Maximum Surface Concentration Research Articles

Further Validation of the Quartic Concentration Profile Approximation for Describing Intraparticle Transport in Cyclic Adsorption Processes

Models developed previously by the authors that describe nonlinear adsorption, and simultaneous pore and surface diffusion in a single particle, that are based on intraparticle quartic and parabolic concentration profile approximations, and that utilize the summation of the gas and adsorbed phases approach in the material balance formulations, were further validated under more diverse, yet more realistic, cycling conditions. Periodic square, sinusoidal and triangular wave functions were used to more accurately represent the periodic boundary conditions that the external surface of an adsorbent particle may be exposed to during repeated adsorption and desorption cycles in a fixed bed adsorber. Analytical solutions that describe the periodic uptake and release of the adsorbate by the adsorbent were obtained for all three periodic wave functions, and for both the quartic and parabolic profile approximations. By comparing the predictions obtained from both models with the exact numerical solution, the superiority of the quartic model over the parabolic model was clearly demonstrated for all wave functions, and for a wide range of adsorbate-adsorbent systems and bulk concentrations. Excellent agreement between the quartic and exact models was obtained in most cases. In general, the predictions improved as the wave function changed more gradually with time (triangular more gradual than sinusoidal and sinusoidal more gradual than square), as the degree of mathematical linearity of the adsorbate-adsorbent system increased, and as the maximum external surface concentration decreased (an isotherm nonlinearity effect). Subtle differences in the predictive ability of the new approximate models, stemming from the use of the different wave functions, were exposed. Overall, these results exemplify the importance of comparing the predictive ability of new approximate models that describe intraparticle transport under more diverse cycling conditions than are typically utilized in the literature, which has been dominated by the square wave function.

Dispersion within a model urban area

This paper analyzes the data collected in a field study conducted in Dugway Proving Ground, Utah, from 17 to 26 July 2001. The experiment was designed to simulate a source in an urban area modeled at a scale of roughly 1:10. The model urban canopy was constructed with 55-gallon drums laid out in a 5 by 9 array. Propylene (C3H6), the tracer, was released and sampled at 3 receptor arcs both within the barrel array and over flat terrain. Turbulence, velocity, and temperature were measured with sonic anemometers. A comparison of observations made with and without the obstacle array indicated that the obstacle array significantly increased lateral and vertical dispersion. The arc maximum surface concentrations observed within the array were well explained by a dispersion model based on that of Van Ulden [1978. Atmospheric Environment 12, 2125–2129] and modified by Venkatram [2004. Atmospheric Environment 38, 1337–1344] to use on-site measurements of mean wind and turbulence. The major conclusion of this study is that estimating dispersion within the urban canopy requires flow information below the canopy top.

The Reactive Chemisorption of Alkyl Iodides at Cu(110) and Ag(111) Surfaces: A Combined STM and XPS Study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.

Adsorptive voltammetry of Hg(II) ions at a glassy carbon electrode coated with electropolymerized methyl-red film

The glassy carbon electrode coated with electropolymerized methyl-red film, 1.2 × 10 −6 m in thickness, (PMRE) showed high sensitivity towards Hg(II) ions. PMREs were adopted to accumulate and detect Hg(II) ions in a pH 2.56 Britton–Robinson buffer solution. Cyclic voltammogram of the accumulated Hg species on PMREs exhibited an anodic wave at 0.64 V and a cathodic wave at 0.13 V, due to the oxidation of accumulated Hg species on PMREs and the reduction of Hg(II) ions in the solution, respectively. For this heterogeneous adsorption of Hg(II) ions onto PMREs, the maximum surface concentration, adsorption equilibrium, and Gibbs energy change were evaluated to be 5.12 × 10 −6 mol m −2, 3.7 × 10 5 l mol −1, and −30.1 kJ mol −1, respectively. The anodic peak current at 0.64 V was linear with the concentration of Hg(II) ions in the range of 1.1 × 10 −10 to 1.1 × 10 −7 M with a detection limit of 4.4 × 10 −11 M. The proposed method was utilized successfully for the detection of Hg(II) ions in the lake water.

