Substrates In Aqueous Solution Research Articles

Effect of TiO 2 nanoparticles on the electropolymerization of polypyrrole

We report here the first electrochemical polymerization of polypyrrole (PPy) on gold substrates in aqueous solutions containing additives of 1 mM rutile TiO 2 nanoparticles under the irradiation of UV light. As a result, PPy/TiO 2 composites were prepared with an increased conductivity of 135 S cm −1, raising one order as compared with that of pure PPy. Furthermore, the thermal stability of the PPy-based composites was improved, as shown from thermogravimetric analysis (TGA). The effects of TiO 2 on the PPy electropolymerization under UV irradiation were examined by its nucleation and growth mechanism and the onset potential of polymerization. The results indicate that the nucleation and growth are instantaneous two-dimensional and three-dimensional processes before and after nuclei overlapping, respectively, for the composite electropolymerization, which are distinguishable from those of pure PPy electropolymerization before nuclei overlapping.

Formation of Calcium Phosphate Film on Ti Substrate in Aqueous Solutions in the Control of Temperature and Ion Activity

Formation of Calcium Phosphate Film on Ti Substrate in Aqueous Solutions in the Control of Temperature and Ion Activity

A novel low-temperature growth and characterization of single crystal ZnO nanorods

One-dimensional (1D) single crystal ZnO nanorods directly grown on ZnO nanostructured substrates by soft chemical method is proposed. Employing a novel two steps procedure, the ZnO nanorods with a diameter about 45 nm can be successfully grown on substrate in aqueous solution at attractive low temperature in 90 °C, permitting large-scale fabrication with a relatively low cost and a potential application in optoelectronic nanodevices. The structures, appearances and optical properties of ZnO nanorods also have been characterized.

Heterogeneous nucleation and growth of crystalline micro-bubbles around gas cavities formed in solution

Perfect single three dimensional crystals with the shape of bubbles can nucleate and grow encompassing gas filled cavities previously formed on solid substrates in aqueous solutions. Bubbles are double walled, both internal and external faces belonging to the cleavage calcite rhombohedron which separate the growth solution from the enclosed gas cavity. The bubbles thickness amounting to a few nanometers, while the size of the bubble is strictly related to that of the cavity ( of the order of a few micrometers), the surface-to-volume ratio is the highest obtained till now for 3D single crystals. A tentative model of nucleation and growth mechanisms of these peculiar objects is proposed. Growth morphology modifications due to the effect of impurities ( Li + and acetate ions) or to foreign gas bubbling in the growth solution (methane) are also described. A peculiar attention is paid to both mechanical and thermodynamic conditions to be fulfilled in order to nucleate crystals around gas cavities from solution and melt. By analysing growth and patterning of Cacarbonate hollow shells in reverse microemulsions along with the growth from melt of sodium disilicate around gas microcavities, we found the criterion ruling the nucleation and growth of poly- or single crystals around a gas cavity and outlined the major role played by the nature of the substrate. Finally, is proposed a new interpretation of hollow shaped crystals, recently found in methane-derived carbonate rocks (Western Alps-Italy) and in late Cenozoic dolomites of the Bahamas.

Single asperity tribochemical wear of silicon nitride studied by atomic force microscopy

Nanometer scale single asperity tribochemical wear of silicon nitride was examined by measuring the wear of atomic force microscope tips translated against a variety of substrates in aqueous solutions. We show that the chemical nature of the substrate plays an important role: significant wear was observed only when the substrate surface is populated with appropriate metal-hydroxide bonds. Mica and calcite substrates, whose water-exposed cleavage surfaces lack these bonds, produced little if any tip wear. As a function of contact force FN and scan duration t, the length of the tips in this work decreases approximately as (FNt)0.5. We propose that pressure-induced intermediate states involving hydroxyl groups form on both the tip and the substrate; chemical reactions subsequently form transient bridging chemical bonds that are responsible for tip wear.

Hydroxyapatite coating on titanium by means of thermal substrate method in aqueous solutions

Hydroxyapatite (HAp) films were formed on a titanium substrate in aqueous solutions by the thermal substrate method controlling the substrate temperature and a cathodic electrolysis method supplying hydroxide ions. A local increase in temperature on substrate and the supply of calcium, phosphate, and hydroxide ions near the substrate accelerate the HAp film formation on the substrate in aqueous solutions at ambient temperature and pressure. The HAp can be directly coated only on the substrate quickly by heating the substrate in an aqueous solution. In the cathodic process, reduction of hydrogen peroxide forms hydroxide ions, which results in the formation of a flat, plate-like HAp film.

