Initial Surface Concentration Research Articles

Effect of evaporation on the limit of detection of explosive traces by active spectral imaging

When developing methods for standoff detection of explosive traces, an important issue is determination of the limit (minimum) amount of a substance available for detection. This limit depends on the features of the measuring complex realization, on the physicochemical properties of the substance, and on the dynamics of sublimation (evaporation) of the substance from the surface. In this work, we consider the problem of detection limit for a measuring complex based on the method of active spectral imaging. In realization of this method, the distance to the object and the measurement time after the appearance at the surface of a substance imprint are important. How long this trace amount of substance can be detected, taking into account its evaporation? An explosive imprint is a thin layer on a surface (usually considered as a fingerprint with traces of the substance). We theoretically consider the dynamics of the change in the area of this imprint on the basis of a mathematical model of evaporation of a thin layer. Based on mathematical modeling, we can estimate the amount of a specific substance available for detection under the given conditions and the availability time. We identified the main parameters that affect the rate of film evaporation and the range of parameter change and calculated the kinetic parameters of sublimation of thin films of some explosives. The performed calculations allowed us to draw conclusions about the detection limit of substances for the active spectral imaging method depending on the distance to the measurement object and the time after the appearance of a surface trace with an initial surface concentration and area specific to a fingerprint.

Enhanced Activity and Stability of PbO2 Electrodes by Modification with Octadecyl Phosphonic Acid

A novel Ti/Sn-SbOx/PbO2-ODP electrode was constructed using a β-PbO2 layer modified with octadecyl phosphonic acid (ODP) through a co-electrodeposition method. Scanning electron microscopy with an energy dispersive spectrometer, X-ray diffraction and contact angle goniometry confirmed that ODP was successfully embedded into the β-PbO2 layer of Ti/Sn-SbOx/PbO2-ODP, increasing the water contact angle at the electrode surface from 75.9° to 133.9°. The Ti/Sn-SbOx/PbO2-ODP electrode exhibited excellent electrocatalytic oxidation of ciprofloxacin (CIP). The pseudo first-order rate constant for CIP removal was 3.92 h−1 at an electrolytic potential of 10 V, 2.6 times greater than that when using the unmodified electrode. Moreover, electrode stability was greatly improved after ODP modification. The Ti/Sn-SbOx/PbO2-ODP electrode had longer accelerated life (324 h) than the Ti/Sn-SbOx/PbO2 electrode (91 h). The enhanced electrocatalytic performance of Ti/Sn-SbOx/PbO2-ODP was attributed to improved utilization of hydroxyl radicals (•OH) resulting from higher oxygen evolution potential. Furthermore, the hydrophobic surface of Ti/Sn-SbOx/PbO2-ODP was beneficial for stronger adsorption of CIP, as evidenced by the initial surface concentration test.

Surfactant and dilatational viscosity effects on the deformation of liquid droplets in an electric field

HypothesisA conducting droplet suspended in an insulating continuous phase, e.g. an aqueous electrolyte in an oil, is deformed by an applied electric field to nonspherical equilibrium shapes, and can even break-up under strong fields. Many technologies use electro-deformation to manipulate fluid dispersions, with surfactants present on the droplet interfaces forming stabilizing monolayers. While surfactants lower the interface tension which facilitates electro-deformation, the monolayer elasticity resists deformation. High molecular weight surfactants, with large dilatational viscosities, can potentially retard the deformation dynamics. NumericsA boundary integral method simulates the dynamic interfacial deformation of a perfectly conducting droplet in a dielectric in a uniform field. The interface contains an insoluble monolayer which is a Newtonian fluid with constant dilatational viscosity obeying a Langmuir state equation. A range of initial surfactant surface concentrations are studied, with elasticity proportional to concentration. FindingsEquilibrium drop deformations, unaffected by surface viscosity, are strongly resisted by elasticity at high surface concentrations, and field strengths necessary for break-up increase with elasticity. Dilatational viscosity scales with the ratio, κ∗, the surface viscosity (divided by the droplet radius) to the bulk viscosity, and can extend the deformation time. Extended times are described by a time rescaling proportional to κ∗.

