Asphaltene adsorption mechanism under shear flow probed by in situ neutron reflectivity measurements

We propose here a method to adapt a rheometer with a cone/plate geometry to a neutron reflectometer in order to perform in situ reflectivity measurements. This study allowed us to probe the influence of the shear rate on the mechanism of asphaltenes adsorption, the heaviest and most polar compounds of crude oil, using bad solvent conditions. Such experiment aims at describing at a local scale the surface modifications induced by flowing crude oils (pipe transportation or production through porous media). Without shearing, in a 34%/66% xylene/dodecane mixture for which the asphaltenes flocculation is achieved in bulk, the nanoaggregates are able to be adsorbed on a hydrophilic surface as multilayers, with a surface excess much larger than for good solvent conditions. Moreover, the thickness of these multilayers increases almost linearly with time, in accordance with QCM experiments. In shear rate conditions, the adsorption process is however strongly limited since the surface excess of the adsorbed layers is around twice lower at 2600 s^?1 than at 1200 s^?1.     

Studies of the electrical double layer and of adsorbed species on electrodes using modulated specular reflectance spectroscopy

The optical properties of adsorbed hydrogen on a platinum cathode have been studied. The results are interpreted in terms of r and s-adatom formation for the two types of adsorption. The reflectivity change in the double layer region of potential is ascribed to an electroreflectance contribution from the metal. Studies of the simple double layer formed between KF solution and a lead or mercury electrode show that the reflectivity changes caused by the diffuse double layer are small compared with contributions from the compressed water in the inner layer and from the electroreflectance effect.     

Adsorption of hydrophobic polyelectrolytes at the air/water interface: Conformational effect and history dependence

We present an experimental study of the adsorption of hydrophobic highly charged polyelectrolytes on a neutral and hydrophobic surface, the air/water interface. The polymer was a randomly sulphonated polystyrene with charge fractions between 0.3 and 0.9 and the adsorbed layers were characterised by Langmuir through measurements, ellipsometry and X-ray reflectivity. The adsorption rate is always very slow and the resulting layers are very thin (< 3 nm). A maximum of adsorption with the charge fraction is observed which we relate to the conformation of the chains in solution. We show that adsorption is partially irreversible, strongly hysteretic and that the state of an adsorbed layer depends on its history. Received 16 June 2000     

Theoretical study of the effect of spin-selective adsorption of water molecules on MgO surface

The effect of spin-selective adsorption of water molecules on the surface of MgO crystal is theoretically studied. The study is performed using two different approaches, i.e., quantum-chemical simulation and an analytical calculation in a quasiclassical approximation. The adsorption energy is calculated using the B3LYP density functional and the 6–311G* basis set. The calculated value of the adsorption energy 0.70 eV agrees well with an experimental value of 0.65 eV. It is established that the energy difference of adsorbed ortho- and para-water molecules is negligible and, thus, the difference of the adsorption energies is completely determined by the energy difference of free molecules in ortho- and para-states. It follows from the analytical calculation that this result is essentially general and is related not only to an MgO surface, but to any other surface on which the energy barrier for rotation of the adsorbed molecule is much larger than the corresponding rotational constant. Based on this, the conclusion is reached that the effect of spin-selective adsorption on these surfaces should not be observed under normal conditions.     

Competitive adsorption of polymers on metal nanoparticles

We investigate the effect of system properties and adsorption sequence on competitive adsorption of poly(methyl methacrylate) (PMMA) and polystyrene (PS) on narrowly polydispersed cobalt (Co) nanoparticles (D ∼ 27 nm). The adsorbed layer composition is studied using thermo-gravimetric analysis (TGA). We find that adsorbed layers of PS are completely displaced by PMMA when the solvent is a common good solvent. An adsorbed layer of only PMMA is also obtained through competitive adsorption from a common good solvent. However, in a selective solvent that is poor for PS, sequential adsorption leads to the formation of mixed layers.     