Electrochemical deposition of ZnTe thin films

The electrodeposition of zinc telluride (ZnTe) thin films on tin conductive oxide substrates in aqueous solution containing TeO2 and ZnSO4 was studied. The electrodeposition mechanism was investigated by cyclic voltammetry. The appropriate potential region where formation of stoichiometric ZnTe semiconductor occurs, was found to be −1.1 V versus saturated calomel electrode, and the pH was maintained at 3.5 ± 0.1. The crystallographic structure of the deposited film is seen to be cubic with a preferential orientation along the (111) phase. Optical absorption studies reveal a bandgap of 2.26 eV and optical constants (n and k) are evaluated. Scanning electron microscopy imaging was used to study the surface morphology. The composition of ZnTe films was analysed using an electron microprobe analyser, and the results are discussed.

Environmental photocatalytic processes with POM. The photodecomposition of atrazine and photoreduction of metal ions from aqueous solutions

Polyoxometalate anions (POM), resulting from the condensation of tungstate anions in strongly acidified solutions, can absorb efficiently light in the UV‐near Vis region.The produced excited state is a very powerful oxidative reagent which can, mainly via OH radical attack, oxidize and mineralize a great variety of organic pollutants in aqueous solutions, while the photocatalytic circle is closed by reoxidation of the reduced POM, mainly, by O2. Metal ions can also serve as oxidants and close the photocatalytic cycle. In the process metal ions are reduced, precipitated and removed from the solution. Thus photocatalytic treatment for decontamination of waste waters from both organic and inorganic pollutants (metal ions) can be, in principle, achieved. Atrazine, a widely used herbicide with s‐triazine structure, is photodecomposed to cyanuric acid in presence of in aqueous solutions. However, no precipitation of metal ions is obtained when atrazine is the sole organic substrate in aqueous solutions. The low solubility of atrazine produces insufficient concentration of reduced POM to cause precipitation.

Kinetic study of the reactions of oxoiron(IV) with aromatic substrates in aqueous solutions

AbstractThe kinetics of the reactions of oxoiron(IV) (FeO2+) with phenol, nitrobenzene, m‐, o‐, and p‐nitrophenol in 1 M HClO4 was investigated by the stopped‐flow technique. The rate constants of these reactions decrease with increasing the one‐electron reduction potentials of the corresponding radical cations of the substrates and with the Hammett parameter of the NO2 group in the phenol ring. A reaction mechanism is proposed, which accounts for the observed trends and for the nature of the reaction products. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 488–494, 2002

Water-soluble calix[n]arenes as receptor molecules for non-polar substrates and inverse phase transfer catalysts

Water-soluble calix[n]arenes are powerful receptors for non-polar substrates in aqueous solution. The inclusion of the guests is studied by 1H NMR titration experiments and molecular modeling studies combined with ab initio NMR shift calculations. The complexation is mainly enthalpically driven. The tested water-soluble calix[n]arenes are promising candidates for carrier molecules for the transport of non-polar substrates through bulk water as well as inverse phase transfer catalysts as proven for the Suzuki coupling of iodobenzene with phenyl boronic acid. In all cases the efficiency of the tested macrocycles is higher than that for β-cyclodextrin.

Adsorption-Induced Conformational Changes of α-Helical Peptides

Peptides that have α-helical structures in solution have been adsorbed to anionic and cationic colloidal silica substrates in aqueous solution, and the orientation and conformation of the adsorbed peptides have been studied by 1H NMR and circular dichroism. The peptides, which are composed of aspartate, alanine, and arginine segments, adsorb to charged colloidal silica such that the complementary-charged amino acid side chains are adjacent to the substrate surface. The intramolecular interactions that stabilize the α-helical structure are perturbed by the intermolecular interactions established upon adsorption, and the peptides undergo a conformational change upon adsorption to the substrate surface. Partial helicity loss propagates from one or both of the peptide termini, although the adsorbed molecules retain some α-helical structure.