Determination of the time-dependent change in the vapor concentration of 2,4,6-trinitrotoluene during the sublimation of its traces from the glass surface

The results of the measurements of 2,4,6-trinitrotoluene (TNT) vapor concentration over its trace amounts, called thin films, on the glass surface with a concentration of 100 ng/cm2 in a square area with a side of 1 cm over time are presented. The trace amounts of TNT on the glass were formed by applying a solution of TNT in the acetonitrile diluted with the chemically pure acetone, followed by the evaporation of the solvents. In order to measure the TNT vapor concentration, an EKHO-V-IDTS portable multibacillary gas-chromatograph with preliminary TNT vapor concentration was used. A sampling of the TNT vapor above the object was carried out with a remote vortex sampler. The vapor sample was taken from a distance of 2 cm from the glass surface. The concentration in the mode of the complete capture of TNT vapors was carried out to the stainless-steel wire mesh. The vapor concentration was determined from the chromatographic peak amplitude. It was found that the concentration of vapor over the examined surface with an area of 1 cm2 decreases from 10-13 to 10-14 g/cm3 within 2.6 ± 0.3 hours. TNT vapor concentration value of 10-14 g/cm3 corresponds to the threshold concentration of TNT vapor for the modern detectors. Based on the assumption that the vapor concentration is proportional to the amount of the TNT mass on the surface for the considered trace amounts of TNT, it was estimated that the initial surface concentration of trinitrotoluene of 100 ng/cm2 on the glass surface decreases to 12 ng/cm2 within 2.6 ± 0.3 hours due to sublimation into an open half-space. It was shown that the use of vortex sampling of vapor intensifies the sublimation of TNT from the glass surface.

Electrochemical treatment of phenol-containing wastewater by facet-tailored TiO2: Efficiency, characteristics and mechanisms

Electrochemical oxidation is widely used for water and wastewater treatment. Anodic material is crucial and the shape-tailored {001}-exposed TiO2 has been proven to be an ideal electrode material for pollutant oxidation. In this work, the electrochemical treatment of wastewater containing typical p-substituted phenols by facet-tailored TiO2 is studied in terms of efficiency, characteristics and mechanisms. Experimental results demonstrate that the anodic oxidation of p-substituted phenols becomes more difficult with the increasing Hammett's constant (σ) of phenols, while their degradation rates (k) increase continuously with the initial surface concentration (Γ). Phenols are degraded mainly by surface-bound ·OH and direct electron transfer on the TiO2/Ti electrode, rather than by bulk-free ·OH suspended in the aqueous phase. Theoretical calculations reveal that the surface-bound ·OH-mediated oxidation mechanism is attributed mainly to the strong surface bond strength between shape-tailored TiO2 and water molecule as well as the reactive ·OH. Such strong interactions are associated with the higher density of atomic steps, edges and kinks of low-coordinate surface atoms with a large number dangling bonds on the high-energy {001} polar facet. For practical treatment of real wastewater with different matrixes, the facet-tailored TiO2/Ti electrode exhibits both a high efficiency and a fast kinetics. Our findings provide a new chance to degrade phenolic pollutants in wastewater and offer atomic-scale insights into the preparation, modification and application of TiO2-based anodic materials for electrochemical water treatment.

Sunlight Photolysis of Safener Benoxacor and Herbicide Metolachlor as Mixtures on Simulated Soil Surfaces.

