Mechanism Of Water Adsorption Research Articles

Fluorination-Induced Changes in Hydrophobicity of Silicon Carbide-Derived Nanoporous Carbon

We report the effect of fluorine doping on hydrophobicity of microporous silicon carbide-derived carbon (SiCDC), exploring water vapor adsorption in virgin and fluorinated samples experimentally and using semiempirical isotherm models. The surface chemistry of the virgin carbon and samples fluorinated to three different levels is characterized by X-ray photoelectron spectroscopy and 19F nuclear magnetic resonance analysis techniques. Besides having different textural features such as surface area and pore size distribution, the virgin and fluorinated SiCDCs show different surface chemistries in terms of the nature and quantity of the C–F bonds. The comparison of the characterization results of the samples and model parameters is used as the methodology to understand the water adsorption mechanism in virgin and fluorinated SiCDCs. We demonstrate that with increasing fluorination level the hydrophobic character of the low and medium fluorinated SiCDC samples remains almost constant while the highly fluorine...

Molecular Dynamics Calculation on Composition B Adsorption Water

To research the adsorption mechanism of water on composition B crystal surfaces and the effect on mechanical properties and sensitivity of explosive, the crystal model of composition B was established by Material Studio (MS). The adsorption process was simulated and the mechanical properties, adsorption energy of different crystal surfaces, trigger bond length, interaction energy of trigger bond and cohesive energy density were got and compared. The results show that the (0 1 0) crystal surface has the best adsorption capacity, the mechanical properties decrease after adsorption and it is more obvious with the increasing of adsorbed gas number, which indicates that the mechanical properties of composition B become worse. The maximum trigger bond length decreases, while the interaction energy of trigger bond and cohesive energy density increase with the increasing of gas number, thus illustrating that the sensitivity of composition B decreases.

Mechanism of water adsorption in the large pore form of the gallium-based MIL-53 metal-organic framework

Water adsorption in the large pore (lp_empty) form of Ga-MIL-53 was studied by TGA, DSC and in situ XRD and FTIR at 298 K. The large pore form can be stabilized at room temperature after activation under vacuum at 553 K. The isotherm of water adsorption in this large pore form (pore dimensions: 1.67 × 1.33 nm) is very similar to that measured on the narrow pore (np_empty) form (pore dimensions: 1.97 × 0.76 nm). Such a similarity is rather unusual given that the pore sizes of these two phases are very different. In order to understand the origin of this effect in situ XRD and FTIR measurements were particularly helpful. It was found that the adsorption of even small amount of water (0.05 mol per Ga atom at 0.2 hPa) in the large pore form of Ga-MIL-53 transforms ca. 50% of the solid into a narrow pore int phase, which is assumed to be present as a shell around the lp_empty core. Additional water molecules adsorbed at higher pressures do not interact with the parent lp_empty phase but with the narrow pore int phase. The phase transformations were confirmed by FTIR revealing significant band displacements in the corresponding pressure ranges. Such easy pore shrinking which occurs at very low water pressure (<0.2 hPa) can have undesirable consequences in working conditions, as for example in separation adsorption processes, because the large pore structure of Ga-MIL-53 can be preserved only under anhydrous conditions.

Water adsorption on TiO2 surfaces probed by soft X-ray spectroscopies: bulk materials vs. isolated nanoparticles.

We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs’ surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.

Scanning curves of water adsorption on graphitized thermal carbon black and ordered mesoporous carbon

Adsorption isotherms of water on porous carbons generally show large hysteresis loops whose origin is believed to be different from simple gases adsorption in mesoporous solids. In this paper, we discussed in details the behavior of water adsorption isotherms and their descending scanning curves for two carbons of different topologies, a highly graphitized thermal carbon black, Carbopack F, and a highly ordered mesoporous carbon, Hex. For both solids, very large hysteresis loops are observed, but their behaviors are different. For Carbopack F, the loop extends over a very wide range of pressure and the loop is larger when the descending is started from a higher loading; while for Hex, the hysteresis loop shows distinct steps, the number of which depends on the loading where the descending starts. By carefully analyzing the scanning curves from different loadings, we established the mechanism of water adsorption in Hex as a sequence of three steps: (1) water molecules adsorb on functional groups located at the junctions between adjacent basal planes of graphene layers, (2) growth of water clusters around the functional groups, and (3) bridging of adjacent clusters to form larger clusters, followed by a complete filling of mesopores.

