Stability In Acidic Media Research Articles

Cross-Linked Chitosan/Chitin Crystal Nanocomposites with Improved Permeation Selectivity and pH Stability

This study is aimed at developing and characterizing cross-linked bionanocomposites for membrane applications using chitosan as the matrix, chitin nanocrystals as the functional phase, and gluteraldehyde as the cross-linker. The nanocomposites' chemistry and morphology were examined by estimation of gel content, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and atomic force microscopy (AFM), whereby the occurrence of cross-linking and nanoscale dispersion of chitin in the matrix was confirmed. Besides, cross-linking and chitin whiskers content were both found to impact the water uptake mechanism. Cross-linking provided dimensional stability in acidic medium and significantly decreased the equilibrium water uptake. Incorporation of chitin nanocrystals provided increased permeation selectivity to chitosan in neutral and acidic medium.

Stable PtNiPb/WC Catalyst for Direct Methanol Fuel Cells

Tungsten carbide (WC) was prepared by carbothermal synthesis in 900°C. PtNiPb catalysts with homemade WC and XC-72 carbon black as supports, respectively, were prepared. Their performances were tested through cyclic voltammetric, chrono-amperometric, and current density-time curves. Electrochemical activity for methanol electro-oxidation of PtNiPb/WC catalyst is evidently higher than that of PtNiPb/C before accelerated durability testing. Its activity is also markedly higher than that of the latter after an accelerated durability test. These results indicate that the WC as a support can improve the activity and stability of PtNiPb catalyst due to usage of WC and its stability in acidic medium.

Preparation of platinum-doped hollow spheres and their electrocatalytic activity in water electrolysis

Pure TiO2 hollow spheres were prepared by using poly(styrene-methacrylic acid) latex particles as template; thereafter, titania hollow spheres were coated by platinum with an appropriate amount of choloroplatinic acid solution to obtain Pt/TiO2 catalysts. The morphology and structure of nonstructural Pt/TiO2 hollow spheres were characterized by BET, XRD, TGA, SEM and TEM analysis. In the samples, a remarkably uniform layer of Pt consisting of particles from 5 to 70 nm in size was formed over TiO2 hollow spheres. We found the electrocatalytic nature of the samples by cyclic voltammetric experiment in acidic solution. The anodic peak current density of 20 wt% Pt-loaded TiO2 hollow particles was observed 2.5 times higher than that of 5 wt% Pt/TiO2 in the same experimental condition. Also, the anodic current density of 20 wt% Pt/TiO2 hollow spheres calcined at various temperatures followed the order: 400 °C≈500 °C>600 °C. The electrocatalytic activity of the Pt-loaded TiO2 hollow spheres depends on the amount of atomic platinum present in the sample; a higher concentration of platinum results in a larger current density value in anodic sweep, resulting in more oxygen production during electrolysis. Pt/TiO2 hollow sphere catalysts have also shown long term electrocatalytic stability in acidic media.

Performance of tin doped titania hollow spheres as electrocatalysts for hydrogen and oxygen production in water electrolysis

The electrocatalytic activity of tin doped TiO 2 hollow sphere particles has been examined in acid solution. Titania hollow spheres have been synthesized by using poly(styrene-methacrylic acid) latex particles as a template, and tin was doped over spheres with the appropriate amount of SnCl 2·2H 2O solution in HCl. The surface characteristics have been evaluated by XRD, SEM and TEM analysis. XRD analysis has shown the presence of rutile SnO 2 and TiO 2 phase in the catalysts; and the peak intensity for SnO 2 phase increased with rise in the calcination temperature indicating the critical growth of tin on the surface of the catalysts. Diameter of pure TiO 2 hollow spheres and Sn doped samples were found from SEM and TEM images in the range of 1–1.2 and 0.4–0.6 μm, respectively. TGA has been performed with the uncalcined 40 wt% Sn doped TiO 2 catalyst. The peaks for hydrogen and oxygen production are present significantly for all the hollow sphere samples in cyclic voltammograms; but peaks for hydrogen production is more prominent in 20 and 40 wt% Sn/TiO 2 catalysts. The performances of the hollow sphere electrocatalysts were evaluated up to current densities of 50 mA cm −2 during anodic polarization measurement. Sn/TiO 2 electrocatalysts have also shown long time electrochemical stability in acidic media.

