Free energy of adsorption of water and calcium on the [10 1 4] calcite surface

Molecular dynamics simulations of the calcite-water interface have shown that the free energy of adsorption of water is relatively small compared to the previously calculated enthalpy of adsorption implying a large entropy change and that the free energy profile of a calcium adsorbing on the surface correlates with the solvent density; these calculations allow us to begin to address the rates of adsorption and desorption which are essential for studying growth and dissolution.     

A discussion on the energies involved in specific adsorption of ions

The change in specific adsorption of I^? ions on the series of metals Au, Hg, Bi, Pb, Cd, and Ga is analyzed using data of specifically adsorbed charge and shift in potential of zero charge. Factors determining the change in adsorbability are discussed in the light of previous formulations. It is shown that the work connected with water desorption as an ion becomes adsorbed, usually neglected or underestimated in previous discussions, is very likely to be the main factor determining the change in adsorbability along the series of metals. A rough estimation of energies involved in water desorption suggests that metal—water surface bonds are probably weak on sp-metals so that they are unable to affect the reactivity of metal surfaces with respect to the gas phase as strong covalent surface bonds are involved, for instance in the hydrogen evolution reaction. Conversely, the strong effect of water desorption on the specific adsorption of ions may be an indication of ion—metal interactions to be substantially independent of the nature of the metal. This suggests that covalent contributions to the surface bond are apparently minor for metals more electropositive than Au.     

Critical Evaluation of Adsorption–Desorption Hysteresis of Heavy Metal Ions from Carbon Nanotubes: Influence of Wall Number and Surface Functionalization

Single‐, double‐, and multi‐walled carbon nanotubes (SWCNTs, DWCNTs, and MWCNTs), and two oxidized MWCNTs with different oxygen contents (2.51 wt % and 3.5 wt %) were used to study the effect of the wall number and surface functionalization of CNTs on their adsorption capacity and adsorption–desorption hysteresis for heavy metal ions (Ni^II, Cd^II, and Pb^II). Metal ions adsorbed on CNTs could be desorbed by lowering the solution pH. Adsoprtion of heavy metal ions was not completely reversible when the supernatant was replaced with metal ion‐free electrolyte solution. With increasing wall number and amount of surface functional groups, CNTs had more surface defects and exhibited higher adsorption capacity and higher adsorption–desorption hysteresis index (HI) values. The coverage of heavy metal ions on the surface of CNTs, solution pH, and temperature affect the metal ion adsorption–desorption hysteresis. A possible shift in the adsorption mechanism from mainly irreversible to largely reversible processes may take place, as the amount of metal ions adsorbed on CNTs increases. Heavy metal ions may be irreversibly adsorbed on defect sites.     

Laws and mechanism of adsorption of cations by different ion-exchange forms of silica gel

Using radioactive tracer method, the regularities of adsorption of over 30 mono-, di- and trivalent cations including transition metal and lanthanide ions on the H, Ca- and Al-forms of silica gel are established. It has been shown that the affinity of cations of the same charge to the silica gel surface depends both on the nature of the adsorbing ion and the nature of exchangeable cation on the surface. Adsorption of alkali earth metal ions on the Ca-form of silica gel increases with a decrease of their radius i.e. an inversion of the sequence of adsorption compared to H- or Al-form of this adsorbent or polymeric cation-exchange resins takes place. For lanthanide ions the sequence of adsorption is the same for all ion-exchange forms of the silica gel studied, namely, an increase of adsorption with a decrease of their crystallographic radius, i.e. from La ³⁺ to Lu ³⁺ takes place. The laws observed are explained by taking into account the fact that adsorption of cations by silica gel is determined by both electrostatic interactions and additional covalent/donor–acceptor interactions between the surface and cations. The latter is due to formation of π-bonds between the electron pair in surface oxygen and vacant p-, d- or f-orbital of adsorbing cations.     

Free energy partitioning analysis of the driving forces that determine ion density profiles near the water liquid-vapor interface

Free energy partitioning analysis is employed to explore the driving forces for ions interacting with the water liquid-vapor interface using recently optimized point charge models for the ions and SPC/E water. The Na(+) and I(-) ions are examined as an example kosmotrope/chaotrope pair. The absolute hydration free energy is partitioned into cavity formation, attractive van der Waals, local electrostatic, and far-field electrostatic contributions. We first compute the bulk hydration free energy of the ions, followed by the free energy to insert the ions at the center of a water slab. Shifts of the ion free energies occur in the slab geometry consistent with the SPC/E surface potential of the water liquid-vapor interface. Then the free energy profiles are examined for ion passage from the slab center to the dividing surface. The profiles show that, for the large chaotropic I(-) ion, the relatively flat total free energy profile results from the near cancellation of several large contributions. The far-field electrostatic part of the free energy, largely due to the water liquid-vapor interface potential, has an important effect on ion distributions near the surface in the classical model. We conclude, however, that the individual forms of the local and far-field electrostatic contributions are expected to be model dependent when comparing classical and quantum results. The substantial attractive cavity free energy contribution for the larger I(-) ion suggests that there is a hydrophobic component important for chaotropic ion interactions with the interface.     