Benoxacor is a safener paired with the high-use herbicide S-metolachlor. Commercial formulations containing both compounds are sprayed onto soil pre-emergence to enhance yields of corn. In this study, we evaluated the sunlight photolysis of metolachlor and benoxacor, individually and as mixtures, in three different reaction environments: in water and on two soil-simulating surfaces (quartz and kaolinite). When irradiated individually, benoxacor degraded at least 19 times faster than metolachlor in each reaction environment, consistent with its higher molar absorptivity within the solar spectrum than metolachlor. When metolachlor and benoxacor were irradiated as mixtures, benoxacor promoted metolachlor degradation on quartz and, to a lesser extent, in water, but not on kaolinite. On quartz, at a benoxacor/metolachlor molar ratio of 0.1:1, metolachlor degraded 1.8 times faster than in the absence of benoxacor; as the benoxacor/metolachlor ratio increased, metolachlor degradation rate also increased. The photolysis rate of benoxacor depended on its initial surface concentration and was promoted by metolachlor. Benoxacor photoproducts were capable of absorbing sunlight and serving as photosensitizers for metolachlor degradation. These results illustrate how a safener can influence the photochemistry of its coformulated herbicide and suggest that such mixture effects should be considered when evaluating the environmental fate of agrochemicals.

Model-Based Interpretation of Groundwater Arsenic Mobility during in Situ Reductive Transformation of Ferrihydrite.

Arsenic (As) release and mobility in groundwater is coupled to the iron (Fe) cycling and the associated transformation of Fe-oxides present in sediments. Recent in situ experiments have provided observations on arsenic mobilization and co-occurring reductive mineral transformation when placing As-loaded ferrihydrite-coated sand for 80 days in wells of an As-contaminated aquifer of Northern China. However, the complex temporal change in solid-associated arsenic and the multiple geochemical processes occurring when the flowing groundwater contacts the As-loaded ferrihydrite-coated sand hamper a detailed evaluation of the experimental data set. In this study, we develop a modeling approach that allows a quantitative interpretation of arsenic release and ferrihydrite transformation observed during the in situ experiments. The model accounts for the interplay of abiotic and biotic geochemical processes (i.e., surface complexation, reductive dissolution, formation of secondary iron minerals, and arsenic sequestration into the newly formed minerals) involved in the transformation of Fe-oxides and controlling arsenic mobility. The results show the capability of the proposed approach to reproduce the temporal trends of solid arsenic and ferrihydrite concentrations, as well as the spatial variability of mineral transformation, observed in different wells using a common set of surface complexation parameters and kinetic rate constants. The simulation outcomes allowed us to disentangle the specific contribution of the different mechanisms controlling the release of arsenic. It was possible to identify an initial rapid but minor release of As (13-23% of the initial surface concentration) due to desorption from ferrihydrite, as well as the reduction of adsorbed As(V) to As(III) upon contact with the flowing anoxic groundwater. Successively, reductive dissolution of ferrihydrite caused the decrease of the amount of the Fe mineral phase and led to a major depletion of solid-associated arsenic. The produced Fe(II) catalyzed the ferrihydrite conversion into more crystalline Fe(III) oxides (i.e., lepidocrocite and goethite) through Ostwald ripening, and resulted in the formation of siderite and mackinawite upon reaction with carbonates and sulfides naturally present in the groundwater. The model results also showed that, whereas the decrease in surface sites during reductive dissolution of ferrihydrite promoted arsenic mobilization, the mineral transformation limited As release through its sequestration into the newly formed secondary mineral phases.

The direct observation of surface martensite formation upon cooling to temperatures close to ambient in a heat treated AISI 301 stainless steel

Martensitic transformation, either athermal or strain-induced, in metastable AISI 301 austenitic steel is of high technical interest as this facilitates manipulation of mechanical properties. This work is part of a project aimed at identifying the influence of variables that determine the athermal α′-martensitic transformation in this steel in order to ensure consistent martensite formation and optimized mechanical properties. The influences of initial austenitic grain size and surface concentrations of interstitial elements (C, N) on martensite start temperature were investigated. The surface concentrations of both carbon and nitrogen were decreased through decarburisation and the formation of martensite upon cooling from ambient temperatures was directly observed using a cryogenic Scanning Electron Microscope (cryo-SEM). It was found that martensite formed on the heat-treated surfaces at much higher temperatures than that expected for AISI 301 steel. The observations were confirmed using electron backscatter diffraction (EBSD). Other analytical techniques including glow-discharge optical emission spectroscopy (GDOES), and Neutron Diffraction were carried out to explain the observations.