Giant thermopower effects from molecular physisorption on carbon nanotubes

Results are presented of in situ studies of the thermoelectric power and four-probe resistance of single-walled carbon nanotube films during the adsorption of cyclic hydrocarbons C(6)H(2n) (n=3-6). The size of the change in these transport parameters is found to be related to the pi electron population of the molecule, suggesting the coupling between these pi electrons and those in the nanotube wall may be responsible for the observed effects. A transport model for the SWNT film behavior is presented, incorporating the effects of a new scattering channel associated with the adsorbed molecules.     

Influence of Pb adatom on adsorption of oxygen molecules on Pb(111) surface: a first-principles study

Using first-principles calculations, we systematically study the influence of Pb adatom on the adsorption and the dissociation of oxygen molecules on Pb(111) surface, to explore the effect of a point defect on the oxidation of the Pb(111) surface. We find that when an oxygen molecule is adsorbed near an adatom on the Pb surface, the molecule will be dissociated without any obvious barriers, and the dissociated O atoms bond with both the adatom and the surface Pb atoms. The adsorption energy in this situation is much larger than that on a clean Pb surface. Besides, for an adsorbed oxygen molecule on a clean Pb surface, a diffusing Pb adatom can also change its adsorption state and enlarge the adsorption energy for O, but it does not make the oxygen molecule dissociated. And in this situation, there is a molecule-like PbO2 cluster formed on the Pb surface.     

L-天冬氨酸在银胶体中吸附状态的表面增强拉曼光谱研究

利用表面增强拉曼光谱 (Surface EnhancedRamanScattering,SERS)研究了L 天冬氨酸在银溶胶体中的吸附状态及其浓度变化对表面增强拉曼散射效应的影响 ,并探讨了L 天冬氨酸在银溶胶表面的吸附作用特点和规律。实验结果表明 ,L 天冬氨酸在银溶胶中有明显的SERS信号 ,经过分析表明 ,该化合物能够吸附在银表面 ,这种吸附是通过羧基和氨基中的氮原子与银结合来实现的 ,L 天冬氨酸分子中带有负电荷的羧基和氨基中带有孤对电子的氮原子都能与银原子配位 ,其中羧基在银表面的增强为电荷转移机制增强 ,具有化学吸附的特征 ;氨基在银表面的增强为电磁场增强机制 ,为物理吸附。而且SERS强度随着L 天冬氨酸浓度的变化而改变 ,当其浓度为 10 - 3mol·L- 1 时增强效果较好 ,当浓度降低 ,增强作用也逐步变弱     

A quantum-mechanical study of the vinyl fluoride adsorbed on the rutile TiO2(1 1 0) surface

The adsorption of vinyl fluoride on the rutile TiO₂(1 1 0) surface has been simulated, on the basis of a recently proposed experimental model, using hybrid-exchange density functional theory. Different surface coverages have been considered and the lateral interaction between adsorbed vinyl fluoride molecules has been quantified through a simple model of nearest and next nearest neighbouring molecules. The vibrational frequencies of the adsorbed molecule have been calculated and are found to be in excellent agreement with those observed providing support for the proposed adsorption model. The effect of the adsorption on the electronic structure of the molecule and the surface have been characterised by computing electrostatic potential maps and the local density of states.     

In-situ FTIR reflection spectroscopy for the semiconductor electrolyte interfaces