Current and potential distributions on positive plates with conductive Pb 3O 4 and BaPbO 3 in their formation and discharge

The positive plates with conductive materials, Pb 3O 4 and BaPbO 3, in automotive lead-acid batteries were investigated by measuring their current and potential distributions in the course of formation and discharge. It is found that these two conductive materials, especially Pb 3O 4, enhance the formation in the initial stage greatly and that they make the current and potential distributions more uniform. In the discharge, the addition of Pb 3O 4 increases the capacity of the positive plate, but it is unfavorable to the paste curing and causes poor contact between active mass (AM) particles so that the polarization increases greatly at 3 C discharge rate. The BaPbO 3 additive improves not only the formation but also the discharge performance because of its stability in acidic media and at high polarization. The violent charge at high polarization around the plates in the initial formation can lead to poor AM contact and high polarization resistance.

Studies of adsorption of alkaline trypsin by poly(methacrylic acid) brushes on chitosan membranes

AbstractPoly(methacrylic acid)‐grafted chitosan membranes (chitosan‐g‐poly(MAA)) were prepared in two sequential steps: in the first step, chitosan membranes were prepared by phase‐inversion technique and then epichlorohydrin was used as crosslinking agent to increase its chemical stability in acidic media; in the second step, the graftcopolymerization of methacrylic acid onto the chitosan membranes was initiated by ammonium persulfate (APS) under nitrogen atmosphere. The chitosan‐g‐poly(MAA) membranes were first used as an ion‐exchange support for adsorption of trypsin from aqueous solution. The influence of pH, equilibrium time, ionic strength, and initial trypsin concentration on the adsorption capacity of the chitosan‐g‐poly(MAA) membranes have been investigated in a batch system. Maximum trypsin adsorption onto chitosan‐g‐poly(MAA) membrane was found to be 92.86 mg mL−1 at pH 7.0. The experimental equilibrium data obtained for trypsin adsorption onto chitosan‐g‐poly(MAA) membranes fitted well to the Langmuir isotherm model. The adsorption data was analyzed using the first‐ and second‐order kinetic models, and the experimental data was well described by the second‐order equation. More than 97% of the adsorbed trypsin was desorbed using glutamic acid solution (0.5M, pH 4.0). In addition, the chitosan‐g‐ poly(MAA) membrane prepared in this work showed promising potential for various biotechnological applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Preparation and characterization of thin organosilicon films deposited on SPR chip

The paper reports on the preparation and characterization of organosilicon thin polymer films deposited on glass slides coated with 5 nm adhesion layer of titanium and 50 nm of gold. The polymer was obtained by the decomposition of 1,1,3,3-tetramethyldisiloxane precursor (TMDSO) premixed with oxygen induced in a N 2 plasma afterglow using remote plasma-enhanced chemical vapor deposition (PECVD) technique. The film thickness was controlled by laser interferometry and was 9 nm. The chemical stability of the gold substrate coated with the organosilicon polymer film (p-TMDSO) was studied in different acidic and basic solutions (pH 1–14). While the gold/polymer interface showed a high stability in acidic media, the film was almost completely removed in basic solutions. The resulting surfaces were characterized using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), water contact angle measurements, cyclic voltammetry, and surface plasmon resonance (SPR).

Development of Zirconium Based Non-Platinum Cathode for Polymer Electrolyte Fuel Cell

Zirconium based compounds (ZrO x N y ) have been investigated as a new cathode of polymer electrolyte fuel cells without platinum. The ZrO x N y film were prepared by the radio frequency magnetron sputtering method under Ar + O 2 + N 2 atmosphere with heating the sputtered material. The effect of the heat treatment on both the catalytic activity for the oxygen reduction reaction (ORR) and the properties of ZrO x N y has been examined. The solubility of ZrO x Ny was less than 5x10 -8 mol dm -3 in 0.1 mol dm -3 sulfuric acid at 30 °C under atmospheric condition. This indicated that ZrO x N y had a high chemical stability in acidic media. The ORR current density on ZrO x N y which was heated at 800 °C was about 30 times larger than that on ZrO x N y deposited at 50 °C. It was found that heating the sputtered material during the deposition was very effective to increase the catalytic activity for ORR. The higher temperature of heat treatment caused the change of crystalline structure and the increase of the crystallinity of ZrO x N y .

