same water activity. However, the presence of silica reduces the strength of interaction between monolayer water molecules and the CSH surface as revealed by the net isosteric heat of sorption (35 kJ/mol for cellulosic pulp and 26 kJ/mol for CSH with 35% silica). Silica confers a higher surface energy to CSH contributing to its polar component and surface wettability (contact angle with water) when compared to cellulosic pulp. The average diameter of cellulose microfibrils and the interfacial cellulose-silica areas are assessed on the basis of the analysis of sorption isotherms.

Ultrafast vibrational dynamics of interfacial water

We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.

Pore Morphology of Porous Polymer Particles Probed by NMR Relaxometry and NMR Cryoporometry

The pore size distribution (PSD) and pore connectivity (PC) within porous polymer particles are probed by combining NMR cryoporometry and NMR relaxometry (spin-spin relaxation). With water as a probe molecule, the constant K in the so-called Gibbs-Thompson equation and the surface relaxivity (rho2) were determined to be K = (420 +/- 50) KA and rho2 = (0.44 +/- 0.01) x 10(-6) ms(-1), respectively. Also, the thickness of the interface layer was estimated to be of the order of one monolayer of water molecules. A detailed analysis of the complete set of NMR data enabled the morphology or pore structure to be probed, and is thoroughly discussed in the text.

The Structure of Water Monolayers on a Hydroxylated Chromium Oxide Surface

We have investigated the structure of monolayer water molecules on a hydroxylated (001) plane of Cr2O3 using GCMC simulations. The water molecules are adsorbed commensurately in the center of three surface hydroxyl groups by directing the proton of a water hydroxyl group towards the solid surface. Another type of water is adsorbed by bridging anchored water molecules, whereby, a two-dimensional (2D) condensed layer is formed. It was ascertained that 2D condensed monolayer water is not in a crystalline state below the 2D critical temperature (319 K), but is composed of many hexagonally linked rings. Above the 2D critical temperature, the hexagonal alignment of water changes to form a one-dimensional linked water structure.

The humidity dependence of ozone deposition onto a variety of building surfaces

Abstract Measurements of the dry deposition velocity of O3 to material samples of calcareous stone, concrete and wood at varying humidity of the air, were performed in a deposition chamber. Equilibrium surface deposition velocities were found for various humidity values by fitting a model to the time-dependent deposition data. A deposition velocity-humidity model was derived giving three separate rate constants for the surface deposition velocities, i.e. on the dry surface, on the first mono-layer of adsorbed water and on additional surface water. The variation in the dry air equilibrium surface deposition velocities among the samples correlated with variations in effective areas, with larger effective areas giving higher measured deposition velocities. A minimum for the equilibrium surface deposition velocity was generally measured at an intermediate humidity close to the humidity found to correspond to one mono-layer of water molecules on the surfaces. At low air humidity the equilibrium surface deposition velocity of O3 was found to decrease as more adsorbed water prevented direct contact of the O3 molecules with the surface. This was partly compensated by an increase as more adsorbed water became available for reaction with O3. At high air humidity the equilibrium surface deposition velocity was found to increase as the mass of water on the surface increased. The deposition velocity on bulk de-ionised water at RH=90% was an order of magnitude lower than on the sample surfaces.

Water ordering around methane during hydrate formation

The structure of water around methane during hydrate crystallization from aqueous solutions of methane is studied using neutron diffraction with isotopic substitution over the temperature range 18 °C to 4 °C, and at two pressures, 14.5 and 3.4 MPa. The carbon–oxygen pair correlation functions, derived from empirical potential structure refinement of the data, indicate that the hydration sphere around methane in the liquid changes dramatically only once hydrate has formed, with the water shell around methane being about 1 Å larger in diameter in the crystal than in the liquid. The methane coordination number in the liquid is around 16±1 water molecules during hydrate formation, which is significantly smaller than the value of 21±1 water molecules found for the case when hydrate is fully formed. Once hydrate starts to form, the hydration shell around methane becomes marginally less ordered compared to that in the solution above the hydrate formation temperature. This suggests that the hydration cage around methane in the liquid may be different from that when hydrate is forming and from that found in the hydrate crystal structure. Methane–methane radial distribution functions show that methane molecules can adopt a range of separations during hydrate formation, corresponding to the more distorted nature of the methane–water correlations. There is noticeable ordering of the methane molecules with a monolayer of water molecules between them once hydrate has formed. The dipole moments of the hydrating water molecules lie mostly tangential to the methane–water axis, both before, during, and after hydrate formation.

An HREELS and TPD study of water on TiO 2(110): the extent of molecular versus dissociative adsorption

The interaction of water with the (110) crystal face of TiO 2 (rutile) was examined with high resolution electron energy loss spectroscopy (HREELS) and temperature programmed desorption (TPD). Water desorption occurred in three sequential TPD features as a function of exposure at 270, 175 and 155 K, ascribed to monolayer, second layer and multilayer states, respectively. Estimation of the extent to which water molecularly versus dissociatively adsorbed on the TiO 2(110) surface was attempted by comparing HREELS and TPD data. Very low water exposures (below 7 × 10 13molecules/cm 2) dissociatively adsorbed at 135 K to surface hydroxyl groups ( v(OH) = 3690 cm − 1), but molecular adsorption ( v(OH) = 3420 − 3505 cm − 1 and δ(HOH) = 1625 cm − 1) resulted from additional water exposure. The appearance of molecular water in HREELS coincided with a red-shift in the OH stretching frequency of the hydroxyl, presumably due to the formation of hydrogen-bonding interactions with the adsorbed water molecules. Little or no additional water dissociation occurred during heating of the monolayer as inferred from HREELS. Second layer water on TiO 2(110) was evident in HREELS by OH stretching intensity below 3400 cm − 1 indicative of hydrogen-bonding. However, the OH stretching mode of monolayer water molecules was not red-shifted by the presence of second layer water suggesting that protons on monolayer water molecules were not involved in hydrogen-bonding second layer water molecules. The influence of second layer water on subsequent layers was evident in both HREELS and TPD.