Polarization modulation fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS) and subtractively normalized interfacial fourier transform infrared spectroscopy (SNIF-TIRS) have been applied to the studies of adsorption of ions and neutral molecules on p-silicon and p-gallium phosphide electrodes in aqueous as well as non-aqueous solutions. The potential and concentration dependence of adsorption were observed. Thiourea is adsorbed on silicon through the sulfur atom and adsorption increases in the anodic direction. Thiourea adsorption on silicon follows a Temkin type isotherm and the free energy adsorption was calculated to be − 5.63 kJ/mole. Acetate in aqueous medium is adsorbed as neutral acetic acid molecule on silicon. The potential dependence of adsorption passes through a maximum which lies close to the potential of zero charge. The tetraethylammonium ion in acetonitrile is adsorbed according to a Langmuir isotherm. Acetonitrile is adsorbed strongly on gallium phosphide when used as a solvent for sodium acetate at cathodic potentials. At anodic potentials adsorbed acetonitrile is replaced by acetate ions. Adsorption of acetate ion starts at potentials cathodic to the potential of zero charge, due to specific adsorption. Ammonium ion adsorption on gallium phosphide occurs through two hydrogen atoms and the other two hydrogen atoms project towards the solution. Two peaks corresponding to different N-H deformation vibrations are observed at 1560 and 1700 cm⁻¹. At sufficiently cathodic potentials, adsorbed ammonium ions are reduced.     

Electronics and Structural Properties of Single-Walled Carbon Nanotubes Interacting with a Glucose Molecule: ab initio Calculations

The adsorption of glucose molecule on single-walled carbon nanotubes(SWCNTs)is investigated by density functional theory calculations.Adsorption energies and equilibrium distances are evaluated,and glucose binding to the typical semiconducting and metallic nanotubes with various diameters and chirality are compared.We also investigated the role of the structural defects on the adsorption capability of the SWCNTs.We could observe larger adsorption energies for the larger diameters semiconducting CNTs,while the story is paradoxical for the metallic CNTs.The obtained results reveal that the adsorption energy is significantly higher for nanotubes with higher chiral angles.Finally,the adsorption energies are calculated for defected nanotubes for various configurations such as glucose molecule approaching to the pentagon,hexagon,and heptagon sites in the tube surface.We find that the respected defects have a minor contribution to the adsorption mechanism of the glucose on SWNTs.The calculation of electron transfers and the density of states supports that the electronic properties of SWCNTs do not change significantly after the gluycose molecular adsorption.Consequently,one can predict that presence of glucose would neither modify the electronic structure of the SWCNTs nor direct to a change in the conductivity of the intrinsic nanotubes.     

Mechanism of H2S molecule adsorption on the GaAs(100) surface: Ab initio quantum-chemical analysis

Ab initio quantum-chemical cluster calculations within the density-functional theory were carried out to study the mechanism of H₂S molecule adsorption on the gallium-rich surface of GaAs(100). It was shown that adsorption can occur in four stages: molecular adsorption; dissociative adsorption, during which an HS radical is adsorbed on a gallium atom comprising a dimer while the detached hydrogen atom is adsorbed on another surface atom of the semiconductor; hydrogen adatom migration between neighboring surface atoms of the semiconductor; and the formation of a Ga-S-Ga bridge bond and of a hydrogen molecule. The stationary-state energies and energy barriers to transitions between these states were determined. The conclusions drawn based on an analysis of calculated diagrams of the potential energy of the processes that occur are in good agreement with the experimental data available in the literature.     

第一性原理研究气体小分子吸附的石墨烯负载金属原子体系

石墨烯负载的单个金属原子体系(M-gra)具有高的结构稳定性,显正电性的金属原子可作为活性位用在气敏器件和催化材料.本文采用基于密度泛函理论的第一性原理方法研究单个有毒气体小分子(NO和CO)在M-gra表面的吸附特性.研究结果表明:单个NO分子吸附的稳定性高于CO分子,由于其能够从反应衬底获得更多的转移电荷,因此,M-gra衬底对NO分子表现出高的灵敏度.此外,不同小分子吸附能够改变M-gra体系的电荷密度和自旋电荷分布,进而使得气体分子吸附体系表现出不同大小的磁矩.通过对比气体分子吸附前后M-gra体系的磁矩变化,能够有效判断吸附分子和反应衬底的类型.     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.     