Zirconium-Based Compounds for Cathode of Polymer Electrolyte Fuel Cell

Zirconium-based compounds, such as , have been investigated as a cathode of polymer electrolyte fuel cells. was prepared using radio-frequency (rf) magnetron sputtering under an with heating the substrate. The solubility of deposited at was less than in at under atmospheric condition, indicating that had a high chemical stability in acidic media. The effect of the heat-treatment during the film deposition on the catalytic activity for oxygen reduction reaction (ORR) and the properties of has been examined. The ORR current density on deposited at a substrate temperature of was times greater than that at . The dependence of both the ORR current density at and the apparent activation energy of the ORR at on the substrate temperature changed near . The crystalline structure of also changed near . In addition, the catalytic activity for the ORR increased with the increasing crystallinity and decreasing ionization potential of the specimens. Therefore, the crystalline structure, the crystallinity, and the ionization potential might affect the catalytic activity for the ORR.

Particle size dependence of oxygen evolution reaction on nanocrystalline RuO 2 and Ru 0.8Co 0.2O 2− x

Nanocrystalline RuO 2 and Ru 0.8Co 0.2O 2− x were prepared by a sol-gel synthesis. Both electrode materials were phase pure with tetragonal structure of rutile type. Characteristic particle size estimated from XRD pattern ranged between 15 and 45 nm in the case of RuO 2 and 20 and 60 nm in the case of Ru 0.8Co 0.2O 2− x . Both electrode materials show acceptable stability in acid media and are active electrocatalysts for oxygen evolution. Despite similar structure and chemical composition the oxygen evolution on RuO 2 is controlled by charge transfer processes; the oxygen evolution on Ru 0.8Co 0.2O 2− x is controlled by chemical steps of the electrode process. The oxygen evolution is in both cases affected by the particle size. Increasing particle size suppresses oxygen evolution on RuO 2, but enhances it on Ru 0.8Co 0.2O 2− x anodes. The opposite trends in electrochemical behavior of RuO 2 and Ru 0.8Co 0.2O 2− x are attributed to a specific effect of cobalt, which substitutes the Ru in the structure of the surface.

Zirconium Nitride and Oxynitride for New Cathode of Polymer Electrolyte Fuel Cell

Zirconium compounds (ZrOxNy) have been investigated as a new cathode of polymer electrolyte fuel cells without platinum. The electrocatalysts of ZrOxNy were prepared by the radio frequency magnetron sputtering method under Ar + O2 + N2 atmosphere with heating the sputtered material. The effect of the heat treatment on the catalytic activity for the oxygen reduction reaction (ORR) and the properties of ZrOxNy has been examined. The solubility of ZrOxNy was less than 5×10-8 mol dm-3 in 0.1 mol dm-3 sulfuric acid at 30 oC under atmospheric condition. This indicated that ZrOxNy had a high chemical stability in acidic media. The ORR current density on ZrOxNy which was heated at 800 oC was about 30 times larger than that on ZrOxNy deposited at 50 oC. The higher heat treatment caused the change of crystalline structure and the increase of the crystallinity of ZrOxNy.

Separation of 1,4-benzodiazepines by micellar elektrokinetic capillary chromatography

In this work the applicability of micellar elektrokinetic capillary chromatography (MECC) for the determination of benzodiazepines (BZD) has been studied. The applied method was used for the simultaneous separation of 8 BZDs (alprazolam, bromazepam, chlordiazepoxide, diazepam, flunitrazepam, medazepam, oxazepam, nitrazepam), and also for the study of stability in acidic medium. A fast and reliable method has been developed; using a separation buffer composed of sodium tetraborate 25 mM (pH 9.5), SDS (50 mM) and methanol (at least 12%) as an organic modifier.

Preparation of 2-mercaptobenzothiazole-derivatized mesoporous silica and removal of Hg(ii) from aqueous solution

