Water vapour sorption on hydrophilic and hydrophobic nanoporous materials

The aim of this study is to compare water vapour sorption isotherms on various mesoporous materials in their pristine state and after silanisation. Commonly the pristine state is regarded as hydrophilic and the silanised one as hydrophobic. Water vapour sorption experiments are discussed for a highly ordered nanoporous aluminium oxide with straight cylindrical channels of ca. 25 nm diameter and for various controlled porous glasses (CPGs) with disordered pores in the range of 13 nm diameter. The water sorption isotherms exhibit in both cases a hysteresis over the entire humidity range. At higher humidities the pristine materials show capillary condensation whereas for the silanised samples this phase transition does not occur or even a loss of water is recorded as for the silanised Al₂O₃. Surprisingly, for the silanised Al₂O higher water uptake is observed in the low humidity region. Application of the excess surface work (ESW) method delivers a reduced structural component in the long range interaction of the water molecules with a hydrophobic surface. Inverse gas chromatography studies of the silanised CPGs result in an increased short range dispersive part of the surface energy with the increasing degree of silanisation.     

Interactions of xanthines with activated carbon: II. The adsorption equilibrium

In the present work, we have studied the adsorption of xanthine derivatives by activated carbon sorbents in aqueous solutions. The study comprised both kinetic, equilibrium and thermodynamic aspects. The kinetic results were reported in a previous paper; the equilibrium-related results are discussed here. The two types of carbon used exhibit some differences but the equilibrium isotherms obtained are all of the H-3 type in the classification of Giles. This suggests a high affinity of the sorbents for the sorbates. We also found that the overall adsorption process comprises more than one individual adsorption-desorption process of which one leads to the formation of a “monolayer” and the other to the “precipitation” of the sorbate on the sorbent surface (multilayer adsorption); the amount of sorbate adsorbed in monolayer form was seemingly greater in C-A14.     

Destabilizing effect of a surface electric field generated by ionic adsorption on the molecular orientation of nematic liquid crystals

The destabilizing effect of a surface electric field, produced by selective ionic adsorption, on the molecular orientation of a nematic-liquid-crystal sample is analyzed for a cell in the shape of a slab of thickness d. The electric-field distribution considered in the analysis is the one obtained in the limit in which essentially all the positive ions are adsorbed. Because of the coupling of this surface field with the nematic director, the surface anchoring energy depends on the thickness of the sample as well as on the adsorption energy characterizing the surfaces. A relation connecting the threshold field for the destabilization of the homeotropic pattern to the adsorption energy and to the thickness of the sample is established in closed form, after solving a set of two coupled non-linear equations determining the electric-field distribution across the sample. It is shown that the values of surface electric field generated by adsorbed ions that can lead to a destabilization of the homeotropic alignment can be attained by real samples.     

Effect of surface electric field on the anchoring of nematic liquid crystals

We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We also discuss the director distortion when the anchoring energy of the surface is finite. Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997     

A numeric model for the kinetics of water vapor sorption on cellulosic reinforcement fibers

Some natural fibers like flax, hemp and others show excellent mechanical properties that make them a promising choice for the reinforcement of polymers. The increasing research on natural fiber reinforced composites has still left important questions open, mainly concerning the fiber–matrix interface. Compared to the well optimized glass fibers, cellulose fibers show very different interaction with matrix polymers and adhesion promoters. The hydrophilic cellulose structure allows for the penetration of a considerable amount of water into the amorphous regions of the fibers, eventually exceeding 20% by mass, depending on fiber type, preparation and environmental humidity. Even embedded in totally apolar polymers the cellulose partly retains its ability for water sorption, which results in unfavorable effects, such as dimensional changes, decrease in strength, roughening of the surface, etc. The interaction of differently prepared fibers with water vapor and the effect of surface treatment is investigated by measuring the dynamics of water vapor sorption. An exponential model is used for the numerical evaluation of the sorption and desorption kinetics. The model not only allows for an excellent fit of the experimental isotherms, but without any further assumptions it immediately gives evidence of the existence of two distinct mechanisms for the exchange of water vapor, related to different sorption sites. These specific mechanisms are represented by individual sorption–desorption isotherms as components of the total isotherms. The model provides a clearer differentiation of the effects of fiber preparation and modification with respect to interfacial interactions.     

Rigid rod adsorption including long-range dispersion interactions

The Caillé-Ågren analysis of rigid rod adsorption is extended by applying the van Kampen theory of condensation to include long-range dispersion interactions between the adsorbed rods. We discuss in greater detail the characteristics of the phase transition predicted by Caillé and Ågren between states having an isotropic and anisotropic distribution of rods adsorbed parallel to the surface. The maximum density range over which the anisotropic adsorbed phase is stable is determined as a function of the length-to-breadth ratio x of the rigid rods and the strength of the anisotropic dispersion energy. Critical surface adhesion and anisotropic dispersion energies necessary for anisotropic adsorption are also obtained as a function of x. In agreement with Caillé and Ågren the isotropic-anisotropic transition for rigid rod adsorption with attractive forces present is found to be second order. We also discuss the spreading pressure-density or area per molecule isotherms obtained for adsorbed rods having various values of x and surface adhesion and dispersion energies. Whenever feasible we compare our results with the spreading pressure isotherms obtained for monolayers of lyotropic molecules on either aqueous or mercury subphases and obtain qualitative agreement. In particular, the critical density and pressure associated with the two dimensional adsorbed gas-liquid condensation for rigid rods with x = 10 gives reasonable agreement with the critical constants observed by Hawkins and Benedek and Kim and Cannell for the corresponding condensation of pentadecanoic acid monolayers on neutral and acidified aqueous subphases. This agreement suggests that considerable dimerization of the pentadecanoic acid molecules may occur on aqueous surfaces.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Zn吸附到含有氧空位(VO)以及羟基(-OH)的锐钛矿相TiO2(101)表面电子结构的第一性原理计算

基于密度泛函理论的第一性原理平面波超软赝势方法, 计算了Zn吸附到TiO₂(101)清洁表面、含有氧空位(V_O)的缺陷表面以及既含有氧空位(V_O)又含有羟基(-OH)表面的能量、Mulliken重叠布居数以及电子结构, 并找到了Zn在每种表面的最稳定结构(分别为模型(c), 模型(aI)以及模型(aII)). 通过对三种表面稳定结构的分析、对比发现: 首先, Zn原子吸附到清洁TiO₂(101)表面上, 主要与表面氧相互作用, 形成Zn–O共价键; 其次, 当Zn原子吸附到缺陷表面时, 吸附能减小到-1.75 eV, 说明Zn更容易吸附到氧空位上(模型(aI)); 最后, 纵观表面模型的能带结构以及态密度图发现, -OH的引入并没有引进新的杂质能级, Zn吸附此表面, 即Zn-TiO₂-V_O-OH, 使得禁带宽度缩短到最小(1.85 eV), 从而有望提高TiO₂的光催化活性. 关键词： 密度泛函理论 氧空位 羟基 Zn原子     

Monomolecular film and absorption behavior of polymers having carboxyl groups

π-A isotherms of monolayers of various copolymers containing carboxyl groups derived from acrylic acid (AA) were measured. The conformation of polymer adsorbed on the water surface and interaction forces between polymer and water surface (considered to be a model for the surface of inorganic powders) owing to AA groups is discussed based on the π-A isotherms.

Polyacrylic esters having a small amount of AA groups formed the expanded type of monolayer. AA groups cause an increase in the interaction force between the polymer and water surface. Polymethyl methacrylate having a small amount of AA groups formed condensed-type monolayers, while with a large amount of AA groups it showed two types of monolayer collapse. Though polystyrene could not spread as a mono-layer, poly (styrene-acrylic acid) [p(St-AA)] copolymer formed a monolayer when the AA content was over 4 mol %. The adsorbance of P(St-AA) on water, calculated from π-A isotherm, is the same as that obtained on α-Fe₂O₃ independently. This means the conformation of the polymer on the solid surface and the interaction force can be discussed by using the π-A isotherm.     

CO and O coadsorption on Pt3Sn studied by DFT: Changes in the adsorptive properties of the surface with alloying and coverage

Adsorption of CO and coadsorption of O and CO on Pt₃Sn(1 1 1) was studied using periodic DFT calculations. Calculations were performed on Pt(1 1 1) by using the same set of parameters and their results were used as reference basis. The calculations showed that the most stable configuration with the minimum energy for coadsorption of CO and O is CO adsorbed atop Pt and O adsorbed on fcc Pt2Sn hollow site and that the decrease in the adsorption strength of the system at a total surface coverage of 0.5 ML is by 0.063 eV as a result of coadsorption, with respect to the adsorption of one species individually. Results show that the interaction between the adsorbed CO and O is short range on PtSn alloy, contrary to that on pure Pt, and this is mainly related to stronger Sn–O bonds compared to Pt–O bonds which eventually reduce the surface strain at the coadsorption structure. There is a pronounced effect of total surface concentration on the adsorption energy of coadsorbed species; the adsorption strength is not directly proportional to the surface coverage but is also related to the distribution of the coadsorbed species on the surface.     