Multilayer adsorption of water at a rutile TiO2)(110) surface: towards a realistic modeling by molecular dynamics

We present a model combining ab initio concepts and molecular dynamics simulations for a more realistic treatment of complex adsorption processes. The energy, distance, and orientation of water molecules adsorbed on stoichiometric and reduced rutile TiO(2)(110) surfaces at 140 K are studied via constant temperature molecular dynamics simulations. From ab initio calculations relaxed atomic geometries for the surface and the most probable adsorption sites were derived. The study comprises (i) large two-dimensional surface supercells, providing a realistically low concentration of surface oxygen defects, and (ii) a water coverage sufficiently large to model the onset of the growth of a bulk phase of water on the surface. By our combined approach the influence of both, the metal oxide surface, below, and the bulk water phase, above, on the water molecules forming the interface between the TiO(2) surface and the water bulk layer is taken into account. The good agreement of calculated adsorption energies with experimental temperature programmed desorption spectra demonstrates the validity of our model.     

Gas-Phase Reactions of hydrated alkaline earth metal ions, M2+(H2O) n (M = Mg, Ca, Sr, Ba and n = 4–7), with benzene

Gas-phase reactions of hydrated divalent alkaline earth metal ions and benzene were investigated by electrospray ionization Fourier-transform mass spectrometry. Rate constants for solvent-exchange reactions were determined as a function of hydration extent for Mg2+, Ca2+, Sr2+, and Ba2+ clusters containing four to seven water molecules each. All of the strontium and barium clusters react quickly with benzene. Barium reacts slightly faster than the corresponding strontium cluster with the same number of water molecules attached. For calcium, clusters with four and five water molecules react quickly, whereas those with six and seven water molecules do not. Magnesium with four water molecules reacts quickly, but not when five through seven water molecules are attached. The slow reactivity observed for some of these clusters indicates that the cation-pi interaction between the metal ion and benzene is partially screened by the surrounding water molecules. The reactivity of magnesium with seven water molecules is intermediate that of the hexa- and pentahydrate and the tetrahydrate. This result is consistent with the seventh water molecule being in the outer shell and much more weakly bound. The unusual trend in reactivity observed for magnesium may be due to the presence of mixed shell structures observed previously. These results are the first to provide information about the relative importance of cation-pi interactions in divalent metal ions as a function of metal hydration extent. Such studies should also provide a model and some insight into the relative binding affinities of divalent metal ions to aromatic residues on peptides and proteins.     

Adsorption of alkali and alkaline earth radionuclides on zeolite from water solutions

The adsorption of cesium and strontium ions from water solutions on zeolite has been investigated in presence of sodium, potassium, magnesium and calcium ions. Distribution ratios of cesium and strontium on the zeolite were determined in solutions of various compositions and solution volume to sorbent weight ratios (batch factor). Breakthrough curves for zeolite layers are reported.     

Adsorption of cadmium on anatase nanoparticles-effect of crystal size and pH

The adsorption and desorption of Cd(2+) to large and nanometer-scale anatase crystals have been studied to determine the relationship between heavy metal adsorption properties and anatase particle size. A solvothermal method was used to synthesize very fine anatase nanocrystals with average grain sizes ranging from 8 to 20 nm. On a surface area basis, it was found that large and nanometer-scale anatase particles had similar maximum Cd(2+) adsorption capacities, while their adsorption slopes differed by more than 1 order of magnitude. The particle-size effect on adsorption is constant over a pH range of 4-7.5. The desorption of Cd(2+) from both particle sizes is completely reversible. The adsorption data have been modeled by the Basic Stern model using three monodentate surface complexes. It is proposed that intraparticle electrostatic repulsion may reduce the adsorption free energy significantly for nanometer-sized particles.     

