A molecular dynamics simulation study of surface effects on gas permeation in free-standing polyimide membranes

A 141100-atom model of a glassy ODPA–ODA polyimide free-standing membrane, corresponding to a thickness of two average radii of gyration for the 40-mers chains, has been studied using molecular dynamics (MD) simulations. Due to the large-scale of the fully atomistic model, a parallelized particle-mesh technique using an iterative solution of the Poisson equation had to be implemented for the efficient evaluation of the electrostatic interactions. With flattened-chain configurations, the density was found to adjust itself naturally in the middle of the membrane to 95% of the ODPA–ODA experimental value. At the free-standing surfaces, the density profile became sigmoïdal, indicating surface roughness. For comparison, two isotropic bulk models, one at the “normal” density as obtained for ODPA–ODA under ambient conditions and the other one at 95% of the normal-density, were built. Small gas probes were inserted into all three models in order to investigate whether the interfacial structure of the glassy free-standing membrane can influence penetrant transport. Gas diffusion in the low-density part of the interface was found to be very fast. The limiting value for the gas diffusion coefficient D_membrane is only attained when the probes enter more dense regions in the membrane. Indeed, D_membrane compares well with D_bulk obtained for the 95%-density bulk system, i.e. about twice that in the normal-density bulk. Solubility is larger in the membrane than in both bulk models, thus suggesting an effect of chain flattening in addition to the density. Adsorption is particularly high at the free-standing interfaces.     

Development of Surface Fluorescence X‐Ray Absorption Fine Structure Spectroscopy Using a Laue‐Type Monochromator

Surface fluorescence X‐ray absorption fine structure (XAFS) spectroscopy using a Laue‐type monochromator has been developed to acquire structural information about metals with a very low concentrate on a flat highly oriented pyrolytic graphite (HOPG) surface in the presence of electrolytes. Generally, surface fluorescence XAFS spectroscopy is hindered by strong scattering from the bulk, which often chokes the pulse counting detector. In this work, we show that a bent crystal Laue analyzer (BCLA) can efficiently remove the scattered X‐rays from the bulk even in the presence of solution. We applied the technique to submonolayer (～10¹⁴ atoms cm^?2) Pt on HOPG and successfully obtained high signal/noise in situ XAFS data in combination with back‐illuminated fluorescence XAFS (BI‐FXAFS) spectroscopy. This technique allows in situ XAFS measurements of flat electrode surfaces to be performed in the presence of electrolytes.     

Importance of bulk properties in the structure and evolution of cleavage surfaces of quasicrystals

The present work reviews the recent achievements in probing bulk properties of quasicrystals by using cleavage surfaces and surface sensitive techniques. In particular, it is shown that the cluster–subcluster-based structure of the cleavage surface of icosahedral Al–Pd–Mn quasicrystals can be related to the presence of stable atom clusters in the bulk, which force the crack front to circumvent them. Furthermore, by subjecting cleavage surfaces of differently pre-annealed Al–Pd–Mn quasicrystals to a post-cleavage heat treatment, we demonstrate that bulk vacancies migrate toward the surface, where they initiate structure and composition changes. These studies allow us to characterize Al–Pd–Mn quasicrystals with respect to their bulk vacancy concentration. As-grown Al–Pd–Mn quasicrystals are found to contain a supersaturation of all chemical species of vacancies in near stoichiometric composition, whereas long term pre-annealed material has a much lower, and predominantly Al, vacancy concentration. Analogous experiments for decagonal Al–Ni–Co quasicrystals show that as-grown Al–Ni–Co has a lower vacancy concentration than as-grown Al–Pd–Mn.     