Mechanisms of water adsorption into partially saturated fractured shales: An experimental study

The physico-chemical processes of water adsorption into water saturated shale rocks have been extensively studied in the past. However, the physico-chemical processes of water uptake into partially saturated shales, such as water loss to fractured organic-rich shales during hydraulic fracturing, are poorly understood which in turn has raised serious technical and environmental concerns. Therefore, we ran a series of experiments on partially saturated calcareous shale samples, with measured total organic carbon (TOC) of 1.935% and mean vitrinite reflectance (Ro) of 0.52%, to identify the potential mechanisms involved in the water uptake. The physico-chemical properties such as mineral and elemental compositions, initial water saturation and contact angles of solutions with different concentrations were measured. Free and confined adsorption tests were then performed on both the intact and artificially fractured shale samples.The experimental results of this study are explained by two proposed mechanisms; capillary hydration and surface–osmotic hydrations that can combine to influence the amount of water uptake into partially saturated shales. The results show that the crack initiation is a strong function of confining stress and cracks are formed parallel to weak structures of the rock (bedding planes and laminations). The results also reveal that the mineral hydration controls the permeability (and consequently water flow) of existing fractures within the shale sample.

Water at Curved Carbon Surface: Mechanisms of Adsorption Revealed by First Calorimetric Study

Water adsorption isotherms and calorimetrically measured enthalpy of this process are reported for a series of modified chemically single and multiwalled nanotubes. On the basis of calorimetric measurements, entropy of adsorption is calculated and discussed. Next the data are described using popular models of adsorption, and finally a new approach for simultaneous description of water adsorption and enthalpy of this process is discussed. On the basis of the results of this model, four different possible mechanisms of water adsorption in nanotubes are proposed.

Advances on Biomedical Titanium Surface Interactions

When used as an implanted material, titanium (Ti) surface controls the subsequent biological reactions and leads to tissue integration. Cells interactions with the surface, through a protein layer that is being formed from the moment Ti surface comes in contact with blood and its components, and indeed this protein layer formation, are regulated by surface properties such as topography, chemistry, charge and surface energy. Currently, the implementation of nanotechnology, in an attempt to support mimicking the natural features of extracellular matrix, has provided novel approaches for understanding and translating surface mechanisms whose modification and tailoring are expected to lead to enhanced cell activity and improved integration. Despite the fact that there has been extensive research on this subject, the sequence of interactions that take place instantly after the exposure of the implanted material into the biologic microenvironment are not well documented and need further investigation as well as the optimization of characteristics of Ti surface. This review, including theoretical and experimental studies, summarizes some of the latest advances on the Ti surface concerning modifications on surface properties and how these modifications affect biomolecular reactions and also attempts to present the initial adsorption mechanism of water and protein molecules to the surface.

Structures and Mechanisms of Water Adsorption on ZnO(0001) and GaN(0001) Surface

The adsorption structures and mechanisms of water adsorption on ZnO(0001) and GaN(0001) surface are investigated by using the first-principles methods. It is found that the stable adsorption structure at full monolayer (ML) coverage is (2 × 1) reconstructed. A (2 × 1) molecular adsorption is definite for ZnO, and a (2 × 1) dissociative adsorption is also possible for GaN. For these structures the hydrogen bonds between adsorbates are significant besides the covalent interaction with substrate. For the coverage below 0.5 ML for GaN and 0.25 ML for ZnO, the individually adsorbed H2O can easily decompose to OH and H. Both covalent and electrostatic attractions contribute to the stability of dissociative adsorption. For the coverage between the above two cases, molecular adsorption is found to be stable in theory, but the real structure may be greatly dependent on the chemical condition. These results give a detailed description of the interaction between the first water adlayer and ZnO(GaN)(0001) surface.

A first-principles characterization of water adsorption on forsterite grains

Numerical simulations examining chemical interactions of water molecules with forsterite grains have demonstrated the efficacy of nebular gas adsorption as a viable mechanism for water delivery to the terrestrial planets. Nevertheless, a comprehensive picture detailing the water-adsorption mechanisms on forsterite is not yet available. Towards this end, using accurate first-principles density functional theory, we examine the adsorption mechanisms of water on the (001), (100), (010) and (110) surfaces of forsterite. While dissociative adsorption is found to be the most energetically favourable process, two stable associative adsorption configurations are also identified. In dual-site adsorption, the water molecule interacts strongly with surface magnesium and oxygen atoms, whereas single-site adsorption occurs only through the interaction with a surface Mg atom. This results in dual-site adsorption being more stable than single-site adsorption.