Mesoporous silicas (SBA-15 and MCM-41) have been functionalized by two different methods. Using the heterogeneous route the silylating agent, 3-chloropropyltriethoxysilane, was initially immobilized onto the mesoporous silica surface to give the chlorinated mesoporous silica Cl-SBA-15 or Cl-MCM-41. In a second reaction a multifunctionalized N,S donor compound (2-mercaptobenzothiazol) was incorporated to obtain the functionalized silicas denoted as MBT-SBA-15-Het and MBT-MCM-41-Het. Using the homogeneous route, the functionalization was achieved via the one step reaction of the mesoporous silica with an organic ligand containing the chelating functions, to give the modified mesoporous silicas denoted as MBT-SBA-15-Hom or MBT-MCM-41-Hom. The functionalized mesoporous silicas were employed as adsorbents for the regeneration of aqueous solutions at pH 6 contaminated with Hg(ii) at room temperature. Results obtained indicate that mercury adsorption was higher in the mesoporus silicas prepared by the homogeneous method, and the maximum adsorption value (0.24 +/- 0.02 mmol Hg(ii) g(-1)) was obtained for MBT-SBA-15-Hom. The chemically stability in acid medium of the functionalized silicas, possibility its regeneration washing with concentrate HCl, resulting in the reuse of the adsorbent material for several cycles.

UV‐Photocrosslinking of Inulin Derivatives to Produce Hydrogels for Drug Delivery Application

In this work, INU, a natural polysaccharide, has been chemically modified in order to obtain new photocrosslinkable derivatives. To reach this goal, INU has been derivatized with MA thus obtaining four samples (INU-MA derivatives) as a function of the temperature and time of reaction. An aqueous solution of the derivative INU-MA1 was irradiated by using a UV lamp with an emission range from 250 to 364 nm and without using photoinitiators. The obtained hydrogel showed a remarkable water affinity but it underwent a partial degradation in simulated gastric fluid. To overcome this drawback, INU-MA1 was derivatized with SA thus obtaining the INU-MA1-SA derivative designed to produce a hydrogel showing a low swelling and an increased chemical stability in acidic medium. Ibuprofen, as a model drug, was loaded by soaking into INU-MA1 and INU-MA1-SA hydrogels and its release from these matrices was evaluated in simulated gastrointestinal fluids. INU-MA1 hydrogel showed the ability to quickly release the entrapped drug thus indicating its potential as a matrix for an oral formulation. INU-MA1-SA hydrogel showed a pH-responsive drug delivery. Therefore it is a promising candidate for controlled drug release in the intestinal tract.

High temperature fast chromatography of proteins using a silica-based stationary phase with greatly enhanced low pH stability

Reversed-phase liquid chromatography (RPLC) is very widely used for the separation and characterization of proteins and peptides. A novel type of highly stable silica-based stationary phase has been developed for protein separations. A dense monolayer of dimethyl-(chloromethyl)phenylethyl)- chlorosilane (DM-CMPES) on the surface of silica is “hyper-crosslinked” with a polyfunctional aromatic crosslinker through Friedel-Crafts chemistry resulting in stationary phases with extraordinary stability in acidic media. Elemental analysis data confirm the high degree of cross-linking among the surface groups. The hyper-crosslinked phases are extremely stable under highly acidic mobile phase conditions even at a temperature as high as 150 °C. A wide-pore (300 Å) material made in this way is used here to separate proteins by a reversed-phase mechanism and compared to a commercially available “sterically protected” C 18 phase. For small molecules, including neutral and basic compounds, these crosslinked phases give comparable peak shape and efficiency to the commercial phase. Our results show that no pore blockage takes place as commonly afflicts polymer coated phases. In consequence, protein separations on the new phases are acceptable. Using strong ion-pairing reagents, such as HPF 6, improves the separation efficiency. Compared to the commercial phases, these new phases can be used at lower pHs and much higher temperatures thereby enabling much faster separations which is the primary focus of this work. Better efficiency for proteins was obtained at high temperature. However, at conventional linear velocities the instability of proteins at high temperature becomes a problem which establishes an upper temperature limit. Uses of a narrowbore column and high flow rates both solves this problem by reducing the time that proteins spend on the hot column and, of course, speeds up the separation of the protein mixture. Finally, an ultrafast gradient (<1 min) protein separation was obtained by utilizing the high temperature and thus high linear velocities afforded by the extreme stability of these new phases. The phases are stable even after 50 h of exposure to 0.1% TFA at 120 °C. This paper is dedicated to the memory of Csaba Horvath whose work in high temperature HPLC inspired the development of the stationary phases described here.

Synthesis and Properties of Phases in the System BaCO3–PbO–Nb2O5

The phase formation kinetics and phase relations in the BaCO3–PbO–Nb2O5 system are studied. Barium lead niobates are prepared by solid-state reactions, and the composition ranges of solid solutions with different structures are determined. All of the materials are examined by x-ray diffraction, and their stability in acid media is assessed. The electrical conductivity of the Pb-containing niobates is measured. The results are used to analyze property–structure–composition relations. Based on the phase-equilibrium data and the chemical stability and electrical properties of the niobates, the structure types and compositions of niobates are identified which are the most attractive for producing ion-selective materials.