Adsorption equilibrium of supercritical hydrogen on multi-walled carbon nanotubes

Based on the isotherms of excess adsorption amounts of hydrogen on multi-walled carbon nanotubes (MWCNTs) volumetrically measured at the temperatures from 123 K to 310 K under the pressures up to 12.5 MPa, the intermolecular interaction energy among hydrogen molecules was calculated by use of a lattice theory-based approximation adsorption model. The maximum surface concentration of hydrogen molecules upon the MWCNTs was fitted by way of linear regression. The interaction potential of hydrogen–MWCNTs and the isosteric heat of hydrogen adsorption in the low limit surface concentration were determined by use of gas-surface virial coefficients. It shows that hydrogen molecules accumulate mainly in the place where the potential is the minimum within the central tubular pore of the MWCNTs and they are likely to be in a state similar to that of compressed gas. However, there is no evidence to suggest that a significant adsorption of hydrogen molecules occurs on the MWCNTs. The hydrogen–hydrogen interaction energy increases linearly with the increase of adsorption temperature and surface loading, but its variation does not indicate any optimum temperatures for hydrogen storage in terms of adsorption on the MWCNTs.

Surface segregation kinetics of sulfur influenced by various conditions in thin-gauged 3% Si–Fe alloy strips

Abstract The surface segregation kinetics of sulfur has been investigated in detail in 3% Si–Fe alloy strips. The segregation profile of sulfur mostly shows a maximum during annealing at 1200 °C. The maximum surface concentration and the annealing time required to observe a certain concentration of segregated sulfur depend on hydrogen flow rate, heating rate and strip thickness.

Electrochemical quartz crystal nanobalance (EQCN) studies of the adsorption behaviour of an enzyme, mandelate racemase, and its substrate, mandelic acid, on Pt

The adsorption behaviour of the enzyme, mandelate racemase (MR), and its substrate, ( S)-mandelic acid (MA), was studied at a polycrystalline Pt surface in pH 7.4 phosphate buffer at 294 K using the electrochemical quartz crystal nanobalance (EQCN) technique of simultaneous cyclic voltammetry (CV) and frequency measurements. It was shown that the EQCN frequency measurements did not directly monitor the molar mass of the adsorbed protein at anodic potentials, but instead measured changes in the surface oxide in the absence and presence of adsorbed protein. However, at a potential characteristic of the double layer for platinum, EQCN frequency measurements gave a measure of the extent of solvent displacement by the adsorbed protein. The adsorption process was modelled using the Langmuir adsorption isotherm. The values for the Gibbs energy of adsorption, Δ G ADS, obtained with these EQCN frequency measurements gave excellent agreement within experimental uncertainty with those obtained from the simultaneous CV measurements for both the enzyme and substrate, and showed the enzyme to have a greater affinity for the surface than the substrate molecule. The maximum surface concentrations from CV and EQCN frequency measurements gave excellent agreement for the two techniques and showed MA to be adsorbed in a tilted orientation with monolayer coverage. On the other hand, the surface concentrations showed the presence of ∼2.5 monolayers of enzyme from charge transfer CV measurements, while the frequency results showed the presence of a densely packed monolayer from the “footprint” of solvent displacement by the adsorbed enzyme molecules on the electrode surface. The two techniques provide valuable complementary information on the interfacial behaviour of molecules.

Aromatic interactions in the close packing of phenyl-imides at Cu(1 1 0) surfaces

The oxidation of aniline at Cu(1 1 0) surfaces at 290 K has been studied by XPS and STM. A single chemisorbed product, assigned to a phenyl imide (C 6H 5N(a)), is formed together with water which desorbs. Reaction with preadsorbed oxygen results in a maximum surface concentration of phenyl imide of 2.8 × 10 14 mol cm −2 and a surface dominated by domains of three structures described by 4 0 2 2 , 4 0 - 1 2 and 4 0 1 2 unit meshes. However, concentrations of phenyl imide of up to 3.3 × 10 14 mol cm −2 were obtained from the coadsorption of aniline and dioxygen (300:1 mixture) resulting in a highly ordered biphasic structure with 3 0 - 1 2 and 3 0 1 2 domains. Comparison of the STM and XPS data shows that only half the phenyl imides at the surface are imaged. Pi-stacking of the phenyl rings is proposed to account for this observation.

The role of meteorological inputs in estimating dispersion from surface releases

Abstract Currently used models for dispersion in the surface layer are explicitly based on Monin–Obukhov (M–O) theory. Because M–O variables such as the surface friction velocity and M–O length are not always accessible or M–O similarity might not hold at the site of interest, there is a need for a model that uses alternative meteorological inputs. Here, we examine the use of near-surface profiles of turbulence and wind speeds instead of M–O variables. We use data from the Prairie Grass experiment (1958, Project Prairie Grass, a field program in diffusion, Vol. 1, Geophysics Research Paper No. 59. Air Force Cambridge Research Center, Bedford, MA) because of its completeness in terms of meteorological inputs as well as spatial coverage of concentration measurements. The maximum surface concentrations and the crosswind plume spreads are first explained using an M–O theory-based model proposed by van Ulden (1978, Simple estimates for vertical dispersion from sources near the ground. Atmospheric Environment 12, 2125–2129), whose performance has already been documented in the literature. The performance of this model is compared with those of models using alternative meteorological inputs. The results from the study indicate that, under stable conditions, models that use profiles of winds and turbulence do not perform as well as van Ulden's model. On the other hand, under unstable conditions, the differences in model performance are much smaller. This supports other studies that indicate that relatively simple dispersion models can be used to estimate concentrations in urban areas, where the boundary layer is often unstable even during the night.