Enhancement of the Activity and Stability of PbO2 Electrodes by Modifying with Polydimethylsiloxane

The β-PbO2 of a Ti/Sn-SbOx/PbO2 electrode was modified with polydimethylsiloxane (PDMS) (Ti/Sn-SbOx/PbO2-PDMS) through a facile electrodeposition method. The results from scanning electron microscopy, energy-dispersive spectrometry and X-ray diffraction demonstrated that the PDMS was irreversibly adsorbed on the β-PbO2 of Ti/Sn-SbOx/PbO2-PDMS, leading to a decrease in the crystal size, surface cracks and preferential growth trend of β-PbO2. Compared with its PDMS-unmodified counterparts, the Ti/Sn-SbOx/PbO2-PDMS electrode exhibited relatively higher activity, selectivity and stability in the anodic oxidation of p-chlorophenol. Moreover, the Ti/Sn-SbOx/PbO2-PDMS electrode was more stable than the Ti/Sn-SbOx/PbO2 electrode, as evidenced by the accelerated life test. Based on the quantitative measurements of contact angle, hydroxyl radicals (HO•) and the initial surface concentration of various p-substituted phenols, including p-cresol, p-methoxyphenol, phenol, and p-chlorophenol, and with the help of electrochemical characterization of linear sweep voltammetry, it could be concluded that the improved electrochemical activity and stability of Ti/Sn-SbOx/PbO2-PDMS originated from the enhanced hydrophobicity of the electrode surface. Different from a traditional Ti/Sn-SbOx/PbO2 electrode with adsorbed HO• as the dominant active species, the Ti/Sn-SbOx/PbO2-PDMS with a hydrophobic surface favors the generation of more free HO•.

Effects of salt concentration and spreading amounts on surface pressures of layers of latex particles grafted to polymer chains at interfaces between air and aqueous salt solutions

Polystyrene (PS) latex particles grafted to diacetone acrylamide chains were spread in layers at the air-aqueous interfaces of NaCl solutions. Their surface pressures were investigated as a function of the NaCl concentration in the water phase, at different initial surface concentrations of PS latex particles. At low initial surface concentrations of PS latex particles, the isotherms of the monolayers measured during compression-expansion cycles exhibited slightly a negative hysteresis, i.e. a decrease in surface pressure as the number of compression-expansion cycles increased, irrespective of the NaCl concentration. The isotherm obtained upon expansion was almost the same as the compression isotherm for three repeated compression-expansion cycles. On the other hand, at high initial concentrations of latex particles, in which the surface area per particle could reach much smaller values upon compression, the compression-expansion isotherms clearly showed a plateau region. In this region, PS latex particles were laterally close packed at the interface, irrespective of NaCl concentration as well as the number of cycles. The isotherm shape was strongly dependent on NaCl concentration; negative hysteresis was observed when NaCl concentrations were lower than 0.001 M and higher than 0.15 M, except for the plateau region in the expansion cycles. The isotherm recorded using a 0.025 M NaCl subphase exhibited negative hysteresis below the plateau region.

