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 955 records · Page 53Linked to original sources
PubMed1999

Reactions of a Trifunctional Silane Coupling Agent in the Presence of Colloidal Silica Sols in Polar Media.

Reactions of 3-glycidoxypropyltrimethoxysilane (GPS) in a water-rich environment with silica colloids were studied. (29)Si NMR spectroscopy was used to monitor quantitatively the variation in GPS hydrolysis, condensation, and adsorption with pH and water concentration. The results show the competition between adsorption on the silica surface and condensation polymerization of GPS in the bulk liquid and are used to identify conditions favorable to surface modification. The presence of colloidal silica was shown to increase the apparent GPS dimerization rate, suggesting either surface catalysis or an increased local reactant concentration in the electrical double layer around the silica colloids. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Silica Coating on Colloidal Maghemite Particles.

Maghemite colloidal particles are coated with a silica layer using a silicon alkoxide as silica precursor. The coating process is studied by electrophoresis, quasi-elastic light scattering, nitrogen adsorption, and infrared spectrometry analyses. The conditions of complete coverage of the iron oxide particles by silica and the nature of the maghemite-silica interface are discussed. Copyright 1999 Academic Press.

Journal Article↗
PubMed2000

Properties of Poly(N-isopropylacrylamide)-Grafted Colloidal Silica.

Poly(N-isopropylacrylamide-co-3-trimethoxysilylpropyl methacrylate) was prepared by radical polymerization and was grafted onto the surface of spherical colloidal silica. The copolymer, which had on average 1 silyl group per chain, condensed on the silica dispersed in THF at 60 degrees C. The resulting particles had a critical coagulation concentration of calcium chloride of 500 mM at room temperature, indicating robust colloidal stability. By contrast, heating the suspension to 50 degrees C lowered the critical coagulation concentration by more than three orders of magnitude, giving a value of 0.1 mM. Thus, the PNIPAM shell induced temperature-sensitive colloidal stability in the silica dispersion. The coated silica particles were also surface active. However, the surface tension of 50 mJ/m(2) at 25 degrees C is 15% higher than the corresponding value for PNIPAM solutions. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Adsorption of Polyelectrolyte-Nanoparticle Systems on Silica: Influence on Interaction Forces.

In this work, we have studied the interfacial properties of cationic polyelectrolyte (PE) and silica nanoparticle (NP) systems at macroscopic silica surfaces by means of ellipsometry. The influence of adsorbed layers on the interactions between silica surfaces was also investigated using the bimorph surface force apparatus. Added nanoparticles were observed to strongly swell the interfacial polyelectrolyte layers, an effect partly related to neutralization of charged polyelectrolyte groups. The effect was more pronounced for low charged than for highly charged polyelectrolytes. Overall, the presence of nanoparticles seemed to increase the repulsive interaction measured between silica surfaces. The force measured on approach was long range and quite strongly repulsive. On separation, an attractive bridging interaction was measured for polyelectrolyte-covered surfaces. For the low charged polyelectrolyte used in the study, the force turned repulsive on addition of nanoparticles. For the highly charged polyelectrolyte used, a change from a very strong attraction (involving a jump of the surfaces out of contact) to a very long-range elastic attractive force was observed on adding nanoparticles. The long-range elastic force indicates that polymer chains and nanoparticles form a transient network in the gap between the surfaces. The observed difference in the outward force curves may explain why the addition of nanoparticles appears to improve, e.g., shear-resistance and reflocculation characteristics of polymeric flocculants. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Lateral Force Microscopy Study of the Friction between Silica Surfaces.

The friction between a single silica glass sphere and a flat silica glass surface has been investigated using lateral force microscopy. Two types of silica surface were investigated, one fully hydroxylated and the other partially dehydroxlated. These samples have markedly different wetting properties and are known as hydrophilic and hydrophobic silica, respectively. The lateral friction force as a function of applied normal load has been collected for five different-sized spheres for both surface types. Analysis of the friction versus load data indicates that the hydrophilic surfaces have a molecularly smooth contact. For the hydrophobic surfaces, it is not clear whether the friction-load response is caused by a smooth or an asperity-dominated contact zone. Copyright 2000 Academic Press.

Journal Article↗
PubMed2001

Silylating Agents Grafted onto Silica Derived from Leached Chrysotile.

Silica from leached chrysotile fibers (SILO) was silanized with trialkoxyaminosilanes to yield inorganic-organic hybrids designated SILx (x=1-3). The greatest amounts of the immobilized agents were quantified as 2.14, 1.90, and 2.18 mmol g(-1) on SIL1, SIL2, and SIL3 for -(CH(2))(3)NH(2),-(CH(2))(3)NH(CH(2))(2)NH(2), and -(CH(2))(3)NH(CH(2))(2)NH(CH(2))(2)NH(2) groups attached to the inorganic support. The infrared spectra for all modified silicas showed the absence of the Si-OH deformation mode, originally found at 950 cm(-1), and the appearance of asymmetric and symmetric C-H stretching bands at 2950 and 2840 cm(-1). Other important bands associated with the organic moieties were assigned to nu(as)(NH) at 3478 and nu(sym)(NH) at 3418 cm(-1). The NMR spectrum of the solid precursor material suggested two different kinds of silicon atoms: silanol and siloxane groups, between -90 and 110 ppm; however, additional species of silicon that contain the organic moieties bonded to silicon at -58 and -66 ppm appeared after chemical modification. These modified silicas showed a high adsorption capacity for cobalt and copper cations in aqueous solution, in contrast to the original SILO matrix, confirming the unequivocal anchoring of silylating agents on the silica surface. Copyright 2001 Academic Press.

Journal Article↗
PubMed2002

Isomerization of n-hexane over silica-supported heteropoly acids promoted by the reduced Ce-Ni oxides.

The structure and catalytic properties of silica-supported heteropoly acids promoted by the reduