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We study, using Monte Carlo simulations, the interaction between infinite heterogeneously charged surfaces inside an electrolyte solution. The surfaces are overall neutral with quenched charged domains. An average over the quenched disorder is performed to obtain the net force. We find that the interaction between the surfaces is repulsive at short distances and is attractive for larger separations. [ABSTRACT FROM AUTHOR]

Platinum in the form of nanoparticles is the key and most expensive component of polymer electrolyte membrane fuel cells, while trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid. Nafion, which acts as both electrolyte and separator in fuel cells, contains -CF2SO3H groups. Consequently, research on the electrochemical behaviour of Pt in aqueous CF3SO3H solutions creates important background knowledge that can benefit fuel cell development. In this contribution, Pt electro-oxidation is studied in 0.1 M aqueous CF3SO3H as a function of the polarization potential (Ep, 1.10 ≤ Ep ≤ 1.50 V), polarization time (tp, 100 ≤ tp ≤ 104 s), and temperature (T, 278 ≤ T ≤ 333 K). The critical thicknesses (X1), which determines the applicability of oxide growth theories, is determined and related to the oxide thickness (dox). Because X1 > dox for the entire range of Ep, tp, and T values, the formation of Pt surface oxide follows the interfacial place-exchange or the metal cation escape mechanism. The mechanism of Pt electro-oxidation is revised and expanded by taking into account possible interactions of cations, anions, and water molecules with Pt. A modified kinetic equation for the interfacial place exchange is proposed. The application of the interfacial place-exchange and metal cation escape mechanisms leads to an estimation of the Ptδ+-Oδ- surface dipole (μPtO), and the potential drop (Vox) and electric field (Eox) within the oxide. The Pt-anion interactions affect the oxidation kinetics by indirectly influencing the electric field within the double layer and the surface oxide. [ABSTRACT FROM AUTHOR]

The atmospheric CO2 concentration continues a rapid increase to its current record high value of 416 ppm for the time being. It calls for advanced CO2 capture technologies. One of the attractive technologies is physical adsorption‐based separation, which shows easy regeneration and high cycle stability, and thus reduced energy penalties and cost. The extensive research on this topic is evidenced by the growing body of scientific and technical literature. The progress spans from the innovation of novel porous adsorbents to practical separation practices. Major CO2 capture materials include the most widely used industrially relevant porous carbons, zeolites, activated alumina, mesoporous silica, and the newly emerging metal‐organic frameworks (MOFs) and covalent‐organic framework (COFs). The key intrinsic properties such as pore structure, surface chemistry, preferable adsorption sites, and other structural features that would affect CO2 capture capacity, selectivity, and recyclability are first discussed. The industrial relevant variables such as particle size of adsorbents, the mechanical strength, adsorption heat management, and other technological advances are equally important, even more crucial when scaling up from bench and pilot‐scale to demonstration and commercial scale. Therefore, we aim to bring a full picture of the adsorption‐based CO2 separation technologies, from adsorbent design, intrinsic property evaluation to performance assessment not only under ideal equilibrium conditions but also in realistic pressure swing adsorption processes. [ABSTRACT FROM AUTHOR]

In this paper we investigate the adsorption of various probe molecules in order to characterize the porous structure of a series of pillared interlayered clays (PILC). To that aim, volumetric and microcalorimetric adsorption experiments were performed on various Zr PILC samples using nitrogen, toluene, and mesitylene as probe molecules. For one of the samples, neutron scattering experiments were also performed using toluene as adsorbate. Various structural models are proposed and tested by means of a comprehensive computer simulation study, using both geometric and percolation analysis in combination with Grand Canonical Monte Carlo simulations in order to model the volumetric and microcalorimetric isotherms. On the basis of this analysis, we propose a series of structural models that aim at accounting for the adsorption experimental behavior, and make possible a microscopic interpretation of the role played by the different interactions and steric effects in the adsorption processes in these rather complex disordered microporous systems. [ABSTRACT FROM AUTHOR]

The activity in lattice-gas systems with geometric constraints is shown to be the ratio of the number of particles to the number of available sites. The key role of sites available for occupation is emphasized. Available sites may be different for different species and are not necessarily just unoccupied sites. Location-specific or non-local constraints are allowed. An analytical expression for the number of available sites is given for the hard-hexagon model. The utility of an expression for available sites is illustrated for the non-trivial case of a mixed Langmuir/hard-hexagon adsorption system, where the influence of the Langmuir adsorbates on the hard-hexagon phase transition is investigated. The dependence on available sites indicates how to extend these results to the kinetic regime and simulations of kinetic voltammograms for the hard-hexagon model are given as an example. [ABSTRACT FROM AUTHOR]

Using aluminum oxide (Al2O3) films deposited by atomic layer deposition (ALD), the dominant passivation mechanisms at the c-Si/Al2O3 interface, as well as the chemical composition of the interface region, are investigated. The excellent surface passivation quality of thin Al2O3 films is predominantly assigned to a high negative fixed charge density of Qf = - (4 ± 1) × 1012 cm-2, which is located within 1nm of the Si/Al2O3 interface and is independent of the layer thickness. A deterioration of the passivation quality for ultrathin Al2O3 layers is explained by a strong increase in the interface state density, presumably due to an incomplete reaction of the trimethyl-aluminum (TMA) molecules during the first ALD cycles. A high oxygen-to-aluminum atomic ratio resulting from the incomplete adsorption of the TMA molecules is suggested as a possible source of the high negative charge density Qf at the Si/Al2O3 interface. [ABSTRACT FROM AUTHOR]

The behavior of liquids separated by a single graphene membrane has been studied with extensive molecular dynamics (MD) simulations at ambient conditions. With the help of appropriate empirical potentials, we have exploited two liquid phases forming distinct systems; say XGY, where G stands for graphene and X (Y) represents water (W), benzene (B), or acetonitrile (A). Our MD simulations revealed important changes in the wettability patterns of these liquids near the graphene surface. For instance, WGW exhibits strong density oscillations in a thin interfacial region with thickness of ∼2.4 nm. In the case of BGB and AGA the oscillating-density interfacial region extends beyond ∼3 nm and ∼5 nm, respectively, under ambient conditions. More interestingly, our findings indicate that a liquid at one side of the graphene sheet can affect the degree of wetting on the other side, by means of dispersion interactions through the graphene membrane. These systems can offer a useful framework to understand the structural as well as thermodynamic properties of interfaces by considering a real two-dimensional substrate. [ABSTRACT FROM AUTHOR]

The interface between a fluid and a static wall is a useful model for a chemically heterogeneous solid-liquid interface. In this work, we outline the calculation of the wall-fluid interfacial free energy (γwf) for such systems using molecular simulation combined with adsorption equations based on Cahn’s extension of the surface thermodynamics of Gibbs. As an example, we integrate such an adsorption equation to obtain γwf as a function of pressure for a hard-sphere fluid at a hard wall. The results so obtained are shown to be in excellent agreement in both magnitude and precision with previous calculations of this quantity, but are obtained with significantly lower computational effort. [ABSTRACT FROM AUTHOR]

Recent studies have shown that there exist precise connections (or covariance relations) between adsorption properties for substrates with different shapes. This occurs, for example, when a fluid is adsorbed in a linear wedge. In this case, the influence of the geometry is to shift effectively the contact angle from θ to θ-α, where α is the tilt angle. Despite the fact that these relations are obeyed both at mean-field level and also exactly in two dimensions (when fluctuation effects dominate), their fundamental origin has been unclear. Here, we show that they can be traced to a symmetry present in interfacial Hamiltonian models, and further relate this to surface thermodynamics and the nonlocal nature of interfacial interactions in systems with short-ranged forces. [ABSTRACT FROM AUTHOR]

The Axilrod–Teller equation is incorporated into nonlocal density functional theory (NLDFT) to improve the description of low-temperature argon adsorption isotherm on graphitized carbon black. Using the NLDFT based on the Tarazona smoothed density approximation and the test-particle method, the pair and triplet distribution functions for liquid argon were analyzed. The latter allowed us to quantitatively account for the three-body Axilrod–Teller nonadditive term. An analogous scheme to that of Weeks, Chandler, and Andersen is proposed to incorporate the Axilrod–Teller equation into the NLDFT in the framework of mean field approximation. The three-body nonadditive term appeared to decrease the internal energy of liquid argon by about 5%, which is in agreement with molecular simulation results. The new approach has been applied to the vapor-liquid coexistence and to argon adsorption on graphitized carbon black at 87.3 K. The approach has been shown to fit the experimental adsorption isotherm within the relative error of about 4% as opposed to 37% in the case of the standard NLDFT. This is due to weakening of the intermolecular interaction potential by about 20% in the molecular layer nearest to the graphite surface resulted mainly from the positive Axilrod–Teller contribution of triplets comprising one carbon atom. [ABSTRACT FROM AUTHOR]

A pillared interlayered clay is represented by a two-dimensional quenched charged disordered medium, in which the pillar configuration is produced by the quench of a two-dimensional electrolyte and the subsequent removal of the anions (that act as a template). The cation charge is counterbalanced by a neutralizing background that is an ideal representation of the layer’s negative charge in the experimental system. In this paper we investigate the adsorption of electrolyte particles in this charged disordered medium resorting both to the use of the replica Ornstein–Zernike equation in the hypernetted chain approximation and grand canonical Monte Carlo simulations. The theoretical approach qualitatively reproduces the simulated behavior of the adsorbed fluids. Theoretical estimates of the material porosities obtained for various types of pillar distributions are in good agreement with the simulation. We investigate the influence of the matrix on correlation functions and adsorption isotherms. [ABSTRACT FROM AUTHOR]

In nanochannel flows, the thermal resistance at the fluid-solid interface may depend on the flow scenario. In this work, we study the interfacial thermal resistance Rth in nanoscale force-driven flows at different temperatures and fluid-wall interactions. For Ar flows in Cu and Ag channels, the fluid-wall binding energy is strong and it is found that Rth assumes a maximum value as the external force is varied. The maximum value is caused by the fluid adsorption on the solid surfaces and the temperature increase in the fluid due to viscous frictions. However, when the fluid-wall interaction is weak, the maximum value is not observed and the interfacial thermal resistance decreases monotonously with increasing external force. With the presence of fluid adsorption, it is also found that the peak in Rth is more detectable at low temperature than high temperature. [ABSTRACT FROM AUTHOR]