PH dependence of daunorubicin interactions with model DMPC:Cholesterol membranes

Mixed monolayers composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC):Cholesterol 7:3 prepared by the Langmuir-Blodgett method were used as model membranes to investigate the influence of the anticancer drug daunorubicin (DNR) on the properties of the lipid membrane. The Langmuir monolayer experiments revealed that drug – membrane interactions are pH-dependent. The changes in monolayer organization at subphases of pH 7.4 containing daunorubicin visualized by Brewster Angle Microscopy showed that in the presence of the drug the typical morphology observed for phospholipid layers containing cholesterol was no longer seen. It supports the explanation of the mechanism of the drug incorporation into the layers in terms of the competition between DNR molecules and cholesterol in the layer. Increasing surface pressure with time and increasing value of limiting surface pressure with increasing drug concentration in the subphase confirmed incorporation of the drug into the membranes. The interactions between the lipid monolayer and the drug at pH 5.4 were of electrostatic nature between the negative part of the DMPC molecule and positively charged drug, while at pH 7.4 contribution of interactions of daunorubicin with cholesterol was observed. Large differences of the surface–pressure vs. time plots were observed at both pH values when the DMPC:Cholesterol monolayer was not well organized yet. The voltammograms recorded for DMPC:Cholesterol monolayers transferred from the air-water interface onto gold electrode confirmed the presence of the drug in the lipid layer. Based on the charge of the oxidation-reduction peaks corresponding to the redox processes of quinone-hydroquinone group in daunorubicin, the initial surface concentration of the drug in the membrane and the drug release profile to the solution were evaluated.

Release Kinetics of Multiwalled Carbon Nanotubes Deposited on Silica Surfaces: Quartz Crystal Microbalance with Dissipation (QCM-D) Measurements and Modeling

Understanding the kinetics of the release of carbon nanotubes (CNTs) from naturally occurring surfaces is crucial for the prediction of the environmental fate and transport of CNTs. In this study, the release kinetics of multiwalled CNTs (MWNTs) from silica surfaces was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). MWNTs were first deposited on silica surfaces under favorable deposition conditions (1.50 mM CaCl2 and pH 7.1) and the deposited MWNTs were then rinsed at different electrolyte solutions to induce the release of MWNTs from the primary energy minimum. The kinetics of MWNT release was shown to be first order with respect to the deposited MWNTs when complete release took place. A model that accounts for the releasable and unreleasable components of MWNTs was used to fit the experimental data in order to derive the release rate coefficients. When the CaCl2 concentration in the eluent was decreased, a larger fraction of deposited MWNTs was released and the release rate coefficient of the releasable MWNTs also increased. The rise in the surface charges of both MWNTs and silica surfaces with the drop in CaCl2 concentration likely resulted in the decrease in the height of the energy barrier, thus facilitating the release of the nanotubes. Moreover, when the initial surface concentrations of deposited MWNTs were over 1000 ng/cm(2), the release rate coefficient was lower than expected. The reduced release kinetics was likely due to the formation of large surface-bound MWNT clusters which had considerably lower diffusion coefficients than dispersed MWNTs or MWNT aggregates.

The release of lindane from contaminated building materials

The release of the organochlorine pesticide lindane (γ-hexachlorocyclohexane) from several types of contaminated building materials was studied to assess inhalation hazard and decontamination requirements in response to accidental and/or intentional spills. The materials included glass, polypropylene carpet, latex-painted drywall, ceramic tiles, vinyl floor tiles, and gypsum ceiling tiles. For each surface concentration, an equilibrium concentration was determined in the vapour phase of the surrounding air. Vapor concentrations depended upon initial surface concentration, temperature, and type of building material. A time-weighted average (TWA) concentration in the air was used to quantify the health risk associated with the inhalation of lindane vapors. Transformation products of lindane, namely α-hexachlorocyclohexane and pentachlorocyclohexene, were detected in the vapour phase at both temperatures and for all of the test materials. Their formation was greater on glass and ceramic tiles, compared to other building materials. An empiric Sips isotherm model was employed to approximate experimental results and to estimate the release of lindane and its transformation products. This helped determine the extent of decontamination required to reduce the surface concentrations of lindane to the levels corresponding to vapor concentrations below TWA.