Electronics and Structural Properties of Single-Walled Carbon Nanotubes Interacting with a Glucose Molecule：ab initio Calculations

The adsorption of glucose molecule on single-walled carbon nanotubes (SWCNTs) is investigated by density functional theory calculations. Adsorption energies and equilibrium distances are evaluated, and glucose binding to the typical semiconducting and metallic nanotubes with various diameters and chirality are compared. We also investigated the role of the structural defects on the adsorption capability of the SWCNTs. We could observe larger adsorption energies for the larger diameters semiconducting CNTs, while the story is paradoxical for the metallic CNTs. The obtained results reveal that the adsorption energy is significantly higher for nanotubes with higher chiral angles. Finally, the adsorption energies are calculated for defected nanotubes for various configurations such as glucose molecule approaching to the pentagon, hexagon, and heptagon sites in the tube surface. We find that the respected defects have a minor contribution to the adsorption mechanism of the glucose on SWNTs. The calculation of electron transfers and the density of states supports that the electronic properties of SWCNTs do not change significantly after the gluycose molecular adsorption. Consequently, one can predict that presence of glucose would neither modify the electronic structure of the SWCNTs nor direct to a change in the conductivity of the intrinsic nanotubes.     

SiO adsorption on a p(2 × 2) reconstructed Si(1 0 0) surface

We have investigated the adsorption mechanism of SiO molecule incident on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. Stable adsorption geometries of SiO on Si surface, as well as their corresponding activation and adsorption energies are identified. We found that the SiO molecule is adsorbed on the Si(1 0 0) surface with almost no activation energy. An adsorption configuration where the SiO binds on the channel separating the dimer rows, forming a Si-O-Si bridge on the surface, is the energetically most favourable geometry found. A substantial red-shift in the calculated vibrational frequencies of the adsorbed SiO molecule in the bridging configurations is observed. Comparison of adsorption energies shows that SiO adsorption on a Si(1 0 0) surface is energetically less favourable than the comparable O₂ adsorption. However, the role of SiO in the growth of silicon sub-oxides during reactive magnetron plasma deposition is expected to be significant due to the relatively large amount of SiO molecules incident on the deposition surface and its considerable sticking probability. The stable adsorption geometries found here exhibit structural properties similar to the Si/SiO₂ interface and may be used for studying SiO_x growth.     

Oxygen adsorption on ultrathin Co/Ir(1 1 1) films: Compositional anomaly

Adsorption of oxygen on ultrathin Co/Ir(1 1 1) films thinner than 4 monolayers in an ultrahigh vacuum environment was studied. For oxygen adsorption on cobalt films, the complex adsorption kinetics emerges partly due to the incorporation of oxygen. The amount of oxygen adsorbed at the surfaces is higher than that incorporated into the film as revealed from sputter profiling measurements. At room temperature the CoO layer exhibits paramagnetism and could not contribute to the remanent Kerr intensity. As oxygen exposure increases, the reduction of the Kerr intensity is due to the reduction of the effective layer for the magnetic measurements. Compared with oxygen saturated cobalt films, the concentration of adsorbed oxygen per Co atom shows an oscillatory behavior. A compositional anomaly of a great amount of adsorbed oxygen in submonolayer Co coverage occurs because of the maximized number of adsorption and incorporation sites for oxygen on the surface. A larger charge transfer between Co and oxygen was observed for thinner Co overlayers as revealed from the larger chemical shifts of Auger lines.     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

Study of ammonia molecule adsorbing on diamond (1 0 0) surface

The diamond (1 0 0) surface with amino terminations is investigated based on density function theory within the generalized gradient approximation. Our calculated negative electron affinity of diamond (1 0 0) surface with hydrogen termination provides a necessary condition for initiating radical reaction. The results display that the ammonia molecule can form stable C-N covalent bonds on the diamond surface. In addition, due to the lower adsorption energy of one amino group binding on diamond surface, single amino group (SAG) model is easy to be realized in experiment with the comparison of double amino group (DAG) model. The adsorbed ammonia molecule will induce acceptor-like gap states with little change of the valence and conduction band of diamond in SAG model. The adsorption mechanism in the formation of ammonia monolayer on H-terminated diamond (1 0 0) surface, and two possible adsorption structures (SAG and DAG) were especially studied.