We study the behavior of random AB-copolymer melts near a selective surface. We consider the case where the copolymers do not display phase segregation behavior in the bulk but the surface is strongly selective for the A-component and the probability of finding an A-monomer along the chain is p<1. Using self-consistent field theory and scaling arguments, we discuss some aspects of conformational rearrangements and composition selection in the surface layer. For strong selectivity we discuss the formation of a polydisperse brush on the surface. Next, we consider selection mechanisms of chains and sequences of A-species in the surface layer. We used the bond-fluctuation method to simulate copolymer melts at different values of the surface selectivity. Several aspects of the surface layer are analyzed, such as the composition profiles, chemical composition of chains on the surface, chain extension, and dynamics. We find evidence for conformational rearrangements in the surface layer according to the polydisperse brush model, as well as enrichment of A-monomers in the adosorbed chains, stretching of chains in the direction perpendicular to the surface, and selection of multiple A-sequences. Slight but systematic variation of the properties of surface layer at long simulation times indicates that selection processes require very long time scales as expected from theoretical arguments. [ABSTRACT FROM AUTHOR]

The adsorption process of a homopolymer chain nongrafted on an attractive surface is numerically investigated using replica-exchange multicanonical Monte Carlo simulation. Based on the microcanonical analysis, the microcanonical entropy in the adsorption transition shows convex features. Correspondingly, with the coexistence of two phases, negative specific heat is also observed in the region, implying first-orderlike transition. The origin of the negative specific heat is due to the nonextensitivity of the energy in the system. This adsorption process has some similarities to the nucleation and growth mechanism in the crystalline process. Further study reveals that the transition type, either first- or second-orderlike, during chain adsorption is strongly dependent on the chain length, interactions among segments, whether chain grafted on the surface, and force upon segments imposed by surface. [ABSTRACT FROM AUTHOR]

The NO chemisorption dynamics on ordered multilayer iron phthalocyanine (FePc) and quasiamorphous multilayer tetra-t-butyl FePc (ttbu-FePc) films on a Au(111) substrate was investigated using the King and Wells reflection technique. The NO zero coverage or initial sticking probabilities (S0) were measured as a function of sample temperature (Ts) and beam energy (Ei). The experimental results for both films show a monotonic decrease in S0 with increasing Ts and Ei consistent with NO adsorption occurring via a multiple pathway precursor-mediated mechanism in which the adsorbate initially physisorbs to the FePc organics, diffuses, and chemisorbs to the Fe metal center. The saturation coverage is 3% for the multilayer FePc surface and only 2% for the multilayer ttbu-FePc surface consistent with NO chemisorption occurring only on the Fe metal, where NO chemisorbs to 100% of the surface Fe metal centers. The reduced saturation coverage in the ttbu-FePc film is attributed to fewer Fe metal centers in the less dense ttbu-FePc films. A comparison of NO sticking on a multilayer FePc/Au(111) film with NO sticking on a monolayer FePc/Au(111) film shows that S0 is greater on the multilayer FePc film for all Ts and Ei, consistent with an increase in collision inelasticity for NO/multilayer FePc/Au(111). [ABSTRACT FROM AUTHOR]

In the present paper, the multicomponent adsorption of polyatomic species is described as a fractional statistics problem, based on Haldane’s statistics. Site exclusion is characterized by a “mutual exclusion matrix” g, which relates to the sizes of the different species and lattice geometry. The adsorption process has been monitored through total and partial isotherms, energy of adsorption and configurational entropy of the adsorbed phase. The thermodynamic functions calculated for a monomer-dimer mixture were applied to describe the adsorption of methane-ethane mixtures in zeolites. In the case of zero lateral interactions, the present approach was compared to the well-known ideal adsorbed solution theory. The results show that the treatment of this complex problem can be significantly simplified if looked up from the new theoretical perspective. [ABSTRACT FROM AUTHOR]

As is well known, liquid-liquid phase separation can occur either upon heating or cooling, corresponding to lower and upper critical solution phase boundaries, respectively. Likewise, self-assembly transitions from a monomeric state to an organized polymeric state can proceed either upon increasing or decreasing temperature, and the concentration dependent ordering temperature is correspondingly called the “floor” or “ceiling” temperature. Motivated by the fact that some phase separating systems exhibit closed loop phase boundaries with two critical points, the present paper analyzes self-assembly analogs of re-entrant phase separation, i.e., re-entrant self-assembly. In particular, re-entrant self-assembly transitions are demonstrated to arise in thermally activated equilibrium self-assembling systems, when thermal activation is more favorable than chain propagation, and in equilibrium self-assembly near an adsorbing boundary where strong competition exists between adsorption and self-assembly. Apparently, the competition between interactions or equilibria generally underlies re-entrant behavior in both liquid-liquid phase separation and self-assembly transitions. [ABSTRACT FROM AUTHOR]

We interpret the measured sensitivity difference of single walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs) used as dielectric sensors of ammonia gas traces in terms of adsorption sites and effective coverages. The dielectric constant of a regular set of SWNTs deposited on a plane is compared to that of a single MWNT having the same size when the same pressure of ammonia molecules is applied on them. The corresponding resonance frequency shifts in the two sensors display an enhanced sensitivity of SWNT over the MWNT in excellent agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]

