The determination of adsorption isotherms by XPS and ToF-SIMS: their role in adhesion science

The study of the adsorption of characteristic ions or molecules from liquid solutions by XPS or ToF-SIMS is shown to be an important experimental method in adhesion science. The theoretical background to adsorption and experimental procedures necessary to yield high-quality results are described. It is shown that in most cases adsorption data conforms to the Langmuir equation, which enables a parameter to be defined which is proportional to the interaction energy between absorbate and adsorbent. A series of examples of the approach are given involving the adsorption of macromolecules, organosilane adhesion promoters, and other organic molecules on oxidised metal and carbon fibre substrates. The estimation of the surface acidity of carbon fibres by measuring the capacity of the surface for derivatising ions is shown to improve the accuracy of the determination.     

十六烷基三甲基溴化铵在二氧化硅表面吸附的研究

十六烷基三甲基溴化铵在亲水二氧化硅表面的吸附呈现出两阶段的特点，在第一阶段对应单分子层吸附，而第二阶段对应双分子层吸附，用椭圆偏振法测出的十六烷基三甲基溴化镓在二氧化硅表面吸附层的厚度表明了这特性。     

Simulation of dynamic behavior of surfactants on a hydrophobic surface using periodic-shell boundary molecular dynamics

The adsorption and aggregation behaviors of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) on a hydrophobic graphite surface were examined using a novel molecular dynamics (MD) simulation with the periodic-shell boundary condition (PSBC). Differences in the adsorption behavior of SDS and CTAB molecules were clearly shown on the hydrophobic surface. Unexpectedly, the SDS molecules approached the graphite surface with their hydrophilic head groups. This unexpected approach mode was thought to be due to the aqueous layer on the graphite surface. The hydrophobic moiety of SDS molecules repeatedly adsorbed and desorbed on the graphite surface. In addition, SDS molecules kept moving on the graphite surface; thus, they did not form a stable adsorption layer. In contrast to SDS, the hydrophobic moiety of CTAB molecules approached the graphite surface at the primary step of adsorption. The hydrophobic moieties of CTAB molecules came close to each other, whereas the hydrophilic groups separated from one another. This result suggests that the CTAB molecules form molecular assemblies with a curved structure. The simulation results were consistent with the experimental observations. A clear difference between the adsorption behavior of SDS and CTAB molecules was revealed by MD simulations with PSBC.     

Zur moleküladsorption an porösen styrol-divinylbenzol-copolymeren,I

The study of the adsorption of p-nitrophenol on porous crosslinked styrene-divinylbenzene copolymers enables the calculation of the surface area and is in fairly good agreement with results, which have been obtained from the same adsorption materials using the adsorption of nitrogen. The calculations are based on BET-equation. A typical monomolecular adsorption isotherm is obtained by methylene blue adsorption. It can be shown that the surface area is not completely coated with a layer of molecules.     

The surface excess and the rate of copper extraction by hydroxyoximes

The interfacial tension is determined for some model 2-hydroxy-5-alkylbenzophenone E-oximes, and the surface excess is calculated according to the different adsorption isotherms. The relationship between the rate of copper extraction and the surface excess is discussed. It is shown that, depending upon the adsorption isotherm used to calculate the surface excess, different relations both between the extraction rate and the surface excess and between the surface excess and the oxime concentration are obtained. The interfacial activity of hydroxyoximes is only a qualitative parameter suggesting the interfacial mechanism for copper extraction, and it cannot give strong evidence quantitatively supporting this mechanism. The predicted reaction orders against oxime demonstrate that the reaction between the oxime molecules from the sublayer and the absorbed intermediate complex is probably the limiting step.     

Insights into the intrinsic interaction between series of C1 molecules and surface of NiO oxygen carriers involved in chemical looping processes

Understanding and modulating the interaction between various reactive molecules and oxygen carriers are the key issue to achieve process intensification of chemical looping technology. C1 chemical molecules play an important role in many reactions involved with chemical looping processes. However, up to now, there is still a lack of systematic and in-depth understanding of the adsorption mechanism of C1 molecules on the surface of oxygen carriers (OCs). In this work, the intrinsic interaction between a series of C1 molecules composed of CH₄, CO, CO₂, CH₃OH, HCHO and HCOOH and surface of NiO OCs in the chemical looping process have been studied using density functional theory calculations. Various adsorption configurations of C1 molecules and also different adsorption sites of NiO have been considered. The structural features of stable configuration of C1 molecules on the surface of NiO OCs have been obtained. Further, the interacted sites, types and strengths of C1 molecules on the surface of NiO have been directly pictured by the independent gradient model methods. Also, the nature of the interaction between C1 molecule and NiO surface has been investigated with the aid of energy decomposition analysis from a quantitative view.     

