Transmission electron microscopy observations of composite polymer colloid nucleated by functionalized silica

In this work, functionalized nanometric silica particles were engaged in emulsion polymerization of ethyl acrylate. The morphological characterization of this composite material was performed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). We were particularly interested in the state of encapsulation of the silica particles and their distribution in the latex film. Initialy, we successively studied both components of the composite: polymer beads and silica particles differ by their size and also by their contrast. In addition, it was possible to perfrom dark field TEM to study this system of two amorphous phases because their respective amorphous halos are not too close. Hence, we investigated the colloid material in aqueous media and after film formation. Although no ideal encapsulation is observed in the colloid in aqueous media, the distribution of silica in the latex film is good. SAXS results are in good agreement with TEM observations.     

Influence of silica fillers during the electron irradiation of DGEBA/TETA epoxy resins, part III: Solid-state NMR investigations

The influence of silica fillers on chemical modifications of diglycidyl ether of bisphenol A/triethylene tetramine (DGEBA/TETA) epoxy resins induced by electron beam irradiation has been studied by ¹³C CP-MAS (Cross Polarisation and Magic Angle Spinning) NMR. Four kinds of silica filler were investigated: a pure micrometric silica, a treated micrometric silica, a pure nanometric silica and a treated nanometric silica. On the unirradiated epoxy resins, the magnetization transfer curves reveal structural differences due to the kind of silica fillers. A decrease of the epoxy network rigidity in the presence of nanometric silica fillers is shown. During irradiation, the formation of phenolic ends and enamine functions is confirmed. The slowing of the magnetization transfer of the pure and treated micrometric silica filled epoxy resin reveals an important decrease of the rigidity of these resins. On the pure and treated nanometric silica filled epoxy resins, reactions of the reactive species created by the irradiation in the epoxy resin and the silica particles surface are shown.     

全氟辛酸铵与全氟壬酸铵混合物的表面活性

测定了全氟辛酸铵和全氟壬酸铵及其不同比例混合物的0.1mol/L氯化铵水溶液表面张力曲线。讨论了它们的胶团化作用、吸附作用和降低水表面张力的能力。改进了Ingram-Luckhurst自单一表面活性剂活性张力曲线得到混合溶液表面张力曲线的方法。     

Novel Method of Synthesis of Quaternary Ammonium Tribromides and Investigation of Catalytic Role of Benzyltrimethylammonium Tribromide in Oxidation of Alcohols to Carbonyl Compounds

Stable crystalline organic quaternary ammonium tribromides (QATBs) have been easily synthesized by the oxidation of the corresponding organic ammonium bromides (QABs) with ammonium persulfate. The reactions have been performed under solvent-free conditions in the presence of sulfuric acid and silica as supporting agent. Two equivalents of potassium bromide have been used as the source of additional bromides for quantitative conversion of QABs to QATBs. Ammonium persulfate, a cheap and readily available oxidant, carries out the bromide oxidation to tribromide very effectively under solvent-free conditions. The synthesized QATBs have been shown to catalyze the oxidation of alcohols to carbonyl compounds with hydrogen peroxide as oxidant in good yields under mild reaction conditions.     

Surface treatment and porosity control of porous silica microspheres

Summary Porous silica microspheres (PSM) have been treated with ammonium bifluoride to adjust porosity, pore size, remove surface impurities, and minimize surface acidity. The porosities of four silicas having mean pore diameters ranging from 150 to 750 ? have been altered from initial values to the point at which the mechanical strength is insufficient to allow packed columns with acceptable performance. It is shown that a linear relationship exists between a change in porosity and the relative amount of ammonium bifluoride used to treat the silica. This reagent removes silica homogeneously from all pores in a given microsphere in a controllable and predictable manner. This treatment increases the peak capacity and improves chromatographic performance. The surfaces of treated silicas were probed with thiamine in the ion-exchange chromatographic mode. The slopes and intercepts of plots in which retention is plotted against the reciprocal of buffer concentration were both significantly reduced indicating that surface acidity is minimized by this treatment.     

