Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “SILICA”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 937 records · Page 52Linked to original sources

Silica-supported biomimetic membranes.

The hybridization of lipid membranes with inorganic silica-based framework results in mechanically stable biomembrane mimics. This account describes three types of silica-based biomimetic membranes. As the first example, a Langmuir monolayer of dialkylalkoxysilane was polymerized and immobilized onto a porous glass plate. Permeability through the monolayer-immobilized glass was regulated by phase transition of the immobilized monolayer. In the second example, spherical vesicles covalently attached to a silica cover layer (Cerasome) were prepared. The Cerasome was stable enough to be assembled into layer-by-layer films without destruction of its vesicular structure. This material could be an example of the multicellular assembly. Mesoporous silica films densely filling peptide assemblies (Proteosilica) are introduced as the third example. The Proteosilica was synthesized as a transparent film through template sol-gel reaction using amphiphilic peptides.

Biomimetic Materials↗

Dependence and reversal of nitric oxide production on NF-kappa B in silica and lipopolysaccharide-induced macrophages.

In this report the differential regulation of NF-kappa B and nitric oxide (NO) was investigated in the mouse macrophage cell line RAW 264.7 following exposure to a mineral dust (silica) and/or an endotoxin (Lipopolysaccharide, LPS). The results indicated that silica and LPS can significantly induce the activation of NF-kappa B as well as elicit enhanced production of NO in RAW 264.7 cells as part of an early inflammatory response mechanism. A 24-hour time-course study showed that NO release from these cells continued to increase following the initial stimulus by LPS or silica. In contrast, activation of NF-kappa B was maximal at 6 hours and then showed a steady decline to 24 hours. The production of NO was suppressed by protease inhibitor and antioxidant, both of which block the activation of NF-kappa B. Surprisingly, the use of an NO synthase inhibitor resulted in an enhancement of NF-kappa B activation. These findings suggest that NO produced in macrophage cells in response to an inflammatory stimulus like silica or LPS my be linked to a negative feedback role on the activation of NF-kappa B.

Acetylcysteine↗

Essential role of NF-kappa B activation in silica-induced inflammatory mediator production in macrophages.

In this report, we demonstrate that NF-kappa B, a ubiquitous transcription factor, plays an essential role in silica-induced inflammatory mediator production in the mouse macrophage cell line RAW 264.7. Compared to the effect of lipopolysaccharide (LPS), silica mediated a stronger activation of NF-kappa B p50/p50 homodimer at early phase of poststimulation. Furthermore, activation of NF-kappa B by silica and LPS appears to be mediated by different signal transduction pathways. Both silica and LPS increased mRNA expression in these cells for cyclooxygenase II, inducible nitric oxide synthase, tumor necrosis factor-alpha and interleukin-1 alpha. This expression was attenuated along with the inhibition of NF-kappa B activation.

Animals↗

Oxidative DNA damage induced by silica in vivo.

Occupational exposure to silica has often been associated with the development of pulmonary fibrosis and, occasionally, lung cancer. Their development may be mediated by oxidant-induced cellular injury. The short- and long-term effects of a single intratracheal instillation of silica in rats (10 mg/200 microliters/saline per rat) was assessed by measuring 8-hydroxy-2'-deoxyguanosine (oh8dG) levels in lung tissue and peripheral blood leukocytes. Cell differentials, reduced glutathione (GSH), and superoxide dismutase (SOD), lipid peroxide, and total phospholipids in peripheral blood and/or bronchoalveolar lavage fluid (BALF) were also measured. The pulmonary oh8dG levels increased approximately 2.24- 2.86-fold from 1 to 5 days after exposure to silica. It was still elevated 1 and 4 weeks after installation, but the difference was no longer statistically significant. The oh8dG levels in peripheral blood leukocytes were never significantly different, but they were generally higher than in the controls. The low SOD levels in the BALF of exposed rats in the early stage and the higher GSH levels in the late stage may represent protective reactions against the generation of oxygen species. A significant increase in oh8dG levels in lung tissue suggested the possible carcinogenicity of silica.