Kinetics of the formation and structure of oligoperoxide nanolayers and grafted polymer brushes on glass plate surface

Functional oligoperoxide surfactants and coordinating oligoperoxide metal complexes were studied as modifiers of glass flat plates to provide the localization of radical forming sites and other functional fragments in adsorbed polymeric layers of a nanoscale thickness. Both the kinetics of the coating formation and properties of the nanolayers witness the dependence of the packing density of oligoperoxide molecules in the coatings on the oligoperoxide natures, concentrations and conditions of the sorption modification. The availability of definite amount of peroxide groups in formed nanolayer provides the possibility of controlled radical graft polymerization initiated from modified surface leading to reliable surface protection, functionality and targeted surface hydrophilic-hydrophobic properties.      

Investigation of surface properties of lunar regolith: Part I

This paper describes an initial investigation of the surface properties of three lunar soil samples from the Apollo 11, 12 and 16 missions, respectively. We report on density measurements using a helium pycnometer, adsorption isotherms of krypton applied for the determination of specific surface area of the samples and gravimetric measurement of the isotherms of water, heptane and octane. Electron-microscopic photographs are described and discussed.     

Some studies of lead and iron adsorption on the W(100) surface by field emission microscopy

The behaviour of lead and iron adsorbed on the W(100) surface has been studied by probe hole field emission microscopy, field desorption, and by measurement of the total energy distribution (TED) of field-emitted electrons. Lead adsorbed at 300 K which reduces the work function of W(100) can be completely removed at 78 K by field desorption below 3.2 V Å^?1 and the resulting surface has both the work function and TED, which are characteristic of the clean plane. Condensation at 800 K followed by field desorption, results in a plane surface of work function 4.17 eV and an altered TED. This effect is attributed to the microfacetting, which is observed by LEED. The Swanson peak in the W(100) TED which is removed by submonolayer amounts of lead re-emerges at monolayer coverage when lead adopts the (1 × 1) structure. Such behaviour is consistent with the model proposed by Kar and Soven. A spectral peak observed when lead is adsorbed on the reconstructed W(100) surface is thought to derive from the atomic ¹D state. Adsorption of iron on a W(100) surface reduces φ considerably due to dipole formation and efficiently quenches the Swanson peak. Higher coverages introduce other peaks in the TED enhancement curve, and by adopting an energy scale based on the work of Hagstrum, an attempt is made to interpret the observed peaks in terms of the known energy structure of the free iron atom. One of the three spectral peaks is assigned to the 4s² ground state of the iron atom, and the remaining two peaks are tentatively attributed to atomic p-states. It is concluded that while the excited state structure of the iron atom is too complex to permit complete interpretation of the spectra, this approach offers the hope that, for simpler atoms, such features may be interpreted in this way.     

Electrostatic Nature of Size Dependences of Adsorption Properties of Ytterbium Nanofilms Grown on the Surface of Silicon: the CO–Yb–Si(111) System

The adsorption of CO molecules onto ytterbium nanofilms with their thickness varying from 1 to 16 monolayers is studied. The dependences of the number of adsorbed CO molecules (adsorption isotherms) and the work function of ytterbium films on the dose of carbon monoxide are examined. It is demonstrated that both the number of adsorbed molecules and the work function depend (under equal conditions) on the nanofilm thickness; in other words, a size effect is revealed. It is found that this size effect is induced by the electrostatic interaction between the conduction electrons of ytterbium and the electrons localized on the nanofilm surface, which establish bonding between the surface and CO molecules. This interaction depends on the film thickness and limits the number of CO molecules that may be adsorbed onto the surface of a film with a given thickness.

     

Raman spectra of molecules considered to be surface enhanced

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) have been used to study the structure of adsorbed benzene (C₆H₆ and C₆D₆) monolayers on the Rh(111) surface at 300K. A surface bonding geometry is proposed for benzene adsorbed to give a c(2?3×4) rectangular structure, which involves very little perturbation of the molecular structure with the ring plane parallel to the surface. Only one chemical environment for adsorbed benzene is indicated by a single frequency shift of the symmetric CH out-of-plane bending mode. The adsorption site is tentatively assigned to benzene centered over a single Rh atom.     