Determination of the Gibbs excess of H and OH adsorbed at a Pt(1 1 1) electrode surface using a thermodynamic method

The thermodynamics of the so-called perfectly polarized electrode were employed to analyze the total charge densities for a Pt(1 1 1) electrode in a series of solutions (0.1− x) M HClO 4+ x M KClO 4 with a constant ionic strength and a variable hydronium ion concentration. The total charge densities were calculated by integration of cyclic voltammetry curves. A complete thermodynamic analysis using charge and potential as independent variables has been performed. The Gibbs excesses of adsorbed hydrogen and OH were determined without the need to introduce an arbitrary correction for the so-called charging of the double layer and further assumptions about the charge number per adsorbed species. We found that the maximum surface concentration of H amounts to ∼11×10 14 ions cm −2 or to a coverage 0.73 monolayer (ML) in good agreement with previous estimates. The maximum surface coverage of OH is equal to ∼8×10 14 ions cm −2 or to a coverage ∼0.53 ML. Using the thermodynamic method, the charge numbers at a constant potential (electrosorption valency) and at a constant chemical potential (reciprocal of the Esin–Markov coefficient) for adsorbed H and OH were calculated using the Gibbs excess data. The charge numbers are equal to ∼1 for H and to ∼−1 for OH. These values suggest that the adsorbed species is hydrogen atom and discharged OH species.

The influence of electrolyte concentration on the parameters of the Frumkin isotherm in the Cd 2+–I − system

The influence of ionic strength (1–6 mol l −1 NaClO 4) on the parameters of adsorption isotherm(s) was studied using Cd 2+–I − system as a model. Results, based on differential pulse polarographic measurements, simulation of polarograms and signal fitting procedure, indicate that the maximum surface concentration ( Γ max=1×10 −10 to (2.2±0.2)×10 −10 mol cm −2), adsorption constant ( β=3.1×10 5 to (1.55±0.35)×10 7 l mol −1) and Frumkin coefficient ( a=0 to (0.5±0.1)) all increase with increasing electrolyte concentration. According to the model calculations, parallel increase of the adsorption constant and Frumkin coefficient should have the opposite influences on the shape of the reduction signal. In the system of interest, however, the former seems to be more important than the latter. As a result, gradual transformation of a single symmetrical peak to two close peaks can be observed on real polarograms at increasing electrolyte concentration.

The distribution of the chlorinated solvents dichloromethane, perchloroethylene, and trichloroethylene in the global atmosphere.

Dichloromethane, perchloroethylene, and trichloroethylene are commercially important chlorinated solvents whose health and environmental impacts are under scrutiny in the industrial world. Their distributions in the global atmosphere have been computed based on data from the Reactive Chlorine Emissions Inventory (RCEI) project using the Global Balance Environment (GLOBE) model, a 3-D radiative-dynamical-chemical model. Their atmospheric lifetimes, scaled to an observed methyl chloroform lifetime of 4.8 years, are 158 days, 105 days, and 4.3 days, respectively. They have strong interhemispheric gradients, with maximum zonal mean surface concentrations in the winter mid-latitude northern hemisphere of approximately 40 ppt, 9 ppt, and 2.5 ppt, respectively. Their spatial distributions show significant seasonal variability, and are sensitive to vertical mixing by cumulus convection and horizontal mixing by synoptic-scale turbulence. While the model interhemispheric exchange time (1.0 years) and computed atmospheric lifetimes are very sensitive to sub-grid scale diffusion, interhemispheric gradients of the chlorinated solvents are not. The simulated results suggest a greater importance for oceanic emissions of perchloroethylene and trichloroethylene than has previously been assumed.