Study of fluoride treatment of silica-containing raw material

An effect of ammonium hydrofluoride treatment of silica-containing raw material was examined. We described thermodynamics and kinetics of caking raw materials and their crushed materials, ammonium hexafluorosilicate sublimation, and amorphous silica formation. Constants of rate and activation energy of the chemical reactions were calculated.     

Effect of increasing concentration of ammonium acetate as an additive in supercritical fluid chromatography using CO2–methanol mobile phase

The effects of increasing concentrations of ammonium acetate additive in supercritical fluid chromatography were studied on silica, 2-ethyl-pyridine and endcapped 2-ethyl-pyridine stationary phases. The study involved the addition of increasing concentrations of the ammonium acetate either in the mobile phase modifier (methanol) or in the sample solvent. The effects of ammonium acetate on retention and peak shape of the analytes were evaluated. Compounds that exhibited satisfactory chromatographic behaviour in the absence of the additive were virtually unaffected by its presence in the mobile phase or sample solvent. Nevertheless, compounds that exhibited late elution and strongly tailing peak shapes when pure methanol was used showed dramatically improved chromatographic behaviour in the presence of the additive. Shorter retention was observed not only when the modifier was introduced in the mobile phase but also when it was in the sample solvent.     

Designed Precursor for the Controlled Synthesis of Highly Active Atomic and Sub‐nanometric Platinum Catalysts on Mesoporous Silica

The development of new methods to synthesize nanometric metal catalysts has always been an important and prerequisite step in advanced catalysis. Herein, we design a stable nitrogen ligated Pt complex for the straightforward synthesis by carbonization of uniformly sized atomic and sub‐nanometric Pt catalysts supported on mesoporous silica. During the carbonization of the Pt precursor into active Pt species, the nitrogen‐containing ligand directed the decomposition in a controlled fashion to maintain uniform sizes of the Pt species. The nitrogen ligand had a key role to stabilize the single Pt atoms on a weak anchoring support like silica. The Pt catalysts exhibited remarkable activities in the hydrogenation of common organic functional groups with turnover frequencies higher than in previous studies. By a simple post‐synthetic treatment, we could selectively remove the Pt nanoparticles to obtain a mixture of single atoms and nanoclusters, extending the applicability of the present method.     

Preparative separation of isoflavone components in soybeans using high-speed counter-current chromatography

The analysis of ionic micro-contamination is of growing importance in the disc drive industry. Through the use of ion chromatography, cleanliness of drive components can be assessed. An objective to improve quantification of highly reactive inorganic ions that exist within the drive environment was implemented. This paper presents a new extraction technique used to determine low levels of ammonium, by microbore ion chromatography. Various chemical compounds within adhesive formulations can be a source of extractable ammonium. By combining this new extraction method with ion chromatography, the percentage of different chemical compounds within adhesive formulations was correlated to the level of extractable ammonium observed.     

Study of fluoride processing of kyanite concentrates

Specific features of the physicochemical processes occurring in the interaction of kyanite concentrates with ammonium hydrodifluoride, sublimation of ammonium hexafluorosilicate, recovery of alumina and aluminum fluoride, synthesis of amorphous silica particles, and reduction of additional reagents were studied. The thermodynamic parameters, rate constants, and activation energies of the occurring chemical reactions were determined. The optimal conditions of the fluoride processing of kyanite concentrates with integrated recovery of various useful components were found.     

Dependence of Colloidal Forces Between Bitumen and Silica on the Chain Lengths of Ammonium Surfactants

Effects of ammonium surfactants with different hydrocarbon chain lengths (C8, C12, C16, and C18) on the colloidal forces between bitumen and silica were studied by atomic force microscopy. The results showed that the chain length of the ammonium surfactants had a significant impact on both the long-range interaction and adhesion forces. With the addition of surfactants with relative short chains of C8 and C12 in the solutions, the long-range repulsive force decreased or even became strong attractive force, while it became repulsive again in solutions of surfactants with long chains of C16 and C18. It was further observed that addition of Ca²⁺ in various surfactants solutions would either depress or enhance the colloidal interactions based on the surfactant chain lengths. It was believed that variation of the interaction behaviors resulted from the mono-layer or bilayer adsorption of various surfactant molecules on the negatively charged surfaces of bitumen and silica, which affected the surface wettability and the surface charge characteristics and then greatly changed the colloidal interactions. The findings indicated that, to have a high bitumen recovery and good froth quality, the surfactant type and concentration of the di-valent metal ions in the oil sand processing slurry must be well considered.     