A first generation of parallel scanning tunneling microscopy (STM) simulator has been developed to accelerate the production of high quality STM images. An efficient master-slave parallel scheme has been constructed specially suited for large scale problems in which the amount of data communications remains a small fraction of the entire calculation. We apply the new parallel scheme to two examples, benzene adsorption on a metal surface and standing wave patterns on the Cu(111) surface, highlighting the efficiency of our approach. [ABSTRACT FROM AUTHOR]

We describe a method for characterizing self-assembled monolayers (SAMs) in terms of their performance as antirelaxation wall coatings for alkali atom vapor cells. A combination of initial surface analysis and subsequent laser spectroscopy is used to provide insight into the quality of the coating, as well as its performance under the exposure to alkalis as it occurs, for example, when used in applications such as atomic magnetometers or clocks. Fused silica plates coated with octadecyltrichlorosilane SAMs were used to make cubic 87Rb gas cells. The surface was characterized by x-ray diffraction, contact angle measurements, and atomic force microscopy. Measurements of hyperfine resonance linewidths and frequency shifts show that the rubidium vapor atoms collide up to 40 times with the walls of the cells before their coherence relaxes and their adsorption energy is around 0.065 eV. Chemical analysis of the cell indicates some weak reactions between the coating and the rubidium atoms. [ABSTRACT FROM AUTHOR]

Structural and electronic properties for oxygen-adsorbed graphene sheets have been explored using first-principles total-energy calculations within the local density functional theory. A finite energy gap emerges for the oxygen-adsorbed graphene and its value increases with the ratio of O/C, as manifested by experiments. Further, adsorption energy and migration barrier for oxygen atoms on the graphene sheet have been investigated. The results show that isolated oxygen atoms are highly mobile and incline to condense on the graphene sheet. [ABSTRACT FROM AUTHOR]

We present here a first principles density functional theory investigation of the reactivity of Pt(111)-skin catalysts, which are varied from surface alloys with Ni to bulk PtxNi1-x (x=0.25,0.50,0.75) alloys. Molecule (CO, O, and H) adsorption and oxidation of CO+O and H+O reactions were studied and analyzed in detail. Independent of the adsorbates, the interaction between adsorbates and substrates becomes weakened with increase in Ni, due to the downshift of d-band center of surface Pt atoms. Moreover, activation barriers of CO and H oxidation toward atomic oxygen gradually decrease. In term of CO preferential oxidation (PROX) in excess of hydrogen, it turns out that the overall reactivity and selectivity rely on the optimum of various elementary steps involved such as competitive molecular (dissociative) adsorption and oxidation reaction. The present calculations show that Pt3Ni(111) with Pt overlayer is an optimum catalyst for CO PROX in excess of hydrogen. [ABSTRACT FROM AUTHOR]

Results of experimental studies, and theoretical calculations utilizing classical trajectories, have shown that dissociation of H2 on the Pt(211) stepped surface is enhanced at low energies by a molecular trapping mechanism. Because quantum effects can play a large role at the low energies and long lifetimes that characterize molecular trapping, we have undertaken quantum dynamics calculations for this system, the first to treat all molecular degrees of freedom of a gas molecule reacting on a stepped metallic surface. The calculations show that molecular trapping persists in the quantum system, but only at much lower energies than experimentally seen, pointing to possible deficiencies in the potential energy surface. Classical and quasiclassical trajectory calculations on the same potential provide a reasonable picture of reaction overall, but many of the finer details are inaccurate, and certain classical reaction mechanisms are entirely invalid. We conclude that some skepticism should be shown toward any classical study for which long-lived trapping states play a role. [ABSTRACT FROM AUTHOR]

By using first-principles calculations, we theoretically investigate the surface reconstruction caused by the adsorption of Sr on the Si (001) surface. For the Sr coverage below 12 monolayer (ML), the system behavior is dominated by the charge transfer between Sr and silicon that results in the “unbuckling” of Si dimers. At a higher coverage, the surface begins to “undimerize.” At the 1 ML coverage, a geometric size constraint induces a series of reconstructions such as 3×, 5×, 7×, etc., characterized by 1×1 reconstructed patches separated by a dimer row. Calculations of the surface energy as function of coverage suggest that for 1 ML of Sr 3×, 5×, and 7× reconstructions of the surface are preferred under the Sr rich conditions, which is in good agreement with the experiment. [ABSTRACT FROM AUTHOR]

We perform comprehensive long-time monitoring of the p-doping and de-doping of poly(3-hexyl thiophene) under changing external conditions of oxygen, light, and temperature. They are shown to be controlled by the complex adsorption and desorption process with time scales ranging from seconds to weeks. The oxygen doping at atmospheric pressure takes several hours in the dark. The doping is dramatically accelerated to be within seconds with light of wavelength of 500–700 nm. Even at low oxygen pressure of 10-4 torr doping occurs within minutes with light. The de-doping by oxygen desorption takes as long as weeks at room temperature and vacuum of 10-4 torr, but when the temperature is raised to near the polymer glass temperature of 370 K, the de-doping is accelerated to minutes as the enhanced chain motion releases the trapped oxygen. Even though visible and near infrared light causes very efficient doping within seconds or minutes depending on vacuum level, such light-induced doping is not a chemical reaction and is fully reversible by thermal annealing at the end without sacrificing the mobility. For the polymer field-effect transistors, only the carrier density is changed while the mobility remains roughly a constant for all the conditions. [ABSTRACT FROM AUTHOR]