8-Hydroxy-2'-Deoxyguanosine↗

Characterization of Silylated Silicas by Inverse Gas Chromatography: Modelization of the Poly(dimethylsiloxane) Monomer Unit/Surface Interactions Using Poly(dimethylsiloxane) Oligomers as Probes

Inverse gas chromatography (IGC) at infinite dilution was used to characterize the fumed silica surface. The silica was modified by chemical treatment with either dimethyldichlorosilane or trimethylchlorosilane. To observe the silica and poly(dimethylsiloxane) (PDMS) interactions, we used apolar (alkanes) and polar probes (chloroform, benzene, tetrahydrofuran, etc.), but also model molecules of PDMS such as linear and cyclic oligomers of PDMS with two to five monomer units. It is shown that PDMS oligomers are very appropriate to discriminate the silylated silicas.

Journal Article↗

X-Ray Absorption and Electron Paramagnetic Resonance Studies of Cu(II) Sorbed to Silica: Surface-Induced Precipitation at Low Surface Coverages

Both EPR and XAFS spectroscopy were used to characterize the sorption of Cu(II) on silica surfaces at extremely low surface coverage (0.8 and 9.9% monolayer). The binding structure of Cu(II) on silica surfaces revealed by XAFS experiment was compared to that predicted by EPR spectra. Our XAFS results show that Cu(II) hydroxide clusters form on silica surfaces even at surface coverage as low as 0.8% of a monolayer. However, we observed low EPR intensity and line-broadening which has been assigned to dipole-dipole interactions between isolated Cu(II) ions. The results of our XAFS studies indicate that the loss of EPR intensity and line-broadening can be traced to Cu(OH)2 clusters on silica surfaces.

Journal Article↗

Scanning Transmission X-Ray Microscopy: A New Method for the Investigation of Aggregation in Silica

During the preparation of silica by acidification of water glass, primary silica particles form extended and ramified aggregates. The growing aggregates form a gel, a tenuous network of interconnected aggregates. After aging and drying of the wet gel, porous silica is obtained. To study the extremely vulnerable aggregates only noninvasive methods are allowed. Moreover, because of the colloidal scale many methods based on (atomic or molecular scale) spectroscopy are not informative. Scanning transmission x-ray microscopy (STXM), using high-brilliance synchrotron radiation at 3.25 nm (380 eV) and 2.60 nm (480 eV) as an X-ray source, provides a new technique to obtain direct images of wet or solved aggregates at a 50-5000 nm scale. The 50 nm resolution is sufficient to provide excellent images of fractal structures. In this paper the principles of STXM are discussed in relation to investigations of wet gel systems like silica gel.

Journal Article↗

Analysis of Nitrogen Adsorption Isotherms for a Series of Porous Silicas with Uniform and Cylindrical Pores: A New Method of Calculating Pore Size Distribution of Pore Radius 1-2 nm

Two kinds of the porous silicas with uniform and cylindrical pores were prepared from calcined Na4SiO4 and cationic surfactants ([C16H33N(CH3)3]Cl and [C18H37N(CH3)3]Br). In this paper, we call these porous silicas PS-C16 and PS-C18. PS-C16 was calcined in vacuo at 873-1273 K in order to prepare a series of porous silicas having different pore sizes. All nitrogen adsorption isotherms for a series of the porous silicas show no hysteresis in the adsorption and desorption branches. The nitrogen adsorption isotherms were analyzed by the t method, and the pore surface area (Sp), the pore volume (Vp), and the core volume (Vc) were determined. The pore radius (rp), the core radius (rc), and the adsorbed thickness on the pore wall (tpore) were computed from the measured values of Sp, Vp, and Vc. We can determine the relationship between the Polanyi adsorption potential and the core radius in the range of rc = 0.6-1.2 nm. On the basis of the present data, we propose a new method of calculating the pore size distribution in the range of rp = 1-2 nm.

Journal Article↗

Effect of Acids on Water Vapor Uptake by Pyrogenic Silica

Effect of gaseous HCl and HNO3 on the water vapor uptake by pyrogenic silica was studied at different relative humidities (RH) for pure water and different compositions of binary and ternary vapor mixtures. Experiments showed that the ability of silica to uptake water strongly depends on RH and on the type of acids and their concentration in the vapor mixtures. At low acid concentration in the binary mixtures the influence of acids is probably small. Water uptake by silica does not change monotonically with acid concentration: at first it decreases and then starts to grow. However, the presence of acids promotes water uptake, and the effect is very significant at low RH. HCl seems to be more effective acid to enhance water uptake than HNO3 . In the case of ternary mixtures the adsorbed weight of water is a bit larger than that adsorbed from the binary mixtures. Acids are accumulated by silica surface, and the accumulation is larger for nitric acid.