Carbon coatings with low secondary electron yield

Carbon thin films for electron cloud mitigation and anti-multipacting applications have been prepared by dc magnetron sputtering in both neon and argon discharge gases and by plasma enhanced chemical vapour deposition (PECVD) using acetylene. The thin films have been characterized using Secondary Electron Yield (SEY) measurements, Scanning Electron Microscopy (SEM), Nuclear Reaction Analysis (NRA) and X-ray Photoemission Spectroscopy (XPS). For more than 100 carbon thin films prepared by sputtering the average maximum SEY is 0.98 ± 0.07 after air transfer. The density of the films is lower than the density of Highly Oriented Pyrolytic Graphite (HOPG), a fact which partially explains their lower SEY. XPS shows that magnetron sputtered samples exhibit mainly sp2 type bonds. The intensity on the high binding energy side of C1s is found to be related to the value of the SEY. In addition the initial surface concentration of oxygen has no influence on the resulting SEY, when it is below 16%. The thin films produced by PECVD have a much higher maximum SEY of 1.49 ± 0.07.Storage conditions in air, namely wrapping in aluminium foil, preserves the low SEY by more than one year. Such coatings have already been applied successfully in accelerators and multipacting test benches.

Formation of two-dimensional (M) and three-dimensional (V) nanoaggregates of substituted cobalt porphyrin in the Langmuir layers and Langmuir-Schaefer films

Floating molecular layers of cobalt tetra(p-metoxyphenyl)porphyrin (CoTMPP) were studied experimentally and theoretically. The structure of the layers was analyzed within the framework of an original model and method for quantitative analysis of compression isotherms of a monolayer comprised of Major nanoaggregates (M-monolayer). The existence regions as well as the structural parameters and properties of monolayers in stable states were determined. These include the size of M-nanoaggregates and the number of molecules in them, the distance between aggregates, the content of bound water and water between aggregates, the compressibility, and the pressure and current surface concentration ranges corresponding to stable states. Quantitative correlations between the fabrication conditions, nanoaggregate size, and properties of M-monolayers were obtained. The state diagrams and M-isotherms were plotted and a passport of CoTMPP floating layers was compiled. A classification of porphyrin M-monolayers including the description of rarefied and condensed M-aggregates in the floating monolayers was proposed; a quantitative model for such monolayers was developed. Constants governing the structure of CoTMPP monolayers and the formation conditions of stable floating layers with 3D nanoaggregates (V-nanoaggregates) were determined. A specific feature of CoTMPP which can discriminate it from azaporphyrins is as follows: the size of M edge-aqua aggregates and the distance between them are independent on the initial surface concentration. CoTMPP thin films on solid substrates were prepared by various methods including the Langmuir-Schaefer technique. A comparison of UV-Vis spectra of the solution, Langmuir-Schaefer films, and spin-cast films suggests different degrees of CoTMPP aggregation in these systems.

Edible Coatings and Retention of Potassium Sorbate on Apples, Tomatoes and Cucumbers to Improve Antifungal Activity During Refrigerated Storage

Fresh apples, tomatoes and cucumbers were coated with guar gum (GG, 1.0% w/v), pea starch (PS, 4.0% w/v) or potato starch (PotS, 4.0% w/v) containing potassium sorbate (KS, 1.0% w/v) and stored at 4C for 25 days. Coatings weight, thickness, KS surface concentration and yeast and mold counts were determined throughout the storage. GG coatings on apples and tomatoes produced the largest liquid film weight per unit area whereas after drying, PS coatings produced the largest coating weight and thickness. GG coatings deposited the largest initial KS surface concentration whereas PS coatings maintained KS at the surface and both coatings exhibited the greatest antifungal effectiveness. The relationships between KS surface concentration and the yeast and mold counts presented good correlations (R 2 between 0.75 and 0.90) and were linear for PS and PotS coatings and logarithmic for GG coatings indicating that multiple factors affecting depletion of KS from GG coatings.

Tuning nanoscopic self‐assembly of diblock copolymer blends on a two‐dimensional interface

AbstractSpreading amphiphilic diblock copolymers on a two‐dimensional liquid interface has been observed to produce nanoscale features via self‐assembly. Here, we develop a model that incorporates the effects of polymer entanglement and surface diffusion in polymer blends to quantitatively predict the size of experimentally observed structures. Simulations show that different polymers in the blend cooperate to self‐assemble into nanoscale features of varying sizes. Characteristic nanoscopic dimensions can be tuned by adjusting two easily controllable macroscopic quantities: the blend composition and the initial surface concentration. Theoretical predictions are in agreement with experimentally measured feature dimensions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Nanoaggregates in floating layers of azaporphyrins