Gallium adsorption and desorption on c-plane sapphire has been studied by real-time grazing incidence small-angle x-ray scattering and x-ray fluorescence as a function of substrate temperature (680–740 °C) and Ga flux. The x-ray techniques monitor the surface morphology evolution and amount of Ga on the surface. During deposition, nanodroplets of liquid Ga are observed to form on the surface and coarsen. The growth of droplet size during continuous deposition follows dynamical scaling, in agreement with expectations from theory and simulations which include deposition-induced droplet coalescence. However, observation of continued droplet distance scale coarsening during desorption points to the necessity of including further physical processes in the modeling. The desorption rate at different substrate temperatures gives the activation energy of Ga desorption as 2.7 eV, comparable to measured activation energies for desorption from Ga droplets on other substrates and to the Ga heat of vaporization. [ABSTRACT FROM AUTHOR]

Satisfying the conditions of high sensitivity and high selectivity using portable sensors that are also reversible is a challenge. Miniature sensors such as microcantilevers offer high sensitivity but suffer from poor selectivity due to the lack of sufficiently selective receptors. Although many of the mass deployable spectroscopic techniques provide high selectivity, they do not have high sensitivity. Here, we show that this challenge can be overcome by combining photothermal spectroscopy on a bimaterial microcantilever with the mass induced change in the cantilever’s resonance frequency. Detection using adsorption-induced resonant frequency shift together with photothermal deflection spectroscopy shows extremely high selectivity with a subnanogram limit of detection for vapor phase adsorbed explosives, such as pentaerythritol tetranitrate (PETN), cyclotrimethylene trinitramine (RDX), and trinitrotoluene (TNT). [ABSTRACT FROM AUTHOR]

The signal enhancement of an easy to fabricate, nanoporous alumina assisted surface plasmon resonance (SPR) sensor is investigated. It is theoretically shown that the presence of a thin (under 200 nm) porous alumina layer on top of an aluminum film supporting the surface plasmons, may significantly increase (over one order of magnitude) the sensitivity of the SPR method in the case where the adsorption of relatively small molecules is probed. The comparative experimental investigation of self-assembled monolayer formation on planar metal films and porous alumina layers verifies the theoretical predictions. Based on these results, we discuss the extended applicability of this setup in biosensor and other related applications. [ABSTRACT FROM AUTHOR]

Using first-principles computations, we studied NH3 adsorption on a series of zigzag (n,0) single-walled BN nanotubes (BNNTs) and the effect of gas coverage. Tube diameter and NH3 coverage play important roles on the tube-NH3 interaction. Chemisorption of a single NH3 molecule on top of B site is energetically preferable for all the tubes studied, but the adsorption energy decreases sharply with increasing tube diameter, and then gradually approaches the value for NH3 physisorption on BN graphene layer. On the sidewall of (10,0) BNNT, NH3 molecules prefer to pair arrangement on top of B and N atoms opposite in the same hexagon. At low coverages, NH3 molecules are partly chemically bound to BNNTs. With the increase of NH3 coverage, hydrogen bonds form between the adsorbed NH3 molecules or between the NH3 molecules and N atoms in BNNTs. When the coverage reaches 25%, the chemisorption of NH3 transforms to physisorption completely. NH3 adsorption does not modify the overall band structures of BNNTs, irrespective of NH3 coverage, but the band gap is narrowed due to the NH3-tube coupling and tube deformation. [ABSTRACT FROM AUTHOR]

The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, ∼0.5 μm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption. [ABSTRACT FROM AUTHOR]

The adsorption, desorption, and diffusion kinetics of N2 on thick (up to ∼9 μm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption. Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 μm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films (>1 μm), N2 adsorption at 27 K results in a nonuniform distribution, where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is ∼7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Preadsorption of Kr preferentially adsorbs onto the highest energy binding sites, thereby preventing N2 from trapping in the outer region of the film which facilitates N2 transport deeper into the porous film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor mediated, and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed second layer N2 species on top of the first surface bound layer. [ABSTRACT FROM AUTHOR]

Using high resolution S 2p and O 1s x-ray photoelectron spectroscopies, the adsorption of SO2 and its surface bound reaction products on Ru(0001) have been investigated simultaneously while dosing SO2 and while heating the adsorbed species. SO2 is found to adsorb on Ru(0001) at 100 K molecularly in two variants as well as dissociatively and to react to SO3, SO4, SO, and S with increasing coverage. After the monolayer has been saturated, SO2 adsorbs molecularly in multilayers. When heating adsorbed SO2 from 100 K, SO, SO2, and SO4 decompose in a wide temperature range up to 305 K. In contrast SO3 is found to be stable bound to Ru(0001) up to 300 K and to disappear from the surface to below 325 K. At 550 K the surface remains with a saturated atomic sulfur and oxygen layer and some sulfur species in a second layer. Our quantitative analysis of the sulfur amount bound to the surface supports a simple desorption process only for SO4. All other species mainly or partly decompose on the surface. [ABSTRACT FROM AUTHOR]

