Layer-by-layer adsorption of water molecules on the surface of a silver iodide crystal

Computer simulations at the molecular level were used to analyze the mechanism of the nucleation of water condensate from the vapor phase on the surface of a silver iodide crystal at 260 K. The initial stage of the condensation process is the sequential growth of monolayers on the substrate surface without formation of a compact microdroplet. The dependence of the equilibrium work of formation of the condensate film on its thickness exhibits oscillations. The formation of layers close to the substrate surface involves the overcoming of a Gibbs energy barrier.     

Quantum chemical model for the adsorption of organic molecules on metal surfaces in electrolyte solutions

A quantum chemical model is proposed for the adsorption of organic molecules on metal surfaces in electrolyte solutions. This approach is based on the method of electrostatic images modefied for the case of a double electrical layer within the framework of the PPP method. The calculated data are in good accord with the available experimental results. Donbas State Building and Architecture Academy, 2 Derzhavina ul., Makeevka 339023, Donetsk Oblast', Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 5, pp. 284–288, September–October, 1999.     

金与银电极表面甘氨酸分子吸附作用的SERS光谱研究

本文利用表面增强拉曼光谱(SERS)技术研究了甘氨酸在金与银基底表面的吸附作用特征。研究表明甘氨酸分子以COO-的不对称形式吸附于金电极表面,且NH2也是其可能的吸附位点;而在银电极表面,则主要是通过COO-的对称形式而吸附。在此基础上,进一步研究了电极电位与溶液酸碱性对吸附于粗糙化银电极表面甘氨酸分子吸附作用的影响。研究结果表明,甘氨酸分子中去质子化羧基的吸附作用受电位影响较小,而电位对-NH3+吸附作用的影响程度较大。另一方面,溶液pH值对银电极表面的甘氨酸分子吸附行为的影响也较为显著。随着溶液酸性减小羧基倾向于相对于电极表面平行吸附。这是由于随着溶液碱性增大氨基质子化程度的减小,有利于氨基在银电极表面吸附。这将改变分子的吸附构型使其更接近于电极表面。这些变化主要出现在pH值大于10的条件下。     

A study of surface adsorption on silver powders

DSC and TG-DTA techniques were used to investigate micro-sized silver powder particles and the adsorption of ethyl cellulose on these particles in a solution of ethyl acetate. The apparent specific heat of the silver particles was determined, and the kinetics of temperature-programmed desorption (TPD) of these adsorbed silver particles was investigated. Results show that the apparent specific heat and desorption kinetic parameters obtained by thermal analysis techniques could be used to characterize certain physico-chemical properties of such a particulate system.     

Physical adsorption of organic molecules on the surface of layered silicate clay platelets: a thermogravimetric study

Physical adsorption of various adsorbents on the surface of premodified montmorillonite platelets was performed to fully organophilize the inorganic platelets for the purpose of their easy nanoscale dispersion in the polymer matrices during compounding. Different extents of adsorption could be achieved owing to the nature and the functionality of the adsorbents. High molecular weight adsorbents not only enhanced the organic coverage of the platelets but also were observed to contribute toward the thermal stability improvement of the organic modification, thus further fitting the use of such clays for high temperature compounding. The amount of adsorption could also be quantified with respect to the initial amount of adsorbent used in the process. The importance of a clean surface free from any excess surface modification or adsorbent molecules was emphasized. The adsorption process is an effective means to generate such high potential montmorillonites and is much simpler in technique than the common methods of grafting of polymer chains from the clay surface.     

Adsorption of organic molecules on silica surface

The adsorption behaviour of various organic adsorbates on silica surface is reviewed. Most of the structural information on silica is obtained from IR spectral data and from the characteristics of water present at the silica surface. Silica surface is generally embedded with hydroxy groups and ethereal linkages, and hence considered to have a negative charged surface prone to adsorption of electron deficient species. Adsorption isotherms of the adsorbates delineate the nature of binding of the adsorbate with silica. Aromatic compounds are found to involve the pi-cloud in hydrogen bonding with silanol OH group during adsorption. Cationic and nonionic surfactants adsorb on silica surface involving hydrogen bonding. Sometimes, a polar part of the surfactants also contributes to the adsorption process. Styryl pyridinium dyes are found to anchor on silica surface in flat-on position. On modification of the silica by treating with alkali, the adsorption behaviour of cationic surfactant or polyethylene glycol changes due to change in the characteristics of silica or modified silica surface. In case of PEG-modified silica, adsolubilization of the adsorbate is observed. By using a modified adsorption equation, hemimicellization is proposed for these dyes. Adsorptions of some natural macromolecules like proteins and nucleic acids are investigated to study the hydrophobic and hydrophilic binding sites of silica. Artificial macromolecules like synthetic polymers are found to be adsorbed on silica surface due to the interaction of the multifunctional groups of the polymers with silanols. Preferential adsorption of polar adsorbates is observed in case of adsorbate mixtures. When surfactant mixtures are considered to study competitive adsorption on silica surface, critical micelle concentration of individual surfactant also contributes to the adsorption isotherm. The structural study of adsorbed surface and the thermodynamics of adsorption are given some importance in this review.     

