Influence of surface structure on surface analytical statements

The influence of surface structure of technical materials on results and statements of surface analytical methods has been investigated. Especially surface roughness as a typical property of rolled products has been observed. For this purpose samples of steel (technical surface, roughness up to 5 m) and silicon wafers (polished surface) have been analyzed by SNMS and GDOS in order to get information about changes of the surface roughness as function of the sputtering time and their influence on the statements about the depth profiles obtained.     

Relationship between surface tension and surface coverage

Surfactant action is caused in part by a dramatic reduction in surface tension. Using surface excess measurements from a radioactive surfactant, it was possible to show that (a) the surface tension declines only slightly when the occupancy of the air/water interface increases from 0 to 60% of the maximum and (b) the steep drop in surface tension in region B (Figure 1 ), frequently observed to be linear, begins at about 80% occupancy. Surfactant continues to enter the interface cooperatively up to and past the critical micelle concentration. Linearity in region B is not indicative of surface saturation despite a seemingly constant surface excess throughout the region. The disparity between interfacial areas determined by surface tension and by other methods is discussed in terms of these results.     

True molar surface energy and alignment of surface molecules

By use of data for surface tension, for the first time a method is presented for calculating true molar surface properties of liquids: free energy, entropy, and enthalpy. These new data allow full comparison with other molar quantities, such as enthalpy and entropy of vaporization. All data are at the normal boiling point. There are differences in behavior between various classes of nonpolar compounds. Rare gases and tetrahydrides of Group 14 form a separate category. The results agree with the experimental findings that water and alcohol molecules are aligned with the -OH groups attached to the surface. The data indicate that hydrogen peroxide and 1,2-ethanediol also have one -OH group directed toward the surface and one directed out toward the vapor phase. Small straight-chain amines have some structure at the surface, but the larger ones behave like the corresponding alkanes. Very polar compounds, such as nitriles, nitro compounds, and aldehydes, have little or no increased degree of order of molecules at the surface. Except for hydrogen-bonded compounds, molecules at the surface have surroundings similar to those in the bulk liquid.     

Oxygen vacancy O-terminated surface: The most exposed surface of hexagonal WO3 (001) surface

It is known that exposed surface determines material’s performance. WO₃ is widely used in gas sensing and its working surface is proposed to control its sensitivity. However, the working surface, or most exposed surface with detailed surface structure remain unclear. In this paper, DFT calculation confirmed that oxygen vacancy O-terminated surface is the most exposed hexagonal WO₃ (001) surface, judging from competitive adsorption of CO and O₂, working surface determination for CO sensing and comparison of oxygen vacancy formation energies on different h-WO₃ (001) surfaces. It is found that DFT can be a useful alternate for exposed surface determination. Our results provide new perspectives and performance explanations for material research.     

Above the surface multifragmentation of surface scattered fullerenes

C(60) (-) ions were scattered from a gold surface at impact energies of 80-900 eV. The C(n) (-) fragments abundance distribution (odd and even) and the sharp fragmentation threshold observed, point at a prompt shattering event. The measured angle and energy distributions of the C(n) (-) fragments (n=2-12) provide clear evidence for a multifragmentation process where the superheated fullerenes leave the surface "intact" and disintegrate away from the surface.     

The surface charging at low density of protonatable surface sites

The point of zero charge (PZC) of a sparingly soluble metal oxide depends on the density of protonatable surface oxygen atoms. The shift in the PZC is due to protonation/deprotonation of water in the regions free of protonatable surface oxygen atoms originating from the solid. The PZC of alumina increases when the density of protonatable surface oxygen atoms increases. In contrast, the PZC of titania is rather insensitive to the density of protonatable surface oxygen atoms. In surfaces of many materials the regions free of protonatable surface oxygen atoms dominate. These materials have a PZC at pH about 4.     