Insights into the intrinsic interaction between series of C1 molecules and surface of NiO oxygen carriers involved in chemical looping processes

Understanding and modulating the interaction between various reactive molecules and oxygen carriers are the key issue to achieve process intensification of chemical looping technology. C1 chemical molecules play an important role in many reactions involved with chemical looping processes. However, up to now, there is still a lack of systematic and in-depth understanding of the adsorption mechanism of C1 molecules on the surface of oxygen carriers (OCs). In this work, the intrinsic interaction between a series of C1 molecules composed of CH₄, CO, CO₂, CH₃OH, HCHO and HCOOH and surface of NiO OCs in the chemical looping process have been studied using density functional theory calculations. Various adsorption configurations of C1 molecules and also different adsorption sites of NiO have been considered. The structural features of stable configuration of C1 molecules on the surface of NiO OCs have been obtained. Further, the interacted sites, types and strengths of C1 molecules on the surface of NiO have been directly pictured by the independent gradient model methods. Also, the nature of the interaction between C1 molecule and NiO surface has been investigated with the aid of energy decomposition analysis from a quantitative view.     

Adsorption of Bovine Serum Albumin and lysozyme on siliceous MCM-41

The adsorption of Bovine Serum Albumin (BSA) and lysozyme on MCM-41 was studied. In particular, characteristics important to the use of this adsorbent as a chromatographic support for protein separations have been examined. These include the effects of external surface area, solution conditions (pH), and rate of adsorption. Comparative studies were performed on MCM-41 of three pore sizes (24.8 Å, 36 Å, 53.5 Å). It has been shown that differences in external surface area can strongly influence the adsorption capacity. This implies that adsorption on the external surface can be misinterpreted as adsorption in the pores of MCM-41. Thus, high adsorption capacities may not necessarily mean high molecular-sieving capacities. This paper also reports a dependence of protein uptake rates on solution conditions. Calorimetry suggests that under certain solution conditions the adsorption of BSA is a combination of two sequential energetic events. These are postulated to be BSA adsorption followed by reorientation/reconformation of the adsorbed protein on the surface. The slow second step affects the attainment of equilibrium.     

水分子在多孔炭材料上的吸附行为研究进展

多孔炭材料具有较大的比表面积和发达的孔隙结构,是吸附有毒有害气体的关键材料,备受环境、化工、军事化学等领域的关注。多孔炭材料对有毒有害气体的吸附性能受气氛中水分子竞争吸附的影响,研究多孔炭材料对水分子的吸附行为是复杂环境下吸附分离有毒有害气体的基础,对改进多孔炭材料的表面官能团组成和孔结构具有重要的指导意义。基于此,本文综述了国内外关于水分子在多孔炭材料上吸附的机理、过程和影响因素,探讨了水分子作为示踪分子用于多孔炭材料结构表征的潜在可能,并对未来吸附理论的研究方向和指导新型吸附材料设计的应用前景进行展望。     

Computer simulation of proteins adsorption on hydroxyapatite surfaces with calcium phosphate ions

Protein adsorption plays a key role in determining the biological properties of calcium phosphate biomaterials. Calcium (Ca) and phosphate (P) ions are involved in the protein adsorption process and influence the protein adsorption behaviors. In this study, the proteins adsorption on hydroxyapatite (HA) (001), (100), (110), (010) surfaces with Ca, P ions in solution were investigated by Molecular Dynamics (MD) simulation. The results reveal that basic proteins were more favorable to adsorb on HA surface than acidic proteins. HA (110) surface absorbed more acidic proteins than HA (001) and (100). HA (010) surface adsorbed more basic protein. Basic residues play a more important role than acidic residues in the adsorption process, as the basic residues have shown higher absorption energy than acidic residues, and they were more likely to adsorb on HA surfaces than to bind P ions in solution. Most of the active sites of protein, especially for acidic residues, are prefer to interact with HA surface through water molecules. Basic residues are more likely to adsorb onto HA surfaces directly. The presence of Ca, P ions in solution can influence the adsorption behaviors of protein. The formation of Ca, P ion cluster may lead to desorption of proteins. And they can compete with water molecules to bond to HA surfaces, acting as a bridge of protein interacting with HA surfaces instead of water-bridge. This Ca, P ions bridge connection make the adsorption energy of protein weakened. The results of this study provided new information at atomic/molecular level to further interpret the mechanism of protein adsorption on calcium phosphate surfaces with Ca, P ions. It is also helpful for designing new calcium phosphate biomaterials with specific surface properties to adsorb target protein for biomedical applications.     