We have examined the adsorption behavior at ∼110 K of NO on NiO(111) overlayers prepared on a Ni(111) substrate. High-resolution electron-energy-loss spectroscopy shows fundamental changes in the vibrational spectrum for the beam dosed surface in comparison with the background dosed surface. Three vibrational peaks are observed after beam dosing, two of which are not observed after conventional background dosing. The peaks can be assigned to NO stretches for a previously observed NO state, a new NO bonding geometry, and a new NO2 surface species, previously unobserved under NO dosing. The difference is accounted for by increased NO uptake due both to kinetically activated adsorption and to increased exposure. [ABSTRACT FROM AUTHOR]

Metal atoms bonded with Si adatoms on the Si(111)-(7×7) surface undergo migration by hopping adjacent Si-rest atoms with dangling bond. By saturated adsorption of Si(111)-(7×7) surface with C2H5OH, the whole Si-rest atoms and a half of Si adatoms are occupied with Si–H and Si–OC2H5, so that the Zn atoms adsorbed on this surface cannot migrate by hopping. When Zn atoms were deposited on this surface, ca. 5 nm Zn dots were grown in the hexagonal spacing of ca. 5.4 nm width around the corner holes, which work as a mold. This is quite different from the growth of honeycomb layers composed of Zn3 clusters on the clean Si(111)-(7×7) surface. The dots grow up to nine (1.97 nm) to 13 layers (2.64 nm) by keeping their size, which implies a layer-by-layer growth of dots in the mold, where the growth is controlled by the kinetics instead of energetic feasibility. [ABSTRACT FROM AUTHOR]

The adsorption of CO on unreconstructed and reconstructed Ir{100} has been studied, using a combination of density functional theory and thermodynamics, to determine the relative stability of the two phases as a function of CO coverage, temperature, and pressure. We obtain good agreement with experimental data. At zero temperature, the (5×1) reconstruction becomes less stable than the unreconstructed (1×1) surface when the CO coverage exceeds a critical value of 0.09 ML. The interaction between CO molecules is found to be weakly repulsive on the reconstructed surface but attractive on the unreconstructed, explaining the experimental observation of high CO coverage on growing (1×1) islands. At all temperatures and pressures, we find only two possible stable states: 0.5 ML CO c(2×2) overlayer on the (1×1) substrate and the clean (5×1) reconstructed surface. [ABSTRACT FROM AUTHOR]

Optimization of enantioselectivity in heterogeneous catalysis and chiral chromatography is a challenging task for the production of enantiopure chemicals. Enantioselective adsorbents usually consist of a surface with chiral receptors being either chiral molecules linked to the surface or chiral pockets formed by molecular templating of the surface. In both cases, the enantioselectivity is controlled mainly by the strength of the receptor-enantiomer interaction, such that one-to-one correspondence is usually preserved. The authors use Monte Carlo calculations to show that this steric requirement is not a necessary condition for the effective separation of chiral molecules. In particular, they propose a way in which a chiral surface can be constructed by a suitable spatial distribution of active sites for which the classical concept of a chiral receptor is no longer useful. Their calculations indicate that the effectiveness of the separation is affected mainly by the difference in shape of the adsorption energy distribution functions corresponding to the enantiomers. [ABSTRACT FROM AUTHOR]

The single component adsorption of alkanes in carbon slit pores was studied using configurational-biased grand canonical Monte Carlo simulations. Wide ranges of temperature, pressure, alkane chain length, and slit height were studied to evaluate their effects on adsorption. Adsorption isotherms and density and orientation profiles were calculated. The behavior of long alkanes at high temperatures was found to be similar to short alkanes at lower temperatures. This suggests that the isotherms may be related through the Polanyi potential theory. [ABSTRACT FROM AUTHOR]

The authors have measured the contribution of CO adsorbed on different bonding sites to the resistivity of epitaxial Cu(100) films. Through an analysis of the correlation between surface resistivity and temperature-programed desorption measurements, CO on terraces is found to affect the resistance much more strongly than CO on defect sites. Two possible explanations are considered: a reduced scattering cross section of defect CO owing to the different local electronic structure at defect sites or strong scattering by the bare defect that is not significantly affected by the adsorption of CO. [ABSTRACT FROM AUTHOR]

The adsorption states and growth process of the first water (D2O) layer on Rh(111) were investigated using infrared reflection absorption spectroscopy, temperature programed desorption, and spot-profile-analysis low energy electron diffraction. Water molecules wet the Rh(111) surface intact. At the early stage of first layer growth, a (&#221A;3&#221A;3)R30° commensurate water layer grows where "up" and "down" species coexist; the up and down species represent water molecules which have free OD, pointing to a vacuum and the substrate, respectively. The up domain was a flatter structure than an icelike bilayer. Water desorption from Rh(111) was a half-order process. The activation energy and the preexponential factor of desorption are estimated to be 60 kJ/mol and 4.8×1016 ML½/s at submonolayer coverage, respectively. With an increase in water coverage, the flat up domain becomes a zigzag layer, like an ice bilayer. At the saturation coverage, the amount of down species is 1.3 times larger than that of the up species. In addition, the activation energy and the preexponential factor of desorption decrease to 51 kJ/mol and 1.3×1014 ML½/s, respectively. [ABSTRACT FROM AUTHOR]