Interaction between water molecules and zinc sulfide nanoparticles studied by temperature-programmed desorption and molecular dynamics simulations

We have investigated the bonding of water molecules to the surfaces of ZnS nanoparticles (approximately 2-3 nm sphalerite) using temperature-programmed desorption (TPD). The activation energy for water desorption was derived as a function of the surface coverage through kinetic modeling of the experimental TPD curves. The binding energy of water equals the activation energy of desorption if it is assumed that the activation energy for adsorption is nearly zero. Molecular dynamics (MD) simulations of water adsorption on 3 and 5 nm sphalerite nanoparticles provided insights into the adsorption process and water binding at the atomic level. Water binds with the ZnS nanoparticle surface mainly via formation of Zn-O bonds. As compared with bulk ZnS crystals, ZnS nanoparticles can adsorb more water molecules per unit surface area due to the greatly increased curvature, which increases the distance between adjacent adsorbed molecules. Results from both TPD and MD show that the water binding energy increases with decreasing the water surface coverage. We attribute the increase in binding energy with decreasing surface water coverage to the increasing degree of surface under-coordination as removal of water molecules proceeds. MD also suggests that the water binding energy increases with decreasing particle size due to the further distance and hence lower interaction between adsorbed water molecules on highly curved smaller particle surfaces. Results also show that the binding energy, and thus the strength of interaction of water, is highest in isolated nanoparticles, lower in nanoparticle aggregates, and lowest in bulk crystals. Given that water binding is driven by surface energy reduction, we attribute the decreased binding energy for aggregated as compared to isolated particles to the decrease in surface energy that occurs as the result of inter-particle interactions.     

The interaction of water with Ni(111) and H/Ni(111) studied by TPD and HREELS

We have used temperature-programmed desorption in combination with specular and off-specular high resolution electron energy loss spectroscopy to study the interaction of H(2)O and D(2)O with the bare and hydrogen-covered Ni(111) surface. Our results for the bare metal surface agree with previous reports and we are able to relate two prominent features in vibrational spectra to nuclear motions at the surface. Pre-covering Ni(111) with hydrogen alters both adsorption and desorption of water significantly. The strong H-Ni bond does not allow for isotopic exchange with co-adsorbed D(2)O. Strong resemblance of desorption traces and vibrational spectra of submonolayer coverages on H-covered Ni(111) and multilayers on bare Ni(111) suggests that adsorption of hydrogen makes this nickel surface hydrophobic.     

Studies on the Zetapotential of Calcite/p-Sulfonato-calix[4,8]arenes

Zeta potential of calixarenes has been reported for the first time. The water-soluble calixarenes has been used as dispersion media in solid/liquid interface. p-sulfonato-calix[4]arene (PSC4) and p-sulfonato-calix[8]arene (PSC8) were synthesized and characterized by FTIR, NMR, mass spectrometry, and HPLC techniques. It was proved that the zeta potential is a fast and simple measurement to know the adsorption behavior of sufonato calixarnes on calcite. The chemisorption of p-sulfonato-calix[n]arene was confirmed by shift in iso electric point, adsorption studies and FTIR. The calculated free energy of adsorption value and its sign suggests the chemical interaction between the calcite surface and p-sulfonato calix[4]and[8]arene.     

银纳米粒子表面油酸盐吸附的分子模拟

采用分子力学方法模拟了油酸盐分子在银纳米粒子(111)表面的两种主要吸附形态: 双键吸附和羧基吸附, 计算了不同溶剂环境中油酸盐分子的吸附能和振动频率. 结果表明: 作为银纳米粒子保护剂的油酸盐能够稳定地存在于极性和非极性溶剂环境中, 与银表面的作用方式随溶剂极性的改变而改变, 水相中以双键吸附为主, 而在有机相中主要是羧基吸附. 这与红外光谱实验的推论基本吻合. 从而在分子水平上为实验结果提供了有价值的理论支持和微观信息.     

Facile sol–gel synthesis of thiol-functionalized materials from TEOS-MPTMS-PMHS system

Thiol-functionalized sol–gel materials with porous structures were successfully prepared via co-condensation of tetraethoxysilane, (3-mercaptopropyl)-trimethoxysilane, polymeric polymethylhydrosiloxane in the absence of traditional structure-directing agents under proposed basic conditions. The dependence of textural characteristics on the preparation parameters were investigated employing various techniques as such N₂ adsorption/desorption isotherms, SEM, TG-DTG, FT-IR and solid state ²⁹Si MAS NMR measurements. It was shown by these characterizations that thiol-functional groups have been successfully introduced into the porous network and the loadings could be tuned by adjusting the content of corresponding precursor in the gel system. Besides, the as-prepared samples exhibited high surface areas, large pore volume, tunable pore diameters and favorable thermal stability. Adsorption experiments indicated that thiol-incorporated materials have selective adsorption ability toward heavy metal ions from aqueous solutions, and a high capacity in adsorbing heavy metal ion of Pb²⁺ (up to 99.4%) was observed under optimized experimental conditions.     