Investigation of the electronic structure and properties of cluster models of silica by the MNDQ method

Using the LCAO MO SCF method in the MNDO valence approximation, we have carried out a systematic study of the electronic and spatial structure of cluster models of the bulk phase and the hydroxylated and chemically modified surface of silica surface. We propose a technique for taking into account the crystallochemical environment of the clusters modeling the bulk phase of SiO₂, based on passivation of the abnormal valencies at the boundary of the clusters by hydrogen atoms, the geometrical location of which ensures homogeneity in the electron density distribution on the silicon atoms of the model fragment. We give a comparative analysis of the electronic characteristics of the studied cluster models. We consider the nature of the adsorption centers and the properties of the hydroxylated and modified silica surface.Translated from Teoreticheskaya i Éxperimental'naya.Khimiya, Vol. 22, No. 5, pp. 533–545, September–October, 1986.     

Polyvinyl alcohol hydrogels I. Microscopic structure by freeze-etching and critical point drying techniques

Froeze-etching (FE) and critical point drying (CPD) techniques were employed to prepare samples for investigating surface and bulk structures of polyvinyl alcohol (PVA) hydrogels by scanning electron microscopy. The hydrogels were obtained by freezing homogeneous solutions containing PVA polymer in either water or an aqueous solution of dimethyl sulfoxide (DMSO). An oriented porous structure was observed in the PVA hydrogel prepared without DMSO. The structure on the surface was found to be more porous than in the bulk for PVA hydrogels prepared from aqueous DMSO solutions. For given compositions of the hydrogels, samples prepared by FE technique showed a highly porous fibrillar structure on the surface, while those prepared by CPD technique showed a collapsed fibrillar structure with much less porosity. This marked difference indicates a collapse of the surface structure caused by the CPD technique. The CPD technique also led to significant reduction in porosity and loss of fibrillar structure in the bulk. Volume shrinkage of hydrogels caused by dehydration in ethanol may be responsible for the surface collapse as well as alteration of bulk structure. The FE technique reveals a more native structure of hydrogels than the commonly used CPD technique. However, it suffers from disadvantages such as charging and structural damage at high magnifications.     

In situ detection of monovalent copper in aerosols by photoemission

Summary In situ information on nanoparticle surface chemistry and modes of particle growth is obtained in gas suspension by the technique of photoelectric charging of particles (PCP) which depends on the surface chemical compositon as well as the electronic structure of the particles via the spectral dependence of the photoelectric yield. With CuCl particles, photoelectric charging is about 100 times more efficient compared to other divalent transition metal compounds. Therefore, particles containing monovalent Cu can be detected with extremely high sensitivity of below 10 ng/m³. In atmospheric aerosols emitted from volcanoes, the relation of solid state oxidation/reduction in Fe_1–xCu_xCl₂ resulting in monovalent Cu for x<0.4 is important. As an example of the PCP technique this relation is monitored in laboratory generated aerosols. The nanoparticles are also precipitated onto a substrate where their surface chemical composition is analyzed by XPS which is important for the interpretation of the results obtained by photoelectric charging.     

Surface complexation studied via combined grazing-incidence EXAFS and surface diffraction: arsenate on hematite (0001) and (10-12)

X-ray diffraction [crystal-truncation-rod (CTR)] studies of the surface structure of moisture-equilibrated hematite reveal sites for complexation not present on the bulk oxygen-terminated surface, and impose constraints on the types of inner-sphere sorption topologies. We have used this improved model of the hematite surface to analyze grazing-incidence EXAFS results for arsenate sorption on the c (0001) and r (10–12) surfaces measured in two electric vector polarizations. This work shows that the reconfiguration of the surface under moist conditions is responsible for an increased adsorption density of arsenate complexes on the (0001) surface relative to predicted ideal termination, and an abundance of “edge-sharing” bidentate complexes on both studied surfaces. We consider possible limitations on combining the methods due to differing surface sensitivities, and discuss further analysis possibilities using both methods. An erratum to this article can be found at     