Adsorption behavior of acetonitrile on platinum and gold electrodes of various structures in solution of 0.5 M H2SO4

The variation of electrode nature and surface structure (the use of stepped single crystal faces with controlled width of (1 1 1) terraces and monoatomic steps of (1 0 0) or (1 1 0) orientation) allows to determine peculiarities of co-adsorption of acetonitrile molecules, hydrogen adatoms and (bi)sulfate anions. It has been shown that first of all acetonitrile blocks adsorption sites at the steps. Anion adsorption at terraces of stepped platinum surfaces in 0.5 M H₂SO₄ solution with additions of acetonitrile depends on terrace width and the step orientation. This demonstrates the important role of structural factors in competitive adsorption processes. The decrease in adsorption of hydrogen and anions on narrow terraces is substantially due to the influence of acetonitrile molecules placed at the steps or nearby sites. At E < 1.0 V, electrochemical conversion of acetonitrile has not been detected at single crystal Pt surfaces. However, acetonitrile oxidation might proceed on polycrystalline platinum followed by product desorption. On Au(1 1 1) surface acetonitrile adsorption is considerably weaker than that on platinum electrodes.     

碳基吸附剂储氢吸附热的密度泛函理论

为比较狭缝孔、纳米管两种典型碳基吸附剂的储氢性能,应用非局域密度泛函理论、微观物理学、吸附平衡原理、典型吸附剂储氢试验结果计算并分析了典型碳基吸附剂的氢等量吸附热.结果表明,两类吸附剂的氢等量吸附热相差不大,等温线变化规律相似,并且只有在低温（液氮）、适度压力时吸附储氢的质量密度才有望达到DOE标准.     

The formation of coupling agent monolayers on the surface of mica

The interaction of very dilute solutions of a cationic vinylbenzyl silane (CVBS), a methacrylate functional silane (γ-MPS), and an isopropyl-tri-(dioctylpyrophosphato) titanate (IDT) with a mica surface has been studied by carbon analysis. For all three coupling agents the dependence of adsorption with time of treatment follows a step-like curve, where each step corresponds to a monolayer coverage on the mica. Under the operating conditions used it was noted that a particular monolayer is always completed before the next is started. The adsorption of γ-MPS was found to be very sensitive to the time allowed for hydrolysis of the silane, while the adsorption of CVBS was relatively independent of the hydrolysis time when it is varied from 0 to 5400 s. The results indicate that γ-MPS and CVBS near the mica surface form a dense structurally regular multilayer phase with the silane molecules oriented normal to the surface. IDT molecules occupy a surface area on mica which is characteristic of their branched nature.     

Spill-Over Effects on Bimetallic Pt/Ru(0001) Surfaces

The concept of spill-over of adsorbed species has a long tradition in Heterogeneous Catalysis and has been explored also for adsorption on bimetallic surfaces, in particular by the Goodman group. In the present paper, we report results of a comprehensive temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy study on spill-over effects in the adsorption and desorption of CO on structurally well defined bimetallic Pt/Ru(0001) surfaces, where part of the substrate is covered by monolayer Pt islands. While upon adsorption at 90 K, the mobility of CO_ad molecules on the surface is very limited, it is activated when the adlayer is annealed to 150 K or, more directly, if CO exposure is done at 150 K or higher temperatures. This enables diffusion of CO_ad molecules to the Pt free Ru(0001) areas, even at local CO_ad coverages which preclude further adsorption from the gas phase on the Ru parts of the surface. Spill-over processes are shown to have significant impact on the TPD spectra; furthermore they provide an additional adsorption channel for adsorption on the bare Ru(0001) areas, allowing uptake of CO at local coverages where adsorption from the gas phase is precluded. This indicates that the apparent CO saturation coverage of 0.68 ML determined for direct adsorption on Ru(0001) under UHV conditions is limited by kinetics rather than thermodynamics. The data are discussed in comparison with results and interpretations in earlier studies, which indicate that these effects are not limited to the Pt/Ru(0001) surface, but may be found on a wide range of bimetallic systems.     

INFLUENCE OF MOLECULAR PROCESSES AT LIQUID INTERFACES ON DYNAMIC SURFACE TENSIONS AND WETTING KINETICS

The application of theories on the wetting kinetics of surfactant solutions requires accurate knowledge of the adsorption kinetics of the surfactant molecules at the involved interfaces. Studies on dynamic surface tensions give access to a quantitative understanding of the mechanisms governing this part of the complex wetting process. It is shown that besides the surface activity and the bulk concentration of a surfactant, the adsorption mechanism and the peculiarities of the interfacial dynamics play a significant role at the liquid-air interface. Due to interfacial processes, such as change in orientation or aggregation of molecules in the surface layer, differences in the adsorption time of more than one order of magnitude are observed.     