Adsorption of Cationic Surfactants on Highly Dispersed Silica

Comparative study was made of adsorption of cationic surfactants, decamethoxine, ethonium, and cetyltrimethylammonium bromide, on a highly dispersed amorphous silica at different pH values. Contrary to the adsorption of monoquaternary ammonium compound, cetyltrimethylammonium bromide, the adsorption of bis(quaternary ammonium) compounds, ethonium and decamethoxine, was found to be caused mainly by electrostatic interaction. The adsorption of antiseptic decamethoxine is reversible at pH 6; this may be used to design carriers capable of controlled drug release into the surrounding medium.     

The electrochemical determination of ammonium based on the selective inhibition of the low-spin iron(II)/(III) system of Prussian blue

A new method is described for the determination of ammonium in aqueous solutions with electrodes modified by Prussian blue (PB). The specific voltammetric response of PB-modified electrodes to ammonium ions is used for their analytical determination. In the presence of ammonium ions, a concentration-dependent inhibition of the low-spin iron(II/III) system of PB occurs. Only thallium and rubidium ions cause similar inhibition. A useful electrochemical determination method is thus available for detecting ammonium ions in the presence of frequently interfering potassium and sodium ions. Paraffin-impregnated graphite electrodes modified with a mechanically transferred PB layer and bulk-modified PB-composite electrodes are studied. The method is applicable within a concentration range which extends from 4 × 10⁻⁵ mol/l to 10⁻² mol/l NH₄ ⁺. The composite electrode is used in an electrochemical flow-through system in conjunction with the Kjeldahl method. Received: 21 April 1997 / Accepted: 28 May 1997     

Computer modeling of amorphous silica structures

An algorithm for large cluster construction for quantum-chemical modeling of amorphous silica structures is suggested. The algorithm is based on the conception that different species of disperse amorphous silica, namely aerosil, silica gel and aerogel are different polymorphic forms; that these polymorphic modifications can be classified by Libau's crystallochemical classification of silicates; that each classification mode is determined by the manufacturing method. The structure models presented are obtained in accordance with the algorithm and are in line with the chemical reactions forming the basis of commercial manufacturing of the products.     

Water-soluble silica-overcoated CdS:Mn/ZnS semiconductor quantum dots

Highly luminescent and photostable CdS:Mn/ZnS core/shell quantum dots are not water soluble because of their hydrophobicity. To create water-soluble quantum dots by an appropriate surface functionalization, CdS:Mn/ZnS quantum dots synthesized in a water-in-oil (W/O) microemulsion system (reverse micelles) were consecutively overcoated with a very thin silica layer ( approximately 2.5 nm thick) within the same reverse micellar system. The water droplet serves as a nanosized reactor for the controlled hydrolysis and condensation of a silica precursor, tetraethyl orthosilicate (TEOS), using an ammonium hydroxide (NH4OH) catalyst. Structural characterizations with transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) indicate that the silica-quantum dot nanocomposites consist of a layered structure. Owing to the amorphous, porous nature of a silica layer, the optical and photophysical properties of silica-overcoated CdS:Mn/ZnS quantum dots are found to remain close to those of uncoated counterparts.     

Preparation of boehmite—silica colloids: Rods, spheres, needles and gels

A method is presented for the synthesis of amorphous silica rods with a boehmite core. The addition of boehmite needles to a mixture of tetraethoxysilane (TES), ethanol and aqueous tetramethylammonium hydroxide solution produces gels or aggregates. However, when boehmite needles are previously coated with silica in an aqueous sodium silicate solution, they serve in the TES solution as nuclei for the polymerisation of part of the hydrolysed TES into discrete, amorphous silica rods. The rods tend to stick together probably because of the increase in ionic strength caused by the TES hydrolysis. In the absence of boehmite nuclei, silica only forms discrete, non-aggregated spheres, which demonstrates that the Stöber silica sphere synthesis can be performed with a base other than ammonia.     