Hydrophobicity of Hydroxylated Amorphous Fused Silica Surfaces

Understanding the mechanism of water adsorption on silica is important in many fields of science and technology, such as geo- and atmospheric chemistry. Vibrational IR-visible sum-frequency generation (SFG) spectroscopy of hydroxyls (~3100-3800 cm(-1)) at the amorphous SiO2 surface in contact with air of varying relative humidity provides information about the adsorption sites and orientation of water molecules. The similar magnitudes of the resonant and nonresonant contributions to the interfacial second-order susceptibility, χ((2)), allow the phases of the various hydroxyls (SiOH; HOH), and thus their orientations with respect to the surface, to be determined. The surface silanols (SiOH) appear to interact weakly with adsorbed water as indicated by the persistence of the narrow surface silanol (SiOH) peak at ~3750 cm(-1) as the relative humidity of ambient air increases from <5% to >95%. Adsorbed water molecules are represented by two oppositely oriented hydroxyl modes, at ~3350-3400 and ~3650 cm(-1), respectively. The weakly hydrogen-bonded water hydroxyls (~3650 cm(-1)) are oriented toward the silica substrate and are assigned to water molecules that aggregate over the hydrophobic silica areas with exposed siloxane bridges. We believe that this is the first experimental identification of water molecules in contact with siloxane network whose hydrophobic nature has been predicted by molecular dynamics simulations for tetrahedral (SiO4) surface of kaolinite. The SFG data suggest that, at the molecular level, hydroxylated amorphous fused silica has hydrophobic character.

Free Energy and Electronic Properties of Water Adsorption on the SnO2(110) Surface

A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

High Surface Reactivity and Water Adsorption on NiFe2O4 (111) Surfaces

Transition metal-doped ferrites are attractive candidates for a wide range of applications including catalysis and electronic and magnetic devices. Although their bulk characteristics are well-understood, very little is known about their surface properties at the molecular level. Here, we demonstrate high reactivity of NiFe2O4 (111) surfaces, a Ni-doped ferrite, by elucidating the surface structure and water adsorption mechanism using density functional theory with on-site correction for Couloumb interaction (DFT + U). The surface reactivity of NiFe2O4 (111) surfaces (with 0.25 ML Fetet1 and 0.5 ML Feoct2–tet1 terminations) is shown to be significantly higher in comparison with the undoped Fe3O4 (111) surfaces. Dissociation of water is found to be highly favorable with an adsorption energy of −1.11 eV on the 0.25 ML Fetet1 terminated surface and −2.30 eV on the 0.5 ML Feoct2–tet1 terminated surface. In addition, we computed a low activation barrier of 0.18 eV for single water molecule dissociation on the ...

Mechanism of Alcohol–Water Separation in Metal–Organic Frameworks

The metal–organic framework Zn2(BDC)2(TED) (1) has been reported to be water-stable and highly selective toward the adsorption of water and alcohols, suggesting the application of this material as a separation membrane for the production of bioethanol. We have studied the adsorption mechanism of water, methanol, ethanol, and dimethylether in this framework by means of density-functional theory with corrections for London dispersion. We show that the combination of the hydrogen bond between the hydroxyl group in ethanol with the oxy group in 1 and the van der Waals interaction between the ethanol alkyl chain with the phenyl ring in 1 is responsible for the preferential adsorption of ethanol over water in the framework. The calculated enthalpy of adsorption for the four compounds in 1 is in excellent agreement with experimental results. We further note that the computational approach has to be chosen with care: It is essential to account for London dispersion interactions, as well as the use of large models...

First principles calculation on the adsorption of water on lithium–montmorillonite (Li–MMT)

The interaction of water molecules and lithium–montmorillonite (Li–MMT) is theoretically investigated using density functional theory (DFT) based first principles calculation. The mechanism of water adsorption at two different water concentrations on Li–MMT as well as their structural and electronic properties are investigated. It is found that the adsorption stability in Li–MMT is higher in higher water concentration. It is also found that an adsorbed water molecule on Li–MMT causes the Li to protrude from the MMT surface, so it is expected that Li may be mobile on H2O/Li–MMT.

The interfacial modification of rice straw fiber reinforced poly(butylene succinate) composites: Effect of aminosilane with different alkoxy groups

AbstractThe effect of aminosilane on the properties of rice straw fiber (RSF) reinforced poly(butylene succinate) (PBS) composites was studied. RSF was pretreated with four different aminosilane coupling agents, 3‐aminopropyltriethoxysilane (APTES), 3‐aminopropyltrimethoxysilane (APTMES), 3‐(2‐aminoethylaminopropyl)triethoxysilane (AEAPTES), and 3‐(2‐aminoethylaminopropyl)‐trimethoxysilane (AEAPTMES). The results of Fourier transform infrared spectroscopy (FTIR) and ζ potential measurements confirmed that amino groups were introduced to the silane‐treated RSF (TRSF) for all four silane coupling agents. The results also indicated a higher degree of hydroxyl ion adsorption by the fiber surface, which was chemically grafted by silane hydroxyl, was obtained with ethoxy silane than with methoxy silane. This difference might be because the relative rates of hydrolysis and ensuing silanol self‐condensation of methoxy silane were too fast to reduce the number of silanol groups grafted on the fiber surface. The TRSF composite produced clearly enhanced tensile properties for the aminosilane coupling agents having ethoxy groups. The AEAPTES‐RSF‐PBS composite showed the highest tensile strength. It might be because of the higher amino content of AEAPTES than APTES; the amino groups on the surface of TRSF were confirmed by FT‐IR to react with the carbonyl groups of PBS to form a blue‐shifted hydrogen bond. The water absorption process of composites was found to follow the kinetics and mechanisms described by Fick's theory. The aminosilane treatment significantly reduced the moisture diffusion coefficient but did not change the mechanism of water adsorption. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