Structure and oxidation at quasicrystal surfaces

We have investigated the atomic and electronic structure, chemical composition, and oxidation characteristics of the surfaces of icosahedral, Al-rich quasicrystals, using a variety of surface-sensitive techniques (LEED, XPS, STM, AES). We have systematically investigated the way that these traits vary with preparation conditions (e.g. sputtering and then annealing to various temperatures, vs. fracture), with surface symmetry (e.g. 2f vs. 3f vs. 5f surfaces), and with bulk composition (e.g. i-Al–Pd–Mn vs. i-Al–Cu–Fe). We have also compared our results for the quasicrystals with results for crystalline approximants and other related crystalline phases. Our main conclusions are that, under specific conditions of sputter-annealing, the bulk atomic and electronic structures of the clean quasicrystal propagate to the surface. Also, the oxidation chemistry is dominated by that of the primary constituent, aluminum.     

Surface and bulk ordering in thin films of a symmetrical diblock copolymer

Amphiphilic diblock copolymers have the ability to adapt their surface's molecular composition to the hydrophilicity of their environment. In the case of about equal volume fractions of the two polymer blocks, the bulk of these polymers is known to develop a laminar ordering. We report here our investigation of the relationship between bulk ordering and surface morphology/chemical composition in thin films of such an amphiphilic diblock copolymer. Upon annealing in vacuum, the expected lamella ordering in the bulk of the film is observed and we find the morphology of the film surface to be defined by the thickness of the as‐deposited film: If the as‐deposited thickness matches the height of a lamella stack, then the film exhibits a smooth surface. Otherwise, an incomplete lamella forms at the film surface. We show that the coverage of this incomplete layer can be quantified by X‐ray reflectivity. To establish the lamella ordering in the bulk, the film needs to be annealed above the glass temperature of the two blocks. Molecular segregation at the film surface, however, is already occurring at temperatures well below the glass temperature of the two blocks. This indicates that below the glass temperature of the blocks the bulk of the thin film is “frozen,” whereas the polymer chains composing the surface lamella have an increased mobility. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013 , 51, 1282–1287     

Origin of electronic structure and time-dependent state averaging in the multi-configuration time-dependent Hartree–Fock approach to electron dynamics

In this Letter, we discuss problems that can arise in the interpretation of results obtained by quantum dynamical simulations with the multi-configuration time-dependent Hartree–Fock (MCTDHF) method. In particular, we show that an effect, which can be seen as the time-dependent version of the state averaging known from standard quantum chemistry, can modify the electronic structure as derived from such a simulation. We illustrate our findings with numerical calculations for the laser excitation of a water molecule, with a Gaussian Type Orbital basis set.     

Structure of DNA-cationic surfactant complexes at hydrophobically modified and hydrophilic silica surfaces as revealed by neutron reflectometry

In this article, we discuss the structure and composition of mixed DNA-cationic surfactant adsorption layers on both hydrophobic and hydrophilic solid surfaces. We have focused on the effects of the bulk concentrations, the surfactant chain length, and the type of solid surface on the interfacial layer structure (the location, coverage, and conformation of the DNA and surfactant molecules). Neutron reflectometry is the technique of choice for revealing the surface layer structure by means of selective deuteration. We start by studying the interfacial complexation of DNA with dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (CTAB) on hydrophobic surfaces, where we show that DNA molecules are located on top of a self-assembled surfactant monolayer, with the thickness of the DNA layer and the surfactant-DNA ratio determined by the surface coverage of the underlying cationic layer. The surface coverages of surfactant and DNA are determined by the bulk concentration of the surfactant relative to its critical micelle concentration (cmc). The structure of the interfacial layer is not affected by the choice of cationic surfactant studied. However, to obtain similar interfacial structures, a higher concentration in relation to its cmc is required for the more soluble DTAB surfactant with a shorter alkyl chain than for CTAB. Our results suggest that the DNA molecules will spontaneously form a relatively dense, thin layer on top of a surfactant monolayer (hydrophobic surface) or a layer of admicelles (hydrophilic surface) as long as the surface concentration of surfactant is great enough to ensure a high interfacial charge density. These findings have implications for bioanalytical and nanotechnology applications, which require the deposition of DNA layers with well-controlled structure and composition.     