The present study aims to establish a theoretical proposition that the rate of macromolecular adsorption can be augmented with application of transverse electric fields across patterned walls of a microfluidic channel. First, an approximate fully developed velocity profile is derived, which is subsequently utilized to solve the species conservation equation pertaining to a combined advection-diffusion transport. Closed form solutions for the concentration field are subsequently obtained, in consistency with the typical second order kinetics of macromolecular adsorption. It is suggested that the transverse electric field and pattern angles can be regulated as independent parameters to maximize the rate of macromolecular adsorption under the prevailing conditions. [ABSTRACT FROM AUTHOR]

We propose a model for the liquid-liquid (Lα→Lα′) phase transition observed in osmotic pressure measurements of certain charged lamellae-forming amphiphiles. The model free energy combines mean-field electrostatic and phenomenological nonelectrostatic interactions, while the number of dissociated counterions is treated as a variable degree of freedom that is determined self-consistently. The model, therefore, joins two well-known theories: the Poisson-Boltzmann theory for ionic solutions between charged lamellae and the Langmuir-Frumkin-Davies adsorption isotherm modified to account for charged adsorbing species. Minimizing the appropriate free energy for each interlamellar spacing, we find the ionic density profiles and the resulting osmotic pressure. While in the simple Poisson-Boltzmann theory the osmotic pressure isotherms are always smooth, we observe a discontinuous liquid-liquid phase transition when the Poisson-Boltzmann theory is self-consistently augmented by the Langmuir-Frumkin-Davies adsorption. This phase transition depends on the area per amphiphilic head group, as well as on nonelectrostatic interactions of the counterions with the lamellae and interactions between counterion-bound and counterion-dissociated surfactants. Coupling the lateral phase transition in the bilayer plane with electrostatic interactions in the bulk, our results offer a qualitative explanation for the existence of the Lα→Lα′ phase transition of didodecyldimethylammonium bromide (DDABr), but the transition's apparent absence for the chloride and the iodide homologs. More quantitative comparisons with experiment require better understanding of the microscopic basis of the phenomenological model parameters. [ABSTRACT FROM AUTHOR]

We show that the 18-electron rule can be used to design new organometallic systems that can store hydrogen with large gravimetric density. In particular, Ti containing organic molecules such as C4H4, C5H5, and C8H8 can store up to 9 wt % hydrogen, which meets the Department of Energy target for the year 2015. More importantly, hydrogen in these materials is stored in molecular form with an average binding energy of about 0.55 eV/H2 molecule, which is ideal for fast kinetics. Using molecular orbitals we have analyzed the maximum number of H2 molecules that can be adsorbed as well as the nature of their bonding and orientation. The charge transfer from the H2 bonding orbital to the empty dxy and dx2-y2 orbitals of Ti has been found to be singularly responsible for the observed binding of the hydrogen molecule. It is argued that early transition metals are better suited for optimal adsorption/desorption of hydrogen. [ABSTRACT FROM AUTHOR]

Self-assembled monolayers (SAMs) of various thiols have shown the potential to protect freshly fabricated or chemically cleaned GaAs surfaces from oxidization, adsorption of foreign atoms, and/or surface defect formation. We have employed an attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs. A strong infrared (IR) signal was measured for all the investigated GaAs-thiol interfaces. Varying the type of terminal groups, from hydrophilic to hydrophobic, significantly changes the IR intensity of the methylene stretching vibration, indicating different methylene chain orientation in SAMs. Consequently, these SAMs exhibited different passivation performance to the (001) GaAs surface as judged by the intensity of the GaAs-related photoluminescence signal. [ABSTRACT FROM AUTHOR]

We develop a computational methodology for the direct measurement of a wetting transition and its order via the effective interface potential. The method also allows to estimate contact angles in the nonwet state and to study adsorption isotherms. The proposed methodology is employed in order to study the wetting behavior of polymers on top of a brush consisting of identical polymers. In the absence of long-range forces, the system shows a sequence of nonwet, wet, and nonwet states as the brush density is increased. Including attractive long-range interactions we can make the polymer liquid wet the bush at all grafting densities, and both first- and second-order wetting transitions are observed. The latter case is limited to a small interval of grafting densities where the melt wets the brush in the absence of long-range interactions. Second-order wetting transitions are preceded by a first-order surface transition from a thin to a thick adsorbed layer. The interval of second-order wetting transitions is limited at low grafting densities by a surface critical end point and at high grafting densities by a tricritical wetting point. Our study highlights the rich wetting behavior that results when competing adsorbent-substrate interactions of different scales are tuned over a broad range. [ABSTRACT FROM AUTHOR]