Modeling the surface of polystyrene and the adsorption of dye molecules on this surface

Models of the structures of complexes of the Nile red dye (NR) molecule on the surface of various types of polystyrene (PS) were constructed by molecular dynamics (MD). The surface of a polystyrene film and the surfaces of clumps of isolated polystyrene chains were examined as surfaces. The film and the clumps were obtained as a result of molecular-dynamics trajectories. The atomistic version of molecular dynamics was used. The surface of the film was comparatively uniform, and the clumps were irregular ellipsoids with varied local surface form. The variety of the surface forms makes it possible to obtain complexes of PS with local environments having different structures. A method of constructing the solvent-accessible surface (SAS) was proposed as method of evaluating the potential sensor characteristics of the material. In the PS clumps the NR molecule is almost completely submerged in the upper layer, while in the film the aromatic fragment of the dye is partly accessible to the analyte.     

A model study on the adsorption of hydrogen,oxygen and carbon monoxide on organic molecules and polymers

The ab initio Hanree-Fock method and a London-type approximation to calculate the dispersion energy have been employed to study the chemisorption of H₂, O₂ and CO on benzene and model macromolecules built up from phenylene units. Different orientations of the adsorbed molecule relative to the aromatic hydrocarbon have been considered. The intermolecular distance has been optimized with respect to the energy for each composite system. The dependence of the interaction energy on the intermolecular distance and on the number of phenyl units has been investigated.     

Calculation of the adsorption of interacting molecules on a stepped surface

A theory of adsorption which allows simultaneously for the nonuniformity of a surface and for lateral interactions between the adsorbed species is able to account for the main features of a stepped surface. The effect of the above factors on the adsorption isotherm and the heat of adsorption is investigated. A sudden increase in the heat of adsorption is predicted when an ordered distribution of the adsorbed species has been realized.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1943–1948, September, 1989.     

Application of organoclays for the adsorption of recalcitrant organic molecules from aqueous media

Water purification is imperative for the welfare of a healthy population. Water is widely contaminated by recalcitrant organic chemicals such a pesticides, herbicides and hormones. One inexpensive method for purifying water from these types of molecules is through adsorption. One suite of materials for this adsorption is based upon organoclays. This paper reviews the adsorption of organics on organoclays.     

Electroless silver plating of the surface of organic semiconductors

The integration of nanoscale processes and devices demands fabrication routes involving rapid, cost-effective steps, preferably carried out under ambient conditions. The realization of the metal/organic semiconductor interface is one of the most demanding steps of device fabrication, since it requires mechanical and/or thermal treatments which increment costs and are often harmful in respect to the active layer. Here, we provide a microscopic analysis of a room temperature, electroless process aimed at the deposition of a nanostructured metallic silver layer with controlled coverage atop the surface of single crystals and thin films of organic semiconductors. This process relies on the reaction of aqueous AgF solutions with the nonwettable crystalline surface of donor-type organic semiconductors. It is observed that the formation of a uniform layer of silver nanoparticles can be accomplished within 20 min contact time. The electrical characterization of two-terminal devices performed before and after the aforementioned treatment shows that the metal deposition process is associated with a redox reaction causing the p-doping of the semiconductor.     

New insights on the stereodynamics of ethylene adsorption on an oxygen-precovered silver surface

The control of spatial orientation of molecules has a great influence on the stereodynamics of elementary processes occurring both in homogeneous and heterogeneous phases. Nonpolar molecules have so far escaped direct experimental investigations because of their poor sensitivity to several external constraints. Recently, it has been shown that the collisional alignment produced in supersonic expansions coupled with molecular-beam velocity selection can help solve such problems. Here we show that the sticking probability of ethylene, a nonpolar molecule prototypical of unsaturated hydrocarbons, on an O(2)-precovered Ag(001) surface is larger for molecules approaching in a helicopter-like motion than for those cartwheeling. A mechanism involving a weakly bound precursor state is suggested, with helicopter molecules having a lower chance of being scattered back into the gas phase than cartwheels when colliding with preadsorbed ethylene.     

Quantum mechanical model for the dissociative adsorption of diatomic molecules on metal surfaces

A quantum mechanical nonadiabatic theory of dissociative adsorption of diatomic molecules X2 on metal surface is presented. The following reaction coordinates are used to construct crossing diabatic potential energy surfaces (PES): the distance y between the atoms of the X2 molecule, the distance x of the X2 molecular axis from the surface, the set of coordinates describing possible displacements of metal atoms under adsorption. Expression for the rate constant is derived using the model potentials describing vibrations along these coordinates. The calculated dependency of the rate constant W on the reaction heat DeltaE is compared with that in classical approximation. It is shown that quantum effects lead to a weaker dependence of W on DeltaE as compared to that for classical one.     