爆轰裂解法制备纳米石墨粉的吸附性能研究

 在酸含量不同的原材料中,通过爆轰的方法制备纳米石墨粉,并利用BET方程以及BJH方法对所得纳米石墨粉进行比表面积和孔径分布分析。分析结果表明,所得爆轰产物中有的比表面积大致为天然鳞片石墨的5.3~9.2倍,而且随酸含量的增大逐渐增大,产物的等温线中存在吸附滞后现象。其中,增大的比表面积主要由分布在3 nm至7 nm之间的孔引起的,而且在爆轰后,孔径4 nm左右的孔,其数量达到最大值。通过对纳米石墨粉的研究,分析了酸在爆轰过程中的积极作用,并为纳米石墨粉的进一步应用提供了结构信息。     

New insights into the adsorption isotherm interpretation by a coupled molecular simulation—experimental procedure

We present here Grand Canonical Monte Carlo (GCMC) simulation results of nitrogen adsorption at 77 K on a crude model of activated carbon. The material is modeled as slit-like pores of different widths, with smooth surfaces. The individual adsorption isotherms serve as the basis to check the success and limitations of the assumptions made when using the BET model to characterize adsorbent materials, in particular to calculate the monolayer capacity and the C parameter. As done in our previous work with several experimental adsorption isotherms, different linearizations of the BET equation are used. The aim of this work is to quantify, using statistical mechanics tools, the changes in the C factor with surface coverage, showing that C is an intrinsically energetic meaningful quantity. The amount of molecules adsorbed at each pressure is calculated in the first and subsequent layers. We also keep track of the adsorbent-adsorbate and adsorbate-adsorbate energy along the simulations. The C factor is obtained following two different routes: as directly derived from the BET equation, once the monolayer capacity is known, and from the heat of adsorption obtained directly from the simulations. Results from simulations confirm the changes in the C values with surface coverage. In addition, molecular simulations provide independent and consistent ways of calculating the monolayer capacity.     

Special problems and methods in the study of water vapor transport in polymers

Water presents special and unique difficulties in diffusion and sorption measurements because of its high cohesive energy and hydrogen-bonding capacity. In the case of transport measurements in hydrophilic polymers the time-lag method is complicated by the tendency of the water vapor to be adsorbed onto the glass receiving sections of the equipment. On the other hand, with more hydrophilic polymers where nonsteady-state direct sorption methods can be used the large amount of heat liberated during the sorption process again causes considerable difficulty. A discussion of these and other problems concerned with transport studies with water vapor is presented and various practical solutions are offered.     

Lateral interactions and zigzag chains of Ho on the Mo(110) surface

The structure of Ho adsorbed layers on the Mo(110) surface has been studied by low energy electron diffraction (LEED) and scanning tunneling microscope (STM). It has been found that at submonolayer coverages Ho atoms, similarly to studied earlier Gd and Nd, form a rich amount of dilute (n × 2) commensurate structures built from zigzag chains oriented along the [11?0] direction. The formation of zigzag chain structures is initiated by the indirect lateral interaction between adsorbed Ho atoms, which, as is illustrated by Monte Carlo simulations, can be well approximated by a screened Coulomb potential superimposed with Friedel oscillations.     

UV photoemission from physisorbed atoms and molecules: Electronic binding energies of valence levels in mono- and multilayers

Angle-resolved UV photoemission spectra were measured for Ar, Kr, Xe, CO, O₂ and N₂ adsorbed on a Ni(110) surface at 20 K. The different gases were adsorbed also on the Ni(110) surface which had been precovered by mono- and multilayers of the same gases. Upon physisorbing one of these species onto the bare and precovered Ni surface, binding energy shifts up to 3 eV were found. These shifts will be explained by work function changes of the substrate onto which the gas is physisorbed. It will be shown that for the investigated gases the binding energy referred to the vacuum level is an atomic or molecular property which is independent of the substrate, to a first approximation. By physisorption of a known gas the work function of any substrate can be evaluated by UPS. The density of valence states for bulk Ar, Kr and Xe will be discussed. There is evidence that the conduction band can be seen in the secondary electron background of the UP spectra.