Direct Observation of Cation Complexation of 18-Crown-6 by Atomic Force Microscopy Using Chemically Modified Tips

Abstract Intermolecular forces between 18-crown-6 and guest ions have been measured by atomic force microscopy using AFM tips modified chemically with an alkylsilane derivative having 18-crown-6 moiety. The greater adhesion forces were observed with the crown-ether modified tip than with bare tips on mica substrate in aqueous solutions containing K+, probably due to the complexation between 18-crown-6 moiety on tip and K+ in solution. Specific interactions based on cation complexation of 18-crown-6 have been observed in ethanol on mica substrate treated with (3-aminopropyl)triethoxysilane (–NH3+).

Preparation of Polymers by Admiceller Polymerization Method and Their Carbonization.

Radical polymerization was performed of acrylonitrile solubilized in admicelles of sodium dodecylsulfate (SDS), an anionic surfactant, formed on a mica substrate in aqueous solution. An attempt was then made to prepare thin carbon film by stabilizing the polyacrylonitrile film thus formed as precursor and carbonizing it and the surface structure of the sample was observed with an atomic force microscope (AFM) after each of the treating processes. The sample surface after polymerization consisted of layers, each being a smooth ultrathin film at the nanometer level. The acrylonitrile film shrank to form a granular structure on the mica surface after stabilization and the granules aggregated after carbonization. Contact angle measurements on the sample surfcce that untreated mica substrate has a hydrophilic surface, which turns hydrophobic after polymerization and the surface hydrophobicity increases as the treating process proceeds further to stabilization and carbonization.

Microporous Polyoxometalates POMs/SiO2: Synthesis and Photocatalytic Degradation of Aqueous Organocholorine Pesticides

Microporous Keggin-type polyoxometalate materials were synthesized by incorporating H3PW12O40 and/or H4SiW12O40 into a silica matrix via a sol−gel technique. The silica-impregnated polyoxometalates (POMs/SiO2) thus obtained are insoluble and readily separable porous materials with uniform micropores and high specific surface areas. Trace aqueous organocholorine pesticides, hexachlorocyclohexane (HCH) and pentachloroenitrobenzene (PCNB), were totally degraded and mineralized into CO2 and HCl by irradiating a POMs/SiO2 slurry in the near-UV area. The disappearance of the organocholorine compounds follows Langmuir−Hinshelwood first-order kinetics. Studies on the reaction mechanism indicate that photogeneration of OH• radicals originating from neutralization of OH- groups on the catalyst surface with positive photoholes is responsible for the oxidation of the organic substrates in aqueous solution.

Micropatterned solid-supported membranes formed by micromolding in capillaries.

The formation of individually addressable micropatterned solid-supported lipid bilayers has been accomplished by means of micromolding in capillaries. Small unilamellar vesicles were spread on glass slides to form planar supported membranes along microscopic capillaries molded as trenches into a polydimethylsiloxane (PDMS) elastomer. PDMS provides an elastic and transparent carrier for microcapillaries molded from silicon wafers displaying the desired inverse trenches. The so-called master structure has been conventionally etched into silicon by photolithography. The cured PDMS elastomer was briefly exposed to an oxygen plasma, rendering the surface hydrophilic, and subsequently attached to a glass surface in order to form hydrophilic capillaries equipped with flow-promoting pads on either side. One flowpad acts as a reservoir to be filled with the vesicle suspension, while the other one serves as a collector to ensure a sufficient capillary flow to cover the substrate completely. Formation of planar lipid bilayers on the glass slide along the capillaries was followed by imaging the flow and spreading of fluorescently labeled DMPC liposomes with confocal laser scanning microscopy. By means of scanning force microscopy in aqueous solution the formed lipid structures were identified and the height of the lipid bilayers was accurately determined. With both techniques, it was shown that the patterned bilayers remain separated and persist for several hours on the substrate in aqueous solution.