Dynamic surface tension and surface rheology of biological liquids

The paper presents results of dynamic and equilibrium surface tension measurements (using a maximum bubble pressure instrument) of serum and urine samples that were obtained from 80 healthy human of various sexes and ages. These data were compared with surface tension measurements of biological liquids obtained from patients suffering from malignant neoplasm of corpus uteri (n=5) and cervix uteri (n=31). In addition, surface dilatational rheology was determined on 32 samples using a drop shape method. The dilatational rheology data were compared with the dynamic surface tension data. Although some trends were found, no significant correlations exist between surface tension and rheology data and any of the disease states or stages. It is difficult to explain these findings in the framework of known mechanisms. However, our studies demonstrate that dynamic interface tensiometry of human biological liquids provide new insight into the biophysical behavior of these liquids, most likely reflecting compositional changes of them during ageing, the course of cancer and as a consequence of therapeutical interventions.     

Effect of atmospheric-pressure surface discharge on the surface properties of polytetrafluoroethylene films

Alteration of the surface properties of polytetrafluoroethylene thin films by treatment in atmospheric-pressure surface discharge plasma initiated by dc barrier corona in argon has been studied. It has been shown that the plasma treatment of the polymer film surface leads to a substantial reduction of contact angles and an increase in the total surface energy.     

Conversion of a metastable superhydrophobic surface to an ultraphobic surface

Superhydrophobic surfaces in Wenzel and metastable wetting state were prepared and the conversion of such surfaces to ultraphobic surfaces was reported by the application of a fine-scale roughness. Silicon nitride substrates with hexagonally arranged pillars were prepared by micromachining. The two-scale roughness was achieved by coating these substrates with 60 nm silica nanoparticles. The surface was made hydrophobic by silanization with octadecytrichlorosilane (OTS). Wettability studies of the silicon nitride flat surface, silicon nitride pillars, and the surfaces with two-scale roughness showed that a two-scale roughness can effectively improve the hydrophobicity of surfaces with a higher apparent contact angle and reduced contact angle hysteresis when the original rough surface was in a metastable or Wenzel state. This study shows the pathway of converting a metastable hydrophobic surface to an ultraphobic surface by the introduction of a fine-scale roughness, which adds to the literature a new aspect of fine-scale roughness effect.     

Effect of particle-size distribution on the surface appearance of glazed surface

Glazes have great influence on most of the final properties of sanitary wares, such as color, glossy, and surface roughness. Generally, these properties have been changed by chemical composition and firing parameters. In addition, the selection of particle-size distribution of raw glaze can affect on the surface appearance and cleanability of glazed surface. In this paper, raw glazes were milled in ball mill for different time. Prepared glaze slurry was applied to green bodies, later dried and fired. The same glaze was also uniaxially pressed to form cylindrical sample; the specimen was placed on substrate surface and fired in hot stage microscope. The glaze evolution upon heating was recorded by CCD camera, the contact angle between molten glaze and substrate was measured to determine the wettability. The results showed that particle-size distributions have great influence on the melting behavior of glaze and the surface appearance of glazed ceramics.     

超疏水表面研究进展

综述了超疏水表面研究进展,简述了超疏水表面研究的理论基础,归纳总结了超疏水表面的制备方法及存在的问题,并介绍了功能超疏水材料研究的最新进展.指出超疏水表面因其独特的自清洁性能而成为功能材料及表面界面科学领域的研究热点之一,并就其今后的发展进行了展望.     

Ab initio surface phase diagram of the {104} calcite surface

Electronic structure calculations, performed at the density functional theory level, were employed to study the surface termination of the {104} calcite surface in contact with a gaseous phase containing water and carbon dioxide. A surface phase diagram was generated to investigate the change in surface termination as a function of temperature, pressure, and gas-phase composition. This diagram revealed that a nonstoichiometric termination could occur in atmospheric conditions at high relative humidity, hence suggesting that nonstoichiometric surfaces can play a major role in the chemistry of calcite surfaces.     