Journal Article↗

Direct Measurement of Colloidal Forces between Mica and Silica in Aqueous Electrolyte

A refined methodology which allows attachment of submicrometer colloidal particles to atomic force microscopy cantilevers is described. The interactions between particles of this size and both mica and silica surfaces in aqueous electrolyte at pH 5.8 have been studied. The monotonic repulsive force observed was in all cases well fitted by assuming that the force was due to overlap of electrical double layers. The potential of a cleaved mica surface was found to be significantly less than that of the silica surface. However, the mica potential was found to increase, if the mica surface was immersed immediately (within seconds) in electrolyte after cleavage or after plasma cleaning. This behavior is explained by the finite aqueous solubility of adventitious carbon that spontaneously deposits on mica after cleavage in air. The surface potential of the silica spheres was obtained by fitting symmetric interaction data. The trend in zeta potential of the silica spheres with pH, determined by particle electrophoresis, was similar to that obtained from analysis of force-separation data.

Journal Article↗

Chemisorption on Thiol-Silicas: Divalent Cations as a Function of pH and Primary Amines on Thiol-Mercury Adsorbed

The silylating agent 3-trimethoxysilyl-1-propanethiol was immobilized on silica gel. Sulphur analysis showed 0.98 mmol of thiol groups per gram of support. These groups chemisorbed divalent cations: nickel, cobalt, copper, and mercury in the pH range from 1 to 6. The adsorption is more significant for mercury in relation to the other cations in the pH range studied. The primary amines, methylamine, ethylamine, n-butylamine, and benzylamine, were adsorbed on silica-thiol surface and on silica-thiol surface with mercury adsorbed. The values of adsorption constants were calculated using Langmuir isotherms data. Using these values, equilibrium constants and DeltaG values for each adsorption system were also obtained. When mercury is present on the silica surface, the adsorption of n-butylamine and benzylamine is significatively enhanced in relation to the other amines. This behavior suggests that the degree of adsorption of the studied amines is intimately related to their dissociation constants. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗

Interactions in Colloidal Mixtures: Partial Structure Factors in Mixtures of Colloidal Silica and an Anionic Oil-in-Water Microemulsion.

We have investigated the colloidal stability and interactions in a mixture of colloidal silica (LudoxTM) and an oil-in-water microemulsion (AOT/decane/brine). Stable silica-microemulsion mixtures containing large fractions of dispersed phase (phioil + phiLudox > 0.36) can be prepared. For fixed silica and AOT concentrations, the addition of decane increases the size of the microemulsion droplets. Since the surfactant concentration remains constant, this droplet growth is balanced by a decrease in the number density of droplets. Partial structure factors for the mixture are extracted from sets of small-angle neutron scattering (SANS) data using a solvent contrast variation technique. The extracted partial structure factors are compared qualitatively to model calculations for a multicomponent sticky hard sphere (SHS) fluid. A shallow contact potential (approximately 2.6 kT) between the Ludox particles is necessary to obtain agreement between model and experiment in the low-q region of the partial structure factors. The magnitude of this contact potential does not change with decane concentration, in agreement with the macroscopic observation that there is no observable change in the stability of these silica-microemulsion mixtures. Copyright 1998 Academic Press.

Journal Article↗

Aqueous Suspensions of Highly Disperse Silica and Germania/Silica.

Highly disperse germania has been synthesized on the surface of fumed silica by the chemical vapor deposition (CVD) technique. Aqueous suspensions both of unmodified fumed silica and of the obtained germania/silica (GS) samples have been studied by electrophoresis, photon correlation spectroscopy, nuclear magnetic resonance (1H NMR), and quantum chemical semiempirical AM1-SM1 methods. For suspensions of GS samples, electrophoretic mobility, isoelectric point (IEP), particle size distribution, average effective diameter (Def), and free energy changes (DeltaG) of interfacial water were found to depend nonlinearly on the concentration of germania (CGeO2). At small concentrations, the structure of the synthesized germania is clustered rather than layered. For the last samples, maximum reduction of the free energy for interfacial water and larger values of IEP and Def in comparison with GS at high CGeO2 or unmodified silica have been observed. In suspensions, the particle size distributions for silica and GS are uni-, bi-, and trimodal and depend on pH and CGeO2. Values of polydispersity (P) are very rarely smaller than 0.05 and are usually between 0.25 and 0.70, which points to the wide size distribution. The solvation energy of the charged and uncharged GS clusters calculated by AM1-SM1 method decreases with increasing germania concentration. Copyright 1998 Academic Press.