Sulphate and molybdate ions uptake by chitin-based shrimp shells

The removal of sulphate and molybdate anions (among other anions) from mining liquid effluents is attracting much interest because of the strict environmental legislation world-wide and the need for water recycling and reuse. In this work, adsorption of sulphate and molybdate ions on chitin-based materials was investigated. Chitin flakes with various deacetylation degrees (DD) were produced from an industrial shrimp shell waste after demineralisation, deproteinisation and deacetylation steps, without further purification, immobilisation or grinding. Batch adsorption experiments were carried out as a function of pH and the best adsorbent material was selected following its chemical stability in acidic medium, degree of anions uptake and time needed for the deacetylation reaction stage. Thus, detailed sulphate adsorption studies were conducted with a chitin having a 25% DD, at various adsorbent concentrations, medium pH and other operating conditions. Best sulphate removal values (92%) were obtained at equilibrium pH 4.5, 8.5 mg mg −1 chitin/ions ratio and 15 min contact time. The adsorption data followed the Langmuir model and showed saturation values of the order of 3.2 mEq g −1. Chitin also proved to adsorb molybdate ions in the presence of sulphate ions, but reaction required longer equilibration time (60 min for the same 92% removal). Practical examples of removal of these anions were studied in actual mining effluents, attaining values of the order of 71% sulphate and 85% Mo from a Cu–Mo flotation mill effluent and 80% sulphate removal from a coal AMD––acid mines drainage (Mo free). The regeneration of the adsorbent material was possible through the anions desorption in alkaline medium. All results are discussed in terms of solution and interfacial phenomena and the practical aspects of the process, in the mining and metallurgy fields, are envisaged.

Pyranmycins, a novel class of aminoglycosides with improved acid stability: the SAR of D-pyranoses on ring III of pyranmycin.

[reaction: see text] The synthesis of a novel class of aminoglycosides, pyranmycins, is reported along with the structure activity relationship (SAR) of their antibacterial activity against Escherichia coli. Two pyranmycins show prominent activity (9 microM). Pyranmycins also manifest superior stability in acidic media. The SAR information will lead to the future designs of pyranmycin against drug resistant bacteria.

Synthesis of the first amine–cyanocarboxyboranes, isoelectronic analogues of α-cyanocarboxylic acids

The first amine–cyanocarboxyboranes and their esters are synthesized from trimethylamine–bromo(methoxycarbonyl)boranes with tetrabutylammonium cyanide, followed by amine exchange reactions and acidic hydrolysis; pKa values of “boracyanocarboxylic acids” are determined and their stability in acidic medium examined.

Directed Mutagenesis Studies of the Metal Binding Site at the Subunit Interface of Escherichia coli Inorganic Pyrophosphatase

Recent crystallographic studies on Escherichia coli inorganic pyrophosphatase (E-PPase) have identified three Mg2+ ions/enzyme hexamer in water-filled cavities formed by Asn24, Ala25, and Asp26 at the trimer-trimer interface (Kankare, J., Salminen, T., Lahti, R., Cooperman, B., Baykov, A. A., and Goldman, A. (1996) Biochemistry 35, 4670-4677). Here we show that D26S and D26N substitutions decrease the stoichiometry of tight Mg2+ binding to E-PPase by approximately 0.5 mol/mol monomer and increase hexamer stability in acidic medium. Mg2+ markedly decelerates the dissociation of enzyme hexamer into trimers at pH 5.0 and accelerates hexamer formation from trimers at pH 7.2 with wild type E-PPase and the N24D variant, in contrast to the D26S and D26N variants, when little or no effect is seen. The catalytic parameters describing the dependences of enzyme activity on substrate and Mg2+ concentrations are of the same magnitude for wild type E-PPase and the three variants. The affinity of the intertrimer site for Mg2+ at pH 7.2 is intermediate between those of two Mg2+ binding sites found in the E-PPase active site. It is concluded that the metal ion binding site found at the trimer-trimer interface of E-PPase is a high affinity site whose occupancy by Mg2+ greatly stabilizes the enzyme hexamer but has little effect on catalysis.