Fibrinogen adsorption by PS latex particles coated with various amounts of a PEO/PPO/PEO triblock copolymer

Polystyrene (PS) latex particles of different sizes were adsorption coated with the polymeric surfactant Pluronic F108 (PEO129-PPO56-PEO129). The commercial surfactant was found to have a bimodal molecular weight distribution. However, the maximum surface concentrations resulting from adsorption of either the purified high molecular weight component or the composite were identical. An increase in the copolymer surface concentration on 252-nm particles was found to decrease their fibrinogen uptake exponentially. At maximum copolymer surface concentration, the fibrinogen uptake was two orders of magnitude lower than that of bare particles (down from 3.3 mg/m2 to 0.03 mg/m2). This surface protection was equally effective whether the adsorption involved the bimodal polymer surfactant or the purified high molecular weight fraction. The PEO tail mobility was investigated with electron paramagnetic resonance (EPR), and found to increase with an increase in polymer surface concentration. The comparatively slow motion of the PEO chains at low surface concentration indicated that not only the PPO block, but also the PEO blocks interacted hydrophobically with the PS surface. When the copolymer surface concentration was increased, the PEO tails were gradually being released, acquiring higher mobility as the surface became covered by the more strongly binding PPO blocks. Results obtained with F108 coated particles of different sizes showed that particle size had a significant effect on the fibrinogen uptake, with larger particles showing larger fibrinogen uptakes.

Determination of adsorption of an ionic surfactant on latex from surface tension measurements

The adsorption isotherm of sodium dodecyl sulfate (SDS) on polystyrene (PSt) latex is evaluated from surface tension ( γ) measurements of aqueous SDS solutions and PSt latex solutions. A video-enhanced pendant bubble/drop tensiometer is applied for the equilibrium and dynamic surface tension measurement of SDS aqueous solutions and PSt latex solutions. The bulk concentrations of un-absorbed SDS molecules ( C SDS, SDS in water bulk phase) in PSt latex solutions are evaluated from equilibrium surface tension data, which is uniquely related between the equilibrium γ and bulk concentration C SDS for SDS aqueous solutions. From the mass balance calculation, m total= C SDS× V water+ Γ SL× A (here A is the surface area of pendant drop of latex solution), the surface concentration of SDS ( Γ SL) on PSt latex is then determined for aqueous latex solutions with different SDS concentrations. A unique relationship between Γ SL and C SDS is resulted and the Langmuir adsorption isotherm, with maximum surface concentration of SDS at PSt latex ( Γ ∞=5.75×10 −10 mol cm −2) and equilibrium constant a=6.49×10 −6 mol cm −3, describes this dependence well.

Formation and Reductive Desorption of Mercaptohexanol Monolayers on Mercury

The formation and reductive desorption of self-assembled monolayers of 6-mercaptohexanol on mercury has been studied by using cathodic stripping voltammetry and capacitative transients, including the possibility of expanding or contracting the electrode area at the end of the preconcentration step. Experimental evidence shows the existence of three sequential stages during the formation of a thiol self-assembled monolayer. Each of these stages can be associated to the presence of (i) a low surface density state of oxidized thiol molecules, characterized by a single electrodimerization wave, (ii) a high surface density state, characterized by the emergence of a second voltammetric wave, and (iii) an ordered monolayer, which gives rise to a voltammetric spike. On the basis of electrode expansion experiments, a method is described to determine the surface concentrations of oxidized products, which does not require a baseline subtraction of the voltammograms to account for the nonfaradaic current. Quantitative voltammetric fits are consistent with the initial formation of a mixture of noninteracting monomers and dimers of oxidized thiol. The value of the maximum surface concentration and the ability to block the Ru(NH3)63+ electron transfer reveal that oxidized thiol molecules adopt a nearly perpendicular orientation in the high surface density state, which hampers ionic permeation. A theoretical model is proposed to account for the observed voltammetric behavior. The transition from the lower to the higher surface density states is modeled as a chemical step involving the exchange of metal free sites. Capacitative transients are also interpreted in terms of the three-stages model.

Ultrashallow and Low-Leakage p+n Junction Formation by Plasma Immersion Ion Implantation (PIII) and Low-Temperature Post-Implantation Annealing

p+n junctions are fabricated by the combination of BF3 plasma immersion ion implantation (PIII) and low-temperature post-implantation annealing. Ultrashallow junctions and very low leakage currents (∼10-11 A at -1 V in 1 mm2 junctions) have been obtained. Despite the fact that the activation of the PIII layer is not sufficient, the I–V characteristics obtained are comparable to those of junctions formed by conventional ion implantation. By a combination of Si+ preamorphization and BF3 PIII aimed to improve the dopant activation, a maximum surface concentration of activated boron higher than 1020 cm-3 is obtained in 550°C annealing for p+n junctions with 5 nm depth. The leakage currents, however, are increased by the Si+ implantation in low-temperature post-implantation annealing.

