Presence Of Silica Particles Research Articles

Synthesis of Hybrid Colloids Through the Growth of Polystyrene Latex Particles onto Methacryloxy methyl triethoxysilane - Functionalized Silica Particles

AbstractOne area of particular effort recently is the use of colloidal particles as precursors in engineering new materials. Nevertheless, these particles are nearly always spheres. This places limitations on the structures that can be built, especially in making photonic-bandgap materials. Therefore, it is a great challenge to create new colloids with an original shape, i.e. different from the sphere, in a controllable manner.We present here an original approach to create hybrid organic-inorganic colloidal particles with a perfect controlled shape. The synthetic route of these structures, which are composed of spherical silica spheres surrounded by a varying number of polystyrene beads, consists in the emulsion polymerization of styrene in presence of silica particles, which had been surface-modified by a coupling agent containing polymerizable groups.The influence of the size of the silica particle and of the nature of the coupling agent on the resulting colloidal particles morphologies was carefully analyzed. We show in particular that the number of growing polystyrene beads varies homogeneously with the diameter of the mineral spheres. A main advantage of this technique is also that we can precisely control the reaction time, allowing us to tune the final morphology of the hybrid structures. These colloidal assemblies are original building blocks for the elaboration of new functional materials.

Adsorption of concanavalin A on the well-characterized macroporous chitosan and chitin membranes

Macroporous chitosan membranes were prepared by solvent evaporation method in the presence of silica particles which were used as a porogen. These membranes were cross-linked under alkaline conditions, using epichlorohydrin as the cross-linking agent. The macroporous chitin membranes were obtained by acetylating the chitosan membranes with acetic anhydride in methanol. SEM photographs showed a homogeneous pore distribution throughout the membranes with 16.1–17.9 μm average pore sizes. The adsorption and desorption properties of concanavalin A (Con A) on the membranes were investigated by fluorescence spectroscopy. The equilibrium adsorption time was found to be 2 h during the static adsorption experiments. The calculated adsorption rates for chitosan and chitin membranes were r ad=0.613 and 0.541 mg Con A/g dry membrane-minute, respectively at the initial protein concentration of 1.0 mg ml −1. The adsorption isotherms represented a monolayer of Con A at 0.5–2.0 mg ml −1 concentration range. The adsorption studies which were carried out at flow rate of 2 ml min −1 showed that adsorbed quantities of Con A decreased markedly in the presence of flow, and they were similar (∼14–15 mg Con A/g dry membrane) for both chitosan and chitin membranes. The fluorescence studies verified the absence of denaturation in the recovered Con A.

Preparation and characterization of crosslinked PVA/SiO 2 hybrid membranes containing sulfonic acid groups for direct methanol fuel cell applications

Organic–inorganic hybrids based on poly(vinyl alcohol) (PVA)/SiO 2 hybrid membranes containing sulfonic acid groups were prepared using the sol–gel process under acidic conditions. The PVA/sulfosuccinic acid (SSA)/silica hybrid membranes were fabricated from different SSA contents. The proton conductivity and methanol permeability of the hybrid membranes were studied with changing SSA content from 5 to 25 wt.%. It was found that the proton conductivity and the methanol permeability were dependent on the SSA content both as a crosslinking agent, and as a donor of the hydrophilic SO 3H group. Up to an SSA content of about 20 wt.%, both of these properties decrease, and above this SSA content, they begin to increase with increasing SSA content. The proton conductivities of the PVA/SSA/silica membranes were in the range of 10 −3 to 10 −2 S/cm, and the methanol permeabilities ranged between 10 −8 and 10 −7 cm 2/s. The presence of silica particles in the organic polymer matrix, which reduce the free water ratio of the membranes, results in hybrids with markedly reduced methanol permeabilities. These characteristics of the PVA/SSA/silica hybrid membranes are desirable for future applications related to direct methanol fuel cells.

Myelin growth and initial dynamics

During the dissolution of solid surfactants in water, various types of nonequilibrium microstructures have been observed. The most important one is the myelin growth which can be observed when some poorly water soluble surfactants such as phosphatidylcholine (PC), Aerosol-OT (AOT), etc. are contacted with water. In this study initial myelin growth for a period of 2–4 s is studied both qualitatively as well as quantitatively in all the directions with respect to water flow in a PC system using digital video microscopy. Further, overall myelin growth is studied by means of optical microscopy to understand the effect of distance between cover slip and glass slide on myelin growth. Experiments are also performed to study effect of additives (silica) to lamellar phase on diffusion coefficients. It has been observed that the presence of silica particles causes extensive coiling of myelin structures. The mechanism of water transport into the lamellar phase during myelin growth is investigated by using silica in a colloidal range as dopant material.