HPLC of aminoacids on silica coated with ammonium tungstophosphate

Summary The chromatographic behaviour of aminoacids on columns of silica coated with different percentages of ammonium tungstophosphate at several buffer phosphate concentrations is reported. The stationary phase shows a high selectivity towards aminobutyric acids and methyl derivatives of tryptophan due to the distance between the carboxylic group and the –NH ₃ ⁺ group involved in the cation-exchange process and/or to the steric hindrance by a methyl substituent. A relationship between HPLC and HPTLC data has been verified and applied to the prediction of the separation of six aminoacids.     

Ammonium Determination in Water Samples by Using Opa-Nac Reagent: A Comparative Study with Nessler and Ammonium Selective Electrode Methods

A selective and sensitive method based on the ammonium derivatisation with o -phthaldialdehyde (OPA) and N -acetyl-cysteine (NAC) has been developed for ammonium determination in real water samples. The proposed procedure has been compared with ammonium reference methods such as Nessler reagent method and ammonium selective electrode. All procedures have been chemometrically tested and compared in terms of the main analytical properties. These procedures have been used to determine ammonium in unknown water samples. The OPA-NAC reagent method does not present any systematic error (proportional or constant), while Nessler reagent presents both of them for some samples assayed. The ammonium selective electrode is free of corrigible systematic errors, however presents amine interference. The OPA-NAC ammonium method is able to achieve a detection limit (LOD) of 0.07 mg/L in the sample, with a linear dynamic range up to 1.4 mg/L of ammonium.     

亚硫酸根离子在硅和二氧化硅的湿法腐蚀中的作用

运用光刻技术和原子力显微技术(AFM)研究了亚硫酸根离子对硅和二氧化硅在40％(w)氟化铵水溶液中腐蚀速率的影响.结果表明硅和二氧化硅的腐蚀速率和亚硫酸根离子浓度有关.高分辨X射线光电子能谱(XPS)分析在有/没有亚硫酸根的溶液中腐蚀后的硅和二氧化硅表面氟元素的结果表明在这两种溶液中腐蚀得到的表面化学成分是有差别的.实验结果证明亚硫酸根离子在硅和二氧化硅的湿腐蚀中不只是表现为一种除氧剂，还干预了表面腐蚀反应过程.     

Sequential flow injection analysis of ammonium and nitrate using gas phase molecular absorption spectrometry

A flow injection method on the basis of gas phase molecular absorption is described for the sequential determination of ammonium and nitrate. Two hundred microliters of sample solution is injected into the flow line. For ammonium determination, the sample zone is directed to a line in which reacts with NaOH (13 M) and produces ammonia. But for nitrate determination, the sample zone is passed through the on-line copperized zinc (Zn/Cu) reduction column and produces ammonium ion and in the follows ammonia. The produced ammonia in both cases is purged into the stream of N₂ carrier gas. The gaseous phase is separated from the liquid phase using a gas-liquid separator and then is swept into a flow through cell, which has been positioned in the cell compartment of an UV-Vis spectrophotometer. The absorbance of the gaseous phase is measured at 194 nm. Under selected conditions for sequential analysis of ammonium and nitrate, linear relations were found between the peak heights of absorption signals and concentrations of ammonium (10-650 μg ml⁻¹) and nitrate (20-800 μg ml⁻¹). The limit of detections for ammonium and nitrate analysis were 8 and 10 μg ml⁻¹, respectively. The relative standard deviations of repeated measurements of 50 μg ml⁻¹ of ammonium and nitrate were 2.0, 2.9%, respectively. Maximum sampling rate was about 40 samples/h. The method was applied to the determination of ammonium in pharmaceutical products and the sequential determination of ammonium and nitrate in spiked water samples.