Sr掺杂对CaO(100)表面吸附甲醇影响的分子模拟

借助分子模拟手段,研究了锶掺杂对氧化钙表面甲醇吸附行为的影响。构建了甲醇在CaO(100)和CaO(100)-Sr表面吸附的模型,计算了甲醇在氧化钙表面的吸附能和解离活化能,分析了甲醇在氧化钙表面成键的态密度以及锶掺杂前后甲醇在氧化钙表面电荷布局和差分电荷密度,评估了锶掺杂量对氧化钙表面甲醇吸附性能的影响。结果表明,锶掺杂能够显著强化氧化钙对甲醇的吸附性能,降低甲醇的解离活化能,且吸附性能随锶掺杂量的增加而增强;甲醇在氧化钙表面吸附时活化,锶掺杂后活化程度增加。     

Interaction of 2-Amino-, 3-Amino-, and 4-Aminopyridines with Chromium and Manganese Ferrocyanides

The present investigation deals with the interaction of 2-aminopyridine, 3-aminopyridine, and 4-aminopyridine with chromium and manganese ferrocyanides. Chromium ferrocyanide possesses better adsorbing properties than manganese ferrocyanide. Maximum uptake was observed at neutral pH (pH 7.0). The adsorption data obtained at neutral pH are fitted in a Langmuir adsorption isotherm. The adsorption behavior of the aminopyridines studied follows the order 3-aminopyridine >4-aminopyridine >2-aminopyridine. The infrared spectral studies of adsorption adducts indicate that adsorption takes place through interactions between the adsorbate molecule and the outer divalent metal ion of metal ferrocyanides. From these studies, it is clear that metal ferrocyanides and metal ions play a major role in the stabilization of organic molecules through their surface activity in the prebiotic environment. Copyright 2000 Academic Press.     

INVESTIGATION OF THE INTERACTIONS BETWEEN WATER AND MODIFIED SILICA GEL BY IGC AND TPD

In this work, the thermodynamic parameters for the adsorption of water vapor on untreated silica gel and silica gel treated with hygroscopic salts and silane coupling agent were determined by Inverse Gas Chromatography (IGC) in the infinite dilution region. The desorption activation energies of the water vapor on virgin and modified silica gels were estimated by using the Temperature Programmed Desorption (TPD) technique. The interactions between the water and the virgin and modified silica gels were discussed. Results showed that the thermodynamic parameters and desorption activation energy of water vapour on the silica gels increase with decreasing pore size and increasing the surface hydrophilic properties. The desorption activation energy of virgin and modified silica gels was found to increase with increasing the thermodynamic parameters. The larger the adsorption parameters and the desorption activation energy were, the interactions between water and virgin and modified silica gels were.     

Removal of multi-metals from water using reusable pectin/cellulose microfibers composite beads

Pectin (Pec) and cellulose microfibers (CF) extracted from orange waste were combined to form composite beads with enhanced adsorption capacity. Such beads were extensively tested in the removal of multi-metal ions from water. A factorial design approach was conducted to establish the optimum conditions for adsorption of Cd(II), Cu(II), and Fe(II) on Pec-CF beads. Batch adsorption experiments revealed that removal efficiency of such metal ions falls in the range of 94–58% and it followed the order Fe(II) > Cu(II) > Cd(II). The maximum Cd(II), Cu(II) and Fe(II) adsorption capacities calculated from the Langmuir isotherm were 192.3, 88.5 and 98.0 mg/g, respectively. FTIR analysis suggests that the functional groups on Pec-CF beads (binding sites) favor the adsorption of such metal ions. Desorption and reuse experiments demonstrated the beads could be used for at least five consecutive adsorption/desorption cycles. Our finds suggest the Pec-CF beads can serve as an efficient adsorbent for the removal of multi-metal ions from wastewater.     

INVESTIGATION OF THE INTERACTIONS BETWEEN WATER AND MODIFIED SILICA GEL BY IGC AND TPD

1. INTRODUCTIONSilica gel is an amorphous inorganic polymer composed of siloxane (Si-O-Si) groups in the inner region and silanol (Si-OH) groups distributed on the surface [1]. Silanol groups can be easily functionalized by different chemical procedures. …     

Aligned nanogold assisted one step sensing and removal of heavy metal ions

We depict a novel strategy exploiting the chemistry of metal ion adsorption for detection and sequestration of toxic heavy metal from processed water using gold nanoparticles capped with 4-aminothiophenol. The interaction between 4-aminothiophenol capped gold nanoparticles and heavy metal ions was studied as a function of time and concentration using TEM, HRTEM, SEM, EDS, and I-V characterization. Experiments confirmed that pH is one of the crucial controlling parameters. Adsorption capacity was monitored using AAS, UV-vis spectroscopy and I-V measurement. In the absence of any alloy formation between Au and heavy metal ions, the desorption of the heavy metal ions from 4-aminothiophenol capped gold nanoparticles surface by pH modulation serves as a mean of collection of heavy metal ions. Experiments revealed that the concentration of heavy metal ions in processed water after adsorption is below the maximum permissible limit set by the WHO.