Study of surface tension and surface properties of binary alcohol/n-alkyl acetate mixtures

The Butler equation is employed to describe quantitatively the nature, properties, and compositions of surface layers in binary liquid mixtures. Bulk mole fraction, surface molar area, and surface tension of pure components are necessary inputs for this equation. In addition, the UNIFAC group contribution method is applied to account for the nonideality of the bulk liquid as well as that of the surface layer. The average relative error obtained from the comparison of experimental and calculated surface tension values for 12 binary systems is less than 1%. Therefore, the model has good accuracy in comparison with other predictive equations. In addition to finding more information about the surface structure of binary mixtures, surface mole fraction was calculated using relative Gibbs adsorption values and an extended Langmuir model (EL). The obtained results show a good consistency between two models employed, i.e., the Gibbs adsorption model and EL model, based on the UNIFAC method.     

Laser-induced surface migration via surface plasmons

A classical model coupling a charged adspecies to a laser-induced surface plasmon is presented. Such coupling can enhance the rate and specify the direction of surface migration. For the particular case of an atomic oxygen ion of charge ?1 adsorbed on aluminum which is exposed to CO₂ laser radiation of intensity 1 W/cm², the velocity of migration (61.3 μm/s) is five orders of magnitude greater than the usual thermal velocities observed at room temperature.     

Determination of the surface pK of carboxylic- and amine-terminated alkanethiols using surface plasmon resonance spectroscopy

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pKd) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pKd of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pKd values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pKd values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pKd. The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pKd of the surface.     

A topography/chemical composition gradient polystyrene surface: toward the investigation of the relationship between surface wettability and surface structure and chemical composition

In this paper, we have prepared of a topography/chemical composition gradient polystyrene (PS) surface, i.e., an orthogonal gradient surface, to investigate the relationship between surface wettability and surface structure and chemical composition. The prepared surface shows a one-dimensional gradient in wettability in the x, y, and diagonal directions, including hydrophobic to hydrophilic, superhydrophobic to hydrophobic, superhydrophobic to superhydrophilic gradients, and so forth. These one-dimensional gradients have different gradient values, gradient range, and contact angle hysteresis, which lie on both the surface roughness and the surface compositions. From the trend of variation of contact angle hysteresis, it can be concluded that the transition from the Cassie's model to the Wenzel's model occurs both by decreasing surface roughness and by increasing surface hydrophilic compositions. Moreover, the transition is more effective via changing surface chemical composition than changing surface roughness herein.     

Simulating surface diffusion and surface growth in ceramics

We examine the movement of ion pairs on the surfaces of simple oxides. Using temperature-accelerated dynamics the elementary processes involved are identified and the activation energies of these used as input to kinetic Monte Carlo simulations. Results are presented for the motion of BaO and SrO ion pairs on the (100) surfaces of BaO and SrO, respectively, and the formation of island pairs on these surfaces is studied. The simulations reveal the importance of exchange mechanisms in surface diffusion and growth of oxides. The importance of such reactions has been recognised previously for metallic surfaces but not for ionic systems, where it has been assumed that ionic surface diffusion is surface diffusion via the hopping motion of ion pairs from one surface site to another. Exchange mechanisms can dominate transport processes both on terraces and steps for both homoepitaxial and heteroepitaxial growth. We suggest the unavoidable mixing when an exchange mechanism operates must be considered when attempting to grow sharp interfaces in oxide nanostructures.     

Polymer surface science

Molecular level studies of the structure and mechanical properties of polymer surfaces have been carried out by sum frequency generation (SFG) surface vibrational spectroscopy and atomic force microscopy (AFM). The surfaces of different grades of polyethylene and polypropylene have been characterized-including during the glass transition and when mechanically stretched. Copolymers that have hard and soft segments with different glass transition temperatures show phase separation, an effect of hydrogen bonding between the hard and soft segments, that influences their adhesive and friction properties. AFM and SFG show that low surface energy additives migrate to the surface and alter the surface mechanical properties. Polymers, where the chemical nature of the end groups is different from the backbone, show surface segregation of the hydrophobic part of the chain in air and the hydrophilic part in water. Likewise, in miscible polymer blends, surface segregation of the more hydrophobic component in air and the more hydrophilic component in water is observed. This area of surface science requires increased attention because of the predominance of polymers as structural materials and as biomaterials.     

Dynamics of surface catalyzed reactions; the roles of surface defects, surface diffusion, and hot electrons

The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.