Transformation superplasticity of water ice and ice containing SiO2 particulates

Transformation superplasticity is a deformation mechanism that occurs during the phase transformation of an externally stressed material. Pressure‐induced transformation superplasticity, which has long been postulated to occur in olivine in the Earth's interior, is shown to take place in water ice. Pure ice specimens were subjected to hydrostatic pressure cycling between 0 and 300 MPa to reversibly induce the I/II transformation (at 220 or 230 K) or the I/III transformation (at 240 K). When a small uniaxial compressive stress was applied during cycling, the specimens exhibited a uniaxial compressive strain (as large as 22% after a single cycle) proportional to the applied stress, in agreement with observations and theory for transformation superplasticity of metals and ceramics, induced by polymorphic thermal cycling. Additionally, specimens of ice containing 10 vol% SiO2 were deformed by this mechanism during the I/II transformation at 230 K. The presence of silica particles was found to enhance the deformation, rather than strengthen the ice. These experimental results are discussed both qualitatively and quantitatively in terms of existing models of transformation superplasticity and implications for the lithosphere rheology of the icy moons of the outer planets.

RHEOLOGICAL AND DIELECTRIC CHARACTERIZATION OF ELECTRORHEOLOGICAL FLUIDS COMPOSED OF BOTH DISPERSED PARTICLES AND LIQUID DROPS IN A DIELECTRIC MEDIUM

The rheological and dielectric behavior of novel electrorheological suspensions composed of noncolloidal silica gel particles of irregular shape and oil-in-oil emulsions with different electrical conductivity (and permittivity) are studied in this work. The rheological behavior of the suspensions is examined in systems with various compositions of the oil phase under different intensities of constant DC electric fields. Results reveal that the electrorheological (ER) response of the oil-in-oil emulsion increases with the applied voltage but is affected negatively when the concentration of the oil with higher conductivity is increased. When a small volume fraction of silica gel is added, the ER response increases considerably and its magnitude is strongly dependent on the electric field. In this case, however, larger concentrations of the oil with the higher conductivity, in the presence of silica particles, increase the relative viscosity of the suspensions. The dielectric properties of the suspensions are examined to elucidate the relation between the dielectric relaxation process and the relative composition of liquid drop phases. The mechanisms responsible of the ER response are explained in terms of the relationship between the microstructure changes in the suspensions and their rheological and dielectric properties.

Selective liquid-phase semihydrogenation of functionalized acetylenes and propargylic alcohols with silica-supported bimetallic palladium-copper catalysts.

Silica-supported, bimetallic palladium-copper catalysts were prepared in solution under mild conditions by reacting lithium di(4-tolyl)cuprate with palladium acetate in the presence of silica particles. Small bimetallic palladium-copper particles were deposited on the silica surface as confirmed with TEM-EDAX and EXAFS. The new material has been applied as catalyst in the liquid-phase semihydrogenation of mono- and disubstituted alkynes and showed high selectivity toward the cis-alkenes. The influence of addition of quinoline or potassium hydroxide to the semihydrogenation reaction mixture and the effects of exposure of the catalyst to air before use have been investigated.

Interfacial Interaction in Polyimide/Silica Hybrid Composites by Fluorescence Spectroscopy

In this paper, we report that the interfacial interaction in polyimide (PI)/silica hybrid composites can be characterized by a fluorescence spectroscopy. We found that silica shows different effects on the fluorescence behavior of polyimide based on biphenyltetracarboximide (BPDA) depending on the type of diamine, that is, whether it is oxydiphenyldiamine (ODA) or phenylenediamine (PDA). The polyimide/silica hybrid composite films were prepared by thermal imidization and sol−gel method from tetraethoxy silane (TEOS). For the polyamic acid (PAA)/TEOS solutions, the interfacial interaction was discussed in terms of the intermolecular hydrogen bonding between PAA and Si(OH)4 or complexation between a solvent, N-methylpyrrolidinone (NMP), and PAA. Significant changes in the fluorescence wavelength as well as intensity were observed by the presence of silica particles in the case of BPDA−PDA/silica composite films as compared to the case of BPDA−ODA/silica composite films. We found that silica has some interaction with BPDA−PDA but no interaction with BPDA−ODA in PI films. The interfacial interaction between silica and BPDA−PDA or BPDA−ODA was explained by different electron-donating strength of the diamines, here, PDA and ODA.

Electrorheological properties of poly( o-anisidine) and poly( o-anisidine)-coated silica suspensions

Poly( o-anisidine) (PANs) and PANs-coated silica particles were prepared by oxidative polymerization of o-anisidine using ammonium persulphate in the absence and presence of silica particles, respectively. The entire surface of the silica particles was completely covered with PANs polymerized using low concentration of o-anisidine. The effects on the electrorheological behavior of surface modification of the silica particles coated with PANs were studied under oscillatory shear. Electrorheological properties of PANs and PANs-coated silica particle suspensions were found to exhibit higher performance in nonaqueous solutions than those of silica particle suspensions even at high temperatures.