Journal Article↗

Surfactant-Mediated Transfer of Colloidal Silica Particles from Water into an Immiscible Weakly Polar Solvent.

Spontaneous transfer of discrete colloidal silica particles from water into 2,6-dimethyl-4-heptanone (DH) is obtained upon preliminary adsorption of dodecylbenzyldimethylammonium bromide (DBDAB) from the aqueous phase onto the solid. Partitioning of the surfactant between the silica surface and both liquid phases is evaluated through the determination of the isotherm in the aqueous phase, taking into account their mutual solubility. The organosol particles are found to be covered by a statistical monolayer of surfactant molecules presumably hydrated to the extent of 20% on a mole ratio basis. Monodispersion of this stable suspension is ascertained by dynamic light scattering. Evidence of DBDAB association in the monolayer emerges from the conductimetric behavior of the amphiphile in DH and from the reversal of the silica native charge in the organosol, as shown by electrophoretic mobility determinations. Disorganization of the water layer contacting silica upon adsorption of DBDAB is argumented to originate from the presence of sparsely distributed dodecyl chains. Copyright 1998 Academic Press.

Journal Article↗

A New Straightforward Approach to Generate Si-H Groups on Silica.

In this paper it is demonstrated unequivocally by solid-state nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopies that, at high temperature, hydrogen reacts directly with pyrogenic silica to form stable hydride Si-H groups. Such modified silicas should exhibit specific properties far from those of silicas having only surface silanol groups. The interest for hydride-modified silicas arises from the possibility of using their Si-H groups as reducing agents or as active sites for the synthesis of new chromatographic phases. Copyright 1999 Academic Press.

Journal Article↗

Relation between the Variation of the Contact Angle and the Adhesion Force between Silica Surfaces.

Adhesion force and contact angle measurements between an aqueous solution of the nonionic surfactant TN 101 and silica surface have been studied. The variation of the contact angle is a function of the adsorption of the surfactant at the liquid-vapor, solid-liquid, and solid-vapor interfaces. The contact angle is maximum well below the cmc, when a monolayer of surfactant is formed at the silica surface. To this maximum contact angle corresponds a maximum adhesion force between the silica surfaces. This force is due to the hydrophobic interaction between the alkyl chain of the surfactant adsorbed at the silica surface. Copyright 1999 Academic Press.

Journal Article↗

Rheological Evidence for the Silica-Mediated Gelation of Xanthan Gum.

In the present study, the conformational effects of the adsorbed polyelectrolyte xanthan gum on the stability of colloidal silica suspensions were investigated by monitoring the rheological responses. The conformational transition of xanthan molecules was induced by changes in pH, added salts, and annealing temperature. The suspension stability was probed by examining the rheological behavior such as shear viscosity versus shear rate and shear moduli versus oscillatory frequency. The oscillatory shear measurements provided useful information on the structural change in the suspension. The results showed that the silica particles dispersed in the xanthan solution remained stable at pH 9 for a long period, whereas at pH 2, the gel network structure formed within a few days. When the gel structure formed, the storage modulus was larger than the loss modulus over the entire frequency range considered here and exhibited a thixotropic behavior. The heat treatment temperature best for stabilizing the silica suspension was shown about 60 degrees C and the silica sol treated at this temperature sustained its stability for more than 6 weeks. Copyright 1999 Academic Press.

Journal Article↗

Aging Phenomena in Silica-Filled Polydimethylsiloxane.

The aging behavior of silica-filled polydimethylsiloxanes with molecular weights ranging from 9000 to 140,000 g/mol was examined for several filler surface treatments. A fumed silica with 130 g/m(2) surface area was used as received and after treatment with hexamethyldisiloxane and hexamethyldisilazane. The rheological properties, bound rubber, and state of dispersion of these systems were monitored as a function of time. The samples exhibited softening instead of crepe hardening which is typically reported for silica filled systems. These experiments indicate that polymer adsorption onto the surface plays an important role in determining the overall stability of these systems. Significant changes in the dispersion of filler were observed and these changes were dependent on polymer molecular weight and filler surface treatment. The addition of a surface treating agent, either physically adsorbed or covalently bound to the silica surface, that inhibits the adsorption of polymer was found to significantly increase the stability of the filler dispersion and therefore the rheological properties. An increase in the number of bridging chains during aging is responsible for the force which allows the filler migration to occur. Copyright 1999 Academic Press.

Journal Article↗