Enhancement of the electrocatalytic O2 reduction on Pt–Fe alloys

The design of high performance cathode electrocatalysts is essential for polymer–electrolyte fuel cells, which are now attracting enormous interest as a primary power source for zero-emission electric vehicles. We have discovered a significant enhancement of electrocatalytic activity of Pt by alloying with Fe, and found a maximum activity at ca. 50% Fe content, which results in 25 times higher activity than pure Pt activity. It was confirmed experimentally at Pt–Fe bulk alloys that the alloy catalyst surface consists of a pure Pt skin-layer (<1 nm in thickness) that is modified in the electronic structure by that of the bulk alloy. The enhancement could be well explained by the 5d-vacancy of the surface, but not by Pt interatomic distance or roughening of the surfaces.     

Substrate-mediated interactions on solid surfaces: theory, experiment, and consequences for thin-film morphology

When two or more atoms bind to a solid surface, the substrate can mediate an interaction between them. In this review, we discuss the origins of this interaction and the theories that describe substrate-mediated interactions, including recent studies with electronic density-functional theory. We summarize the results of experimental studies, in particular those with scanning–tunneling microscopy, aimed at quantifying substrate-mediated interactions. Over the intermediate range, these interactions can be strong enough to influence the ordering of adsorbates at surfaces. We discuss the results of recent studies, employing kinetic Monte Carlo simulations that probe the ramifications of these interactions for the morphology in thin-film epitaxy.     

Polymer/surfactant interactions at the air/water interface

The development of neutron reflectometry has transformed the study and understanding of polymer/surfactant mixtures at the air/water interface. A critical assessment of the results from this technique is made by comparing them with the information available from other techniques used to investigate adsorption at this interface. In the last few years, detailed information about the structure and composition of adsorbed layers has been obtained for a wide range of polymer/surfactant mixtures, including neutral polymers and synthetic and naturally occurring polyelectrolytes, with single surfactants or mixtures of surfactants. The use of neutron reflectometry together with surface tensiometry, has allowed the surface behaviour of these mixtures to be related directly to the bulk phase behaviour. We review the broad range of systems that have been studied, from neutral polymers with ionic surfactants to oppositely charged polyelectrolyte/ionic surfactant mixtures. A particular emphasis is placed upon the rich pattern of adsorption behaviour that is seen in oppositely charged polyelectrolyte/surfactant mixtures, much of which had not been reported previously. The strong surface interactions resulting from the electrostatic attractions in these systems have a very pronounced effect on both the surface tension behaviour and on adsorbed layers consisting of polymer/surfactant complexes, often giving rise to significant surface ordering.     

Surface phase separation in complex mixed adsorbing systems: an interface-bulk coupling effect

The interfacial thermodynamics and structure of ternary mixtures of the type A+B+solvent are investigated. According to the Gibbs phase rule, the coupling between the bulk phase and the interfacial region-which is related to the reversibility of the adsorption of the corresponding species-is a determinant as to whether phase separation can be observed at the interface. For an n-component adsorbing solution, at least one of the species has to adsorb irreversibly over the experimental time scales in order not to fix more intensive variables than those required to observe surface phase separation. We present results for a lattice model planar interface consisting of the ternary mixture A+B+solvent. The solvent molecules and the type A molecules have fixed chemical potentials at the interface since they are equilibrated with a bulk solution. In contrast, the type B molecules are irreversibly adsorbed at the interface and do not equilibrate with the bulk. Mean-field theory is compared with Monte Carlo simulation. Interestingly, the spinodal line in the interaction-composition plane shows a reentrant on the B-rich phase side. We discuss the implications of these results for surface phase separation of adsorbing mixtures of proteins and low-molecular-weight surfactants.     