Adsorption Fractionation Studies by Size-Resolved Pulsed-Field Gradient Spin-Echo-NMR

In this paper the ability of pulsed-gradient spin-echo nuclear magnetic resonance (PGSE-NMR) to study adsorption fractionation is demonstrated. An inverse Laplace transform applied to the attenuation function yields a representation of the distribution of self-diffusion coefficients. The self-diffusion coefficient distributions of two model systems consisting of bimodal poly(ethylene oxide) (PEO) solutions have been examined in the absence and in the presence of silica particles. The results show that high molecular weight species are preferentially adsorbed over smaller ones as predicted by theory confirming the feasibility of the PGSE-NMR technique to study such distributions.

Silica-poly(dimethylsiloxane) mixtures: n.m.r. approach to the crystallization of adsorbed chains

This work deals with the crystallization of polymer chains within silica-poly(dimethylsiloxane) mixtures, irrespective of the type of crystalline structure that is actually present in the system. An analysis of the transverse magnetization relaxation function is proposed as a method for melting point determination as well as for determination of the extent of crystallization as a function of time during an isothermal crystallization. We first pay attention to pure poly(dimethylsiloxane) (PDMS), for which we have measured the enthalpy of fusion per mole of structural units and defined the isothermal crystallization kinetics. The results obtained for the mixtures are compared to those obtained for pure PDMS. It turns out that the presence of silica particles does not affect the dimensionality of the crystalline growth. However, it reduces the extent of crystallization and has two antagonistic effects on the overall rate of crystallization. The first is a nucleation effect which is dominant at low silica volume fractions, and the second effect is a topological constraint which becomes dominant and hinders the crystalline growth at high silica volume fractions. As a result of the competition between these two effects, the overall rate of crystallization goes through a maximum with increasing silica volume fraction.

Electrorheological property of a polyaniline-coated silica suspension

Silica particles, whose surfaces were completely covered with polyaniline, were prepared by oxidative polymerization of aniline with ammonium persulphate in the presence of silica particles. The electrorheological property of a suspension of polyaniline-coated silica particles dispersed in silicone oil was found to exhibit a high performance even at high temperatures.

Correlation between froth viscosity and flotation efficiency

The amount of hydrophobic materials constituting the mineral froth strongly influences the froth viscosity. Higher froth viscosity, on the other hand, can retard the drainage of physically carried over, hydrophobic particles. In phosphate flotation tests, a direct correlation was observed between froth viscosity and recovery, and an inverse correlation existed between froth viscosity and grade. Phosphate flotation tests in the presence of a silica depressant indicated that higher froth viscosity is an effect rather than a cause for the presence of silica particles in the froth.

Peritoneal Dialysis in the Workplace

Pulmonary alveolar proteinosis developed in a 29-year-old white man within 2 years of working as a cement truck driver. Pulmonary alveolar proteinosis (PAP), an uncommon respiratory disorder characterized by the accumulation of phospholipid material within the alveoli, has been described in association with exposure to silica, aluminum oxide, and a variety of dusts and fumes. Although a link between exposure to Portland cement and PAP has not been previously noted, this type of cement contains nearly 20% silica. Lung biopsy material, originally used to diagnose PAP, was reviewed under electron dispersive spectroscopy. Analysis indicated the presence of silica particles within the alveolar fluid and macrophages. A number of items support a causal relationship between exposure to cement dust and PAP: (1) the temporal sequence between assuming job duties and the development of the illness, (2) improvement following removal from further exposure, (3) dusty, unprotected working conditions, (4) the presence of silica within the cement, and (5) the alveolar fluid from periodic acid-Schiff-positive lung tissue.

Is silica involved in neuritic (senile) plaque formation?

The agent responsible for inducing neuritic (senile) plaque formation in senile dementia of the Alzheimer's type and in the ageing non-demented brain is unknown. Other workers have detected a high concentration of silicon in the rims and cores of senile neuritic plaques. We have therefore looked at whether the reaction of brain tissue to silica particles resembles a neuritic plaque. In this study both fine (10 nm) and coarse (less than 5 microns) particles of silica have been introduced into the brains of rats and mice using a wide range of doses and several methods of administration. The reaction of the brain to the presence of the silica was examined by light and electronmicroscopy up to one year after the injection. The presence of silica particles in the brain resulted in the proliferation of fibrous astrocytes and macrophages and strongly stimulated the production of collagen fibres. Degeneration of some adjacent axons and axon terminals occurred, but there was no detectable deposition of amyloid which is characteristic of senile plaques. Coarse particles of silica invariably produced a more intense reaction than fine particles. The reaction of the brain did not diminish with time within one year of injection. The possible significance of the presence of silica in the plaque as a secondary phenomenon is discussed.