The role of the porosity and oxygen groups on the adsorption of n-alkanes, benzene, trichloroethylene and 1,2-dichloroethane on active carbons at zero surface coverage

The adsorption at low concentration (zero surface coverage) of n-alkanes (from n-C₂ to n-C₅) benzene, trichloroethylene (TCE) and 1,2-dichloroethane (DCE) on active carbons has been measured by inverse gas-solid chromatography (IGC). The results offer new unexpected insights on the nature of the adsorption mechanisms of these systems under conditions very close to those found in the real applications for the elimination of atmospheric pollutants. In particular, the adsorbate-adsorbent interactions are governed mainly by dispersive forces even for those adsorbates with strong dipolar moment (TCE and DCE) which are capable of specific interactions. Although the introduction of large amounts of oxygen groups on the surface of the adsorbent increases the specific component of the adsorption, still the dispersive component is clearly dominant. In addition, the introduction of oxygen groups produces a decrease in the adsorption capacity. Nevertheless, it is shown that the presence of pores with appropriate sizes (close to the molecular dimension) has a more important effect on the retention of the adsorbates than chemical surface groups. Therefore, it is more desirable to have active carbons with accessible pores of the appropriate size than oxygen groups to get a good performance in the adsorption of these molecules.     

Hartree–Fock periodic study of the chemisorption of small molecules on TiO2 and MgO surfaces

We present ab initio periodic Hartree–Fock calculations (CRYSTAL program) of the adsorption of small molecules on TiO₂ and MgO. These may be molecular or dissociative, depending on the acidic and basic properties of the molecules in gas phase and of the nature of the surface oxide. For the molecular adsorption, the molecules are adsorbed as bases on Ti(+IV) sites, the adsorption energies correlate with the proton affinities. The dissociations on the surface correlate with the gas phase cleavages of the molecule; they also depend on the surface oxide; the oxygen atom of MgO, in spite of its large charge, is poorly reactive and dissociation on MgO is not favorable.

The surface hydrosyl of MgO are more basic than the O of the lattice and water is not dissociated under adsorption. As experimentally observed, NH₃ adsorbs preferentially on TiO₂ and CO₂ on MgO. However, this difference of reactivity should not be expressed in terms of acid vs basic behavior, but in terms of hard and soft acidity. MgO surface is a “soft” acidic surface that reacts preferentially with the soft base, CO₂.

Another important factor is the adsorbate–adsorbate interaction: favorable cases are the sequence of H-bonds for the hydroxyl groups resulting from the water dissociation and the mode of adsorption for the ammonium ions. Lateral interactions also force the adsorbed CO₂ molecules to bend over the surface, so that their mutual orientation resembles the geometry of the CO₂ dimer.     

A comparison of the change from inhibiting to enhancing anions in the electrochemical oxidations of ethylene glycol and formaldehyde

Techniques dealing with inhibiting anions have previously been employed to measure their properties. The present approach is to change the behavior of inhibiting anions. Anions, which have long been considered to inhibit reactions, are shown to enhance some electrochemical reaction rates. Fluoroborate, nitrate, and (hydrogen) sulfate are separately shown to increase the current response of the electrochemical oxidation of formaldehyde. The reaction rate of the oxidation of ethylene glycol is increased but for a restrictive set of conditions. Experimental data, as well as theoretical considerations lead to the conclusion that adsorption of the added anions is the slow step of the process that increases the reaction rate. The differences in the response of ethylene glycol and formaldehyde follow well-documented changes of surface CO. These observations are consistent with general properties of the step in the part of the mechanism on the electrode surface that enhances the reaction rate.     

Variable Temperature IR Spectroscopy in the Studies of Oxide Catalysts

The paper deals with the development of cell technique for spectral measurements and new possibilities provided by the studies at variable temperatures for obtaining information about the nature and properties of active sites. Difficulties caused by light scattering on the pressed samples of catalysts are discussed. Spectroscopy at low temperatures enables us to broaden the number of probe molecules for acidic or basic sites and to obtain data on the active site concentrations from the band intensities. The necessary values of absorption coefficients can be specified from their correlation with the frequencies of adsorbed molecules. At high surface coverages the measurements are complicated by lateral interactions between the adsorbed molecules. The application of isotopic dilution enables one to distinguish between static interaction, which modifies the catalytic and adsorption properties of surfaces, and dipole coupling. The latter shifts the bands of probe molecules, distorting the surface acidity data. Co-adsorption of acidic and basic molecules leads to the mutual enhancement of adsorption. This is revealed in the phenomena of induced Brønsted acidity and induced basicity or Lewis superacidity. Another important property of surface sites detectable by variable temperature spectroscopy is linkage isomerism of surface species when molecules form with surface sites complexes of unusual structure. Such complexes, e.g. the energetically less favourable O-bonded isomer of CO, could be considered as activated states or intermediates in catalytic reactions. Analysis of the band intensity dependence on temperature and pressure provides valuable information about the thermodynamics of adsorption.