Electrochemical Studies of the Effect of Temperature on the Adsorption of Yeast Alcohol Dehydrogenase at Pt

The interfacial behavior of yeast alcohol dehydrogenase (YADH) without and with the coenzyme nicotinamide adenine dinucleotide (NADH−YADH) at a Pt surface was studied over the temperature range 273−353 K in a phosphate buffer solution of pH 7.0, using cyclic voltammetry and electrochemical impedance spectroscopy. It was shown that the surface charge density and corresponding polarization resistance, resulting from protein adsorption and its oxidation, respectively, are directly proportional to the amount of adsorbed protein (surface concentration), indicating that adsorption at anodic potentials is accompanied by the transfer of charge, that is, chemisorption through carboxylate groups on the protein. The adsorption process for both proteins was described with the Langmuir adsorption isotherm, which revealed very high affinity of the proteins toward adsorption onto a Pt surface. From the calculated Gibbs energies of adsorption, it was concluded that both proteins strongly adsorb onto the Pt surface via chemisorption. The adsorption process for YADH was found to be exothermic. However, the adsorption of NADH−YADH resulted in an endothermic adsorption process as a result of the presence of the coenzyme, nicotinamide adenine dinucleotide (NAD+/NADH), in NADH−YADH which stabilizes the protein against denaturation. However, the adsorption of both proteins was found to be an entropically governed process, suggesting structural unfolding of the proteins at the electrode surface. The maximum surface concentration values indicate that there is no significant difference in the amount of adsorbed proteins between YADH and NADH−YADH in the whole temperature range investigated.

Advanced Gettering Techniques in ULSI Technology

Progress in silicon technology has been phenomenal since the invention of the transistor some 50 years ago. Device performance has improved by at least a factor of a million in every respect. As the minimum feature size on chips decreases toward 0.1 μm, which should be reached on a mass-production scale in a few years, the device yield is becoming ever more sensitive to defects and impurities. Transition metals, particularly iron, nickel, and copper, are the most common and most detrimental contaminants on a process line, and they can be unintentionally introduced in nearly every process step, including ingot growth, wafer handling, ion implantation, wet-chemical cleaning, high-temperature anneals, or oxidation. To avoid yield losses, the silicon industry has to be very strict with respect to metal-contamination levels on the production line. For instance, for iron, the Semiconductor Industry Association (SIA) Roadmap presently specifies 2.5 ϗ 1010 cm−2 as the maximum tolerable surface concentration, decreasing to 5 ϗ 109 cm−2 by 2004.

A Study on Surfactant Adsorption Kinetics: Effect of Bulk Concentration on the Limiting Adsorption Rate Constant

A limiting adsorption rate constant βl was defined as the value at which the mixed diffusive-kinetic controlled surface tension profile is indistinguishable from the diffusion-controlled one (Hsu et al. J. Chem. Eng. Jpn. 1996, 29, 634). The value of βl depends on surfactant, bulk concentration, the shape of fluid interface, and the process (adsorption or desorption). In this work, the dependence of βl on surfactant and bulk concentration (C) was examined. First, the dependence of βl vs C of eight well-known surfactants was investigated. In the second part, the effects of equilibrium constant (surfactant activity), molecular interaction, and maximum surface concentration were systematically studied. The difference of βl for a planar and a spherical interface with 0.1 cm radius of curvature was also examined. A universal relationship in terms of dimensionless variables was obtained. Using this universal relationship, one needs only the model constants of adsorption isotherm of surfactant to evaluate the value of βl at any concentration and to know the dependence of βl as a function of surfactant concentration.

Surface activity and thermodynamic properties of water-soluble polyester surfactants based on 1,3-dicarboxymethoxybenzene used for enhanced oil recovery

The surface activity and thermodynamic properties for eight low molecular weight nonionic co-polyester (PE) surfactants have been investigated. Surface and interfacial tensions (IFT) of surfactants in aqueous solutions were measured using the spinning drop technique. From these measurements, the critical micelle concentration (CMC), the surface pressure at CMC (YCMC), the maximum surface concentration (Γmax), the minimum area/molecule at the aqueous solution/air interface (Amin), the effectiveness of surface tension reduction (ΠCMC), the alkane carbon number (nmin) and the IFT at nmin (Ymin) were determined. The thermodynamic parameters of micellization (ΔGmic, ΔHmic and ΔSmic) and of adsorption (ΔGad, ΔHad and ΔSad) for these polymeric surfactants were also calculated. Structural effects on micellization and adsorption are discussed in terms of these parameters. The results show that the ΔGad values were more negative than ΔGmic values for these compounds, so that they favored adsorption before the micellization process. They exhibited IFT in the order of 10−3 to 10−4 mN/m against the thin alkane carbon number range 6–9. This range seemed to be prefered for enhanced oil recovery. Copyright © 2000 John Wiley & Sons, Ltd.