An analysis of compression isotherms of floating layers of copper tetra-tert-butylphthalocyanine ( CuPctBu4 ) and copper tetrabenzotriazaporphyrine, obtained at varied initial conditions, is carried out. The structure of layers is determined by three methods: the traditional one (direct estimation of area per molecule from a π-A isotherm), by a quantitative analysis of compression isotherms (with the use of Volmer equation), and by Brewster angle microscopy. It is shown that in layers with edge-on arrangement of molecules (initial surface concentrations N0 > 0.15 μmol.m-2), the layer structure, determined by the quantitative method, may differ drastically from the structure obtained by the traditional one. The equation of state of the floating layer generalized to the case when structural units of 2D gas are molecular aggregates is used to describe the layer of azaporphyrines. Boundaries of existence and characteristics of structure and properties of floating layers in stable states (size of nanoaggregates formed in a layer, number of molecules in them, interaggregate spacing, compressibility) are determined. The quantitative model of floating layer of copper tetra-tert-butylphthalocyanine is proposed. Constants characterizing a stable monolayer, as well as the region where it may be formed, are determined on the base of the model created.

Essential Explanation of the Strong Mineralization Performance of Boron-Doped Diamond Electrodes

Electrochemical oxidation of p-nitrophenol was examined using differentanodic materials, including T/boron-doped diamond (BDD), Ti/SnO2-Sb/PbO2, and Ti/SnO2-Sb anodes. The results demonstrated that Ti/BDD anodes had a much stronger mineralization performance than the other two anodes. Furthermore, it was found that hydroxyl radicals could mainly exist as free hydroxyl radicals at BDD anodes, which could react with organic compounds effectively. This implied that the dominant mechanism for a much higher mineralization capacity of BDD anodes would be attributed to the existence of free hydroxyl radicals in the BDD anode cell rather than adsorbed hydroxyl radicals on the BDD anode. To further corroborate this hypothesis, electrochemical oxidation of p-substituted phenols (p-nitrophenol, p-hydroxybenzaldehyde, phenol, p-cresol, and p-methoxyphenol) was examined at the Ti/BDD, Ti/SnO2-Sb/ PbO2, and Ti/SnO2-Sb anodes, respectively. The study revealed that for Ti/BDD electrodes, the degradation rate of p-substituted phenols (k) increased with the increase of Hammett's constant (sigma), which confirmed the dominance of free hydroxyl radicals at BDD anodes and its effective reaction with organics therein. For Ti/SnO2-Sb/PbO2 electrodes, the degradation rate of p-substituted phenols (k) increased with the increase of initial surface concentration gamma (representing the adsorption capacity of phenols to electrode surface), which indicated that organic compounds mainly reacted with adsorbed hydroxyl radicals at PbO2 anodes. For Ti/SnO2-Sb electrodes, however, k increased with the increase of the integrated parameter S (representing the effects of both sigma and gamma), which implied that organic compounds reacted with both adsorbed hydroxyl radicals and free hydroxyl radicals at SnO2 anodes.

Nanoaggregates of Copper Porphyrazine in Floating Layers and Langmuir−Schaefer Films

Aggregation behavior of unsubstituted copper porphyrazine (CuPaz) on the water surface was studied by analysis of compression curves, Brewster angle microscopy (BAM), and optical spectroscopy. The structure and stability of the CuPaz aqua aggregates in the floating layers are determined by hydration degree that depends on initial surface concentration and surface pressure. Langmuir-Schaefer (LS) films of CuPaz were prepared by deposition of the variously structured floating layers and studied by X-ray scattering technique and optical spectroscopy. Stable and labile structures were detected and compared with the floating CuPaz aqua aggregates. Conditions of formation of the stable four-stacked nanoaggregates in LS films were determined. A model comprising both nucleation of CuPaz on the water surface and structural transformations in the solid films is proposed.