端羟基化聚苯乙烯的表面性质

利用Wilhelmy片技术和躺滴法研究了端羟基化聚苯乙烯的表面性质.结果表明,端羟基化对聚苯乙烯在空气面的接触角基本没有影响(89°),而在玻璃面的接触角则大大降低(66°),其降低幅度与分子量及分子量分布有关.这与动态接触角的测定结果基本一致,而且宽分子量分布的端羟基化聚苯乙烯的前进接触角(θa)随着温度的升高而降低,于40℃时达到最低值.而窄分子量分布样品的动态接触角基本不变.样品与不同温度水接触后表面接触角的变化也基本相似.DMA研究结果表明,样品损耗模量、储能模量和tanδ从40℃开始发生突变,刚好与接触角最低值的温度相对应.这是由于宽分子量分布样品中的较低分子量组分在表面聚集,导致表面分子具有较高的活动能力.接触角随温度的变化趋势可能是聚合物表面分子运动能力增加和结晶程度变化等因素综合作用的结果.     

Rapid determination of aluminum by UV-vis diffuse reflectance spectroscopy with application of suitable adsorbents

Diffuse reflectance spectroscopy (DRS) can be used as a rapid and sensitive method for the quantitative determination of low amounts of aluminum. In this analytical technique, the analyte in samples are extracted onto a solid sorbent matrix loaded with a colorimetric reagent and then quantified directly on the adsorbent surface. Alternatively, colored aluminum complexes formed in solution can also be immobilized onto adsorbent surface and be measured by DRS technique. Octadecyl silica disk, methyltrioctylammonium chloride–naphthalene and MCM-41 were examined as adsorbents. Eriochrome cyanine R and quinalizarin were used as coloring reagents. Optimal sorption conditions were found for each system of analyte–reagent–adsorbent. The concentration of analyte is determined using the appropriate form of the Kubelka–Munk function. We obtained for each of the aluminium–reagent–adsorbent system a calibration curve by plotting the absorbance versus the log 10²[Al³⁺] μg ml⁻¹. The linear dynamic range extends over two orders of magnitude within 0.01–15 μg ml⁻¹ with little differences in the range and in the correlation coefficients among the adsorbents. We consider that for a rapid determination of aluminum a spot-test-DRS combination with a detection limit of 1.0 × 10⁻² μg ml⁻¹ is the more facile and preferred technique.     

Catalytic and physico-chemical properties of La1?xSrxCoO3 perovskites

Catalytic properties of strontium-substituted lanthanum cobaltites La_1–xSr_xCoO₃ in the reaction of CO oxidation were compared with the data about chemical composition of the surface and bulk defect structure. It was found that (1) the initial catalytic activity correlates with the cobalt concentration on the surface; (2) surface composition differs from the bulk one; (3) the steady-state activity is proportional to the density of bulk extended defects. Under our experimental conditions no role of point defects was noted.     

Regularities of the sorption of cycloalkenyl-substituted thiophenes and 2,2′-bithiophenes from water-acetonitrile solutions on hexadecyl silica gel under conditions of high-performance liquid chromatography

Regularities of the sorption of newly synthesized cycloalkenyl-substituted thiophenes and 2,2′-bithiophenes from water-acetonitrile solutions of different compositions on hexadecyl silica gel are investigated by high-performance liquid chromatography (HPLC). The retention factors and Henry constants of adsorption of these substances are determined. We discuss the effect of the molecular structure of the investigated heterocycles, and the nature and positions of the substituents, on the sorption. The equilibrium constants for the quasi-chemical reactions of sorption of the investigated compounds at the interface, and their solvation in a multicomponent bulk solution, are calculated using the Lanin-Nikitin equation. Based on the obtained results, we suggest that structural self-organization in the bulk solution and at the interface with the hydrophobic surface of hexadecyl silica gel plays an important part in the sorption of cycloalkenylsubstituted thiophenes and 2,2′-bithiophenes from multicomponent solutions.