Simultaneous electrochemical formation of Al 2O 3/polypyrrole layers (I): effect of electrolyte anion in formation process

We studied the capability of various kinds of sulfonate-based electrolytes to form a bilayered Al 2O 3/polypyrrole (PPy) film on an aluminum substrate in aqueous solution. As electrolytes, we selected sodium p-toluenesulfonate (monovalent sulfonate), sodium naphthalene-trisulfonate (trivalent sulfonate), poly(sodium 4-styrenesulfonate), poly(sodium vinylsulfonate) (sulfonate polymer), sodium n-dodecylbenzenesulfonate (SDBS), sodium butylnaphthalenesulfonate(BNS), and sodium n-dodecylsulfate (SDS, surfactant sulfonate/sulfate). Among the sulfonates we investigated, SDBS, BNS and SDS appeared to form a barrier-type Al 2O 3 layer with high coulombic efficiency in the absence of pyrrole. In the presence of pyrrole, anodization of these electrolytes resulted in the formation of Al 2O 3 and PPy layers, simultaneously.

Enantioselective Synthesis of (S)-2-Hydroxypropanone Derivatives by Benzoylformate Decarboxylase Catalyzed C−C Bond Formation

Chiral 2-hydroxypropanone derivatives 5a−v, 8a−d, and 10a, b were formed by benzoylformate decarboxylase (BFD) catalyzed C−C bond formation. A donor aldehyde and acetaldehyde as an acceptor were carboligated in aqueous buffer solution with remarkable ease in high chemical yield and good to high optical purity. The substrate range of this thiamin diphosphate dependent enzyme was examined to employ this benzoin condensation type reaction in stereoselective synthesis. The observed dependence of the enantiomeric excess on the substitution pattern could be exploited to design substrates resulting in high selectivity. Best substrates with regard to optical purity were meta-substituted benzaldehyde derivatives. To enable a general and convenient applicability of the BFD-catalyzed C−C bond formation, analytical batch experiments were scaled up to give (S)-2-hydroxy ketones in good to high yields on a preparative scale. Further, the solubility of some of the organic substrates in aqueous solution was increased by the use of cyclodextrin or buffer/DMSO mixtures.

Accelerated Biphasic Hydroformylation by Vesicle Formation of Amphiphilic Diphosphines

The synthesis, aggregation behavior, and catalytic activity of rhodium complexes of a series of Xantphos derivatives with surface-active pendant groups, −4-C6H4O(CH2)nC6H4(SO3Na)− (n = 0, 3, 6) is described. Electron microscopy experiments show that these ligands and their complexes form vesicles in water if the hydrophobic part of the ligand is large enough (n = 3, 6). The formed aggregates are stable at elevated temperatures (90 °C), and their presence leads to a significant enhancement of the solubility of organic substrates in aqueous solution. This enhanced solubility results directly in a higher reaction rate in the rhodium-catalyzed hydroformylation of 1-octene. Furthermore, recycling experiments show that the TOF and the high selectivity toward the more valuable linear aldehyde remains approximately the same in four consecutive runs. This indicates that the aggregates stay intact during the recycling and the active rhodium complex is retained in the water-phase quantitatively.

Analysis of Contact Interactions between a Rough Deformable Colloid and a Smooth Substrate

A model was developed for the effect of van der Waals interactions between a rough, deformable, spherical colloid and a flat, smooth, hard surface in contact. The model demonstrates the significant effect of colloid roughness on removal force. Small changes in colloid roughness produce large changes in the predicted removal force. Several authors attribute discrepancies in the observed interaction force between particles and surfaces to colloid roughness, and our model supports their hypotheses. Experimental data documenting the force required to remove colloids of polystyrene latex from silica substrates in aqueous solution were collected during AFM studies of this system. When colloid roughness exists, as is the case in this work, our model bounds the observed removal force. The predicted range of removal forces is in better quantitative agreement with our removal force data than are forces predicted by classical DLVO theory.

Simultaneous pH–rate profiles applied to the two step consecutive sequence A → B → C : a theoretical analysis and experimental verification †

The absorbance extremum method introduced to resolve the rate constant–reaction step ambiguity problem in two step consecutive reactions is applied to cases where the rate constant in each step is pseudo first order and dependent on catalyst concentration. In particular, acid and base catalyzed reactions of substrates in aqueous solution are examined. Resultant simultaneous pH–rate profile functions are treated in the context of the ambiguity problem and a detailed account of the possible interactions between pH–rate profiles is also given. The merits of this method include its general scope, its applicability to reactions for which it may be experimentally difficult to resolve the ambiguity assignment by other means, and its use of rate data acquired from original kinetic traces without requiring additional information. The theoretical analysis is verified and tested experimentally for the hydration of fluorenylideneketene in dilute aqueous perchloric acid solutions.