High physisorption affinity of water molecules to the hydroxylated aluminum oxide (0 0 1) surface

The adsorption mechanism of water on the hydroxylated (001) plane of α-Al2O3 was studied by measuring adsorption isotherms and GCMC simulations. The experimental adsorption isotherms for three α-Al2O3 samples from different sources are typical type II, in which adsorption starts sharply at low pressures, suggesting a high affinity of water to the Al2O3 surface. Water molecules are adsorbed in two registered forms (bilayer structure). In the first form, water is registered at the center of three surface hydroxyl groups by directing a proton of the water. In the second form, a water molecule is adsorbed by bridging two of the first-layer water molecules through hydrogen bonding, by which a hexagonal ring network is constructed over the hydroxylated surface. The network domains are spread over the surface, and their size decreases as the temperature increases. The simulated adsorption isotherms present a characteristic two-dimensional (2D) phase diagram including a 2D critical point at 365K, which is higher than that on the hydroxylated Cr2O3 surface (319K). This fact substantiates the high affinity of water molecules to the α-Al2O3 surfaces, which enhances the adsorbability originating from higher heat of adsorption. The higher affinity of water molecules to the α-Al2O3 (001) plane is ascribed to the high compatibility of the crystal plane to form a hexagonal ring network of (001) plane of ice Ih.

Physisorption Structure of Water on the GaN Polar Surface: Force Field Development and Molecular Dynamics Simulations

The adsorption mechanism of water on the GaN (0001) polar surface is investigated via both the Density Functional Theory (DFT) method and its derived classical force field. The physisorption binding energy and the adsorption geometry of the water molecule on the clean Ga-terminated surface are analyzed via the first-principle static calculations. The adsorption energy hypersurfaces are then extracted to be used in the fitting of the interaction potentials between water and GaN. Classical molecular dynamics (MD) simulations based on the developed force field are performed for the interfacial system of liquid water and the GaN surface slab. From our computations, the interfacial water exhibits significant oscillatory profiles for the atomic densities and the molecular orientations. Further data analysis suggests a highly confined first layer with the O being locked right upon the surface Ga atoms and the H pointing toward the neighboring O to form the weakened hydrogen bonds. A bilayer configuration with opposite dipole orientations is consequently characterized as the wetting structure on the GaN polar surface and is explained by the anisotropic perturbations from the surface polar sites. Our simulations would be helpful to provide an atomistic picture for the water adsorption configuration on this semiconductor surface and would be useful in the relevant nanofluidic and nanoengineering applications.

Sensor for detection of water presence in gaseous mixtures based on gold nanoparticles stabilized by sodium citrate

The adsorption properties of organic–inorganic sensitive films formed by gold nanoparticles dropped from sodium citrate solutions of different concentrations are investigated using mass-sensitive (QCM) sensors. The obtained sensitive layers, composed of discontinuous assembly of gold nanoparticles and sodium citrate film have demonstrated several special adsorption properties. The most interesting among them are (i) the reverse dependence of the sensitivity on the initial colloid solution concentration, (ii) the difference of the response value for alcohols and water (the sensitivity to water is more than one order higher than to alcohols) and (iii) the character of water adsorption curves (the presence of two stages of adsorption with different kinetics). The physical mechanism of water adsorption on the citrate–gold surface is considered to be concerned with the presence of two processes: adsorption and diffusion of water molecules along the surface of citrate to the gold nanoparticles. Gold nanoparticles in this case serve as drains for water molecules. The results obtained can be applied for the development of sensors for the detection of water presence in gaseous mixtures of different nature.

A periodic DFT study of water and ammonia adsorption on anatase TiO 2 (001) slab

Water and ammonia adsorption mechanisms on anatase TiO 2 (001) slab surface are investigated by means of periodic DFT approach. Molecular and dissociative adsorption energies for water are calculated to be − 15 kcal/mol and − 32 kcal/mol, respectively. Similarly, molecular and dissociative adsorption energies of ammonia on the same surface are found as − 25 kcal/mol and − 20 kcal/mol. A reverse result in this order is reached for the previous case of ONIOM cluster study (− 23 kcal/mol and − 37 kcal/mol, respectively). The vibration frequency values are computed for the optimized geometries of adsorbed water and ammonia molecules on anatase TiO 2 (001) slab surface and compared with the values reported in the literature.