Coronene (C24H12) adsorption on the clean Si(001)-2×1 surface was investigated by scanning tunneling microscopy and by density-functional calculations. The coronene adsorbed randomly at 25 °C on the surface and did not form two-dimensional islands. The scanning tunneling microscopy measurements revealed three adsorption sites for the coronene molecule on the Si(001) surface at low coverage. The major adsorption configuration involves coronene bonding to four underlying Si atoms spaced two lattice spacings apart in a dimer row. The two minor adsorption configurations involve asymmetrical bonding of a coronene molecule between Si dimer rows and form surface species with a mirror plane symmetry to their chiral neighbor species. The two minor bonding arrangements are stabilized by a type-C defect on the Si(001) surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Stable and functionalized SU-101(Bi) derivatives are explored by GCMC and DFT. • Separating CO 2 and C 2 H 2 from C 2 H 4 in the new Bi-MOFs is fully investigated. • Bi-MOF-CHO shows the highest CO 2 /C 2 H 4 and C 2 H 2 /C 2 H 4 selectivity. • Effects of ligand replacement/modification on separation in Bi-MOFs are analyzed. • Fine-tuning pore structure and chemistry dominate in one-step C 2 H 4 purification. One-step C 2 H 4 purification from ternary C 2 H 4 /C 2 H 2 /CO 2 mixtures is of prime importance but challenging in chemical industries, owing to similar physicochemical properties of the three gases. Herein, we investigated the potential of ultramicroporous Bi-MOFs modified with –CHO, –COOH, –NO 2 , -F, -Cl, -Br, and -I for C 2 H 4 purification. These MOFs featured good thermodynamic and thermal stability and exhibited precise regulation of channel sizes and surface structures/electronegativity. Their separation performances for CO 2 /C 2 H 4 , C 2 H 2 /C 2 H 4 , and CO 2 /C 2 H 2 were theoretically investigated by grand canonical Monte Carlo, density functional theory, and transient breakthrough simulations. Results showed that introducing –CHO/–COOH/–NO 2 enlarged the difference of adsorption uptakes between CO 2 , C 2 H 2 , and C 2 H 4 favoring efficient separation of corresponding mixtures, in which Bi-MOF-CHO showed the highest selectivity for CO 2 /C 2 H 4 , C 2 H 2 /C 2 H 4 , and CO 2 /C 2 H 2 , with values of 58.6 (64.5), 11.8 (11.5), and 7.7 (21.5) under 50/50 (1/99) at ambient conditions. The enhanced separation performance is attributed to the synergism of pore size limitation, fine-tuned surface structure, and high surface electronegativity, resulting in special CO 2 recognition, moderate enhancement for C 2 H 2 , but negligible influence on C 2 H 4 adsorption. This work provides an effective strategy to design high-performance adsorbents for one-step C 2 H 4 purification from ternary C 2 H 4 /C 2 H 2 /CO 2 mixtures. [ABSTRACT FROM AUTHOR]

In the first ever attempt to study the adsorption of organic molecules on high-index Si surfaces, we investigated the adsorption of benzene on Si(5 5 12)-(2×1) by using variable-low-temperature scanning tunneling microscopy and density-functional theory (DFT) calculations. Several distinct adsorption structures of the benzene molecule were found. In one structure, the benzene molecule binds to two adatoms between the dimers of D3 and D2 units in a tilted butterfly configuration. This structure is produced by the formation of di-σ bonds with the substrate and of two C==C double bonds in the benzene molecule. In another structure, the molecule adsorbs on honeycomb chains with a low adsorption energy because of strain effects. Our DFT calculations predict that the adsorption energies of benzene are 1.03–1.20 eV on the adatoms and 0.22 eV on the honeycomb chains. [ABSTRACT FROM AUTHOR]

In the present research study, we present the development of a model for predicting the adsorption of binary mixtures of nonpolar molecules, as well as polar molecules, based on density functional theory with mean-field approximation in narrow slit-pores. The first system under consideration is comprised of a binary mixture of nonpolar molecules, modeled by considering intermolecular dispersion forces, whereas the second system comprised of a binary mixture of polar molecules is modeled by considering orientation averaged electrostatic interactions, namely dipole-dipole and dipole-induced dipole interactions as well as dispersion interactions. An explicit equation for the Helmholtz free energy of the pore phase binary fluid mixture is derived. The proposed model is used to simulate the selective sorption of ethane from an ethane-methane mixture and water from a methanol-water mixture in the slit-pore. The simulated results are interpreted by studying the relative contributions of fluid-wall and fluid-fluid interactions. Finally, simulation results obtained are compared with the results of existing models and molecular simulations in the literature. [ABSTRACT FROM AUTHOR]

Thin films of fumaramide [2]rotaxane, a mechanically interlocked molecule composed of a macrocycle and a thread in a “bead and thread” configuration, were prepared by vapor deposition on both Ag(111) and Au(111) substrates. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy were used to characterize monolayer and bulklike multilayer films. XPS determination of the relative amounts of carbon, nitrogen, and oxygen indicates that the molecule adsorbs intact. On both metal surfaces, molecules in the first adsorbed layer show an additional component in the C 1s XPS line attributed to chemisorption via amide groups. Molecular-dynamics simulation indicates that the molecule orients two of its eight phenyl rings, one from the macrocycle and one from the thread, in a parallel bonding geometry with respect to the metal surfaces, leaving three amide groups very close to the substrate. In the case of fumaramide [2]rotaxane adsorption on Au(111), the presence of certain out-of-plane phenyl ring and Au–O vibrational modes points to such bonding and a preferential molecular orientation. The theoretical and experimental results imply that the three-dimensional intermolecular configuration permits chemisorption at low coverage to be driven by interactions between the three amide functions of fumaramide [2]rotaxane and the Ag(111) or Au(111) surface. [ABSTRACT FROM AUTHOR]