混合式格点法研究氧在银或银合金表面的吸附动力学

本文改进了混合式格点法对第一时间步的计算方法, 在保持原有精度的基础上, 减少计算时间约三个数量级。用这一方法, 研究了氧在银及其合金表面的吸附动力学。计算表明: 氧分子在银表面有效吸附的反应阈值是6.29kJ/mol, 这和实验所得的活化能相同。当氧分子动量大于45a.u.或合金中金配比大于0.30时,氧分子均无法在银及其合金表面形成稳定吸附, 这此结果和实验一致。计算中没有发现分子氧直接解离成原子氧的现象。从计算结果中推测, 处在振动激发态的氧分子比处在振动基态的氧分子更容易吸附在银表面。     

Second virial coefficients of adsorption isotherms of organic molecules on porous polymers

The dependence of the specific retention volumes of sorbates on their concentration in the gas phase is investigated. It is shown that the slope tangent of the dependence is related to the second coefficient of the virial expansion of adsorption isotherm. It is established that the free energy of sorbate-sorbate interactions on the surface of porous polymer sorbents is constant for all investigated molecules. An equation for calculating retention volumes at different sorbate concentrations is proposed.     

Analysis of equal conformity of differential capacitance curves upon the adsorption of organic molecules with different models of surface layer

The differential capacitance curves are calculated, by using the model of three parallel capacitors, both for the electrical double layer dense part and the surface layer as a whole, at different ratios of the attraction constants in this model: a ₁₁, a ₁₂, and a ₂₂. It is shown by regressive analysis of the curves that these capacitance curves agree well with the model of two parallel capacitors, provided allowance is made for the linear potential dependence of the effective attraction constant in this model. When allowance is made for the electrical double layer diffuse part, the agreement with the model of two parallel capacitors is still improved, not only at the attraction constant critical value a ₁₂ = 2 but also at its smaller values (1.5 and 1.0). However, at a ₁₂ = 0.5 and rather high concentration of organic substances whose molecules adsorb in two different orientations the curves as if get divided into two parts separated with a maximum that reflects the process of the adsorption layer transformation. Under these conditions the model of two parallel capacitors turns to be inapplicable. It follows from the obtained results that the observed good applicability of the model of two parallel capacitors to the experimental data on the adsorption of numerous aliphatic compounds at electrodes by no means implies that the adsorption layer structure in the studied systems actually corresponds to the physical picture on which the model is based.     

Study of the adsorption of organic molecules on transition metal exchanged zeolites via solid state NMR. Part 2: adsorption of organic molecules on zeolite NaX, CaX, and CaCoX

Literature [Denayer et al. Microporous Mesoporous Mater. 2007, 103, 1 and Denayer et al. Microporous Mesoporous Mater. 2007, 103, 11] shows that zeolite NaX exchanged with Ca(2+) and Co(2+) ions is able to remove cyclopentadiene (CPD) impurities from a 1-octene feed with high selectivity. In the present work, the adsorption of dicyclopentadiene (DCPD), CPD, 1-octene, and n-octane on zeolite X, exchanged with Ca(2+) and/or Co(2+) ions, has been investigated via (1)H magic-angle spinning (MAS) NMR spectroscopy. The liquid adsorbate was dosed under inert atmosphere in an MAS rotor filled with dry adsorbent, at a pore filling degree of 70%. Next, the evolution in time was recorded of the (1)H MAS NMR spectrum and the (1)H spin-lattice and spin-spin relaxation times of the adsorbed components. For the various adsorbate-adsorbent systems, a plot is made of the signal intensity versus the square root of the contact time. It is found that, over the considered time interval, Fickian diffusion takes place. On the basis of the change in time of the spin-lattice relaxation time, a transport diffusion coefficient ranging between 1 and 2 x 10(-15) m(2) x s(-1) is calculated. Moreover, there appear to be two sorption regimes, with different diffusivities. A comparison is made between the (1)H spin-lattice relaxation behavior of DCPD, 1-octene, and n-octane, indicating that 1-octene and n-octane are located closer to the paramagnetic ions than DCPD. The average distance between the adsorbate molecules and the paramagnetic ions is derived from relaxometric data. By analyzing the chemical shifts of the resonance lines, it is found that the pi-interaction of CPD and 1-octene is stronger than that of DCPD.     

Ab initio study on the adsorption and dissociation process of methanol on a silver surface

The geometries of methanol adsorbed on an oxygen-free silver surface, a promoted silver surface and an oxygen preadsorbed silver surface were optimized at the MP2 level and the energies were calculated at the MP4 level. Our calculations showed that weak physisorption of methanol occurs on the clean silver surface, but stable molecular chemisorption occurs in the other two cases. The adsorption and dissociation process of methanol was postulated to occur via two pathways, i.e. the Eley-Rideal mode and the Langmuir-Hinshelwood mode. The calculations also showed that the presence of atomic oxygen at a silver surface is essential for the cleavage of the OH bond in the methanol. The dissociation of methanol in the Langmuir-Hinshelwood mode has a small energy barrier but has no energy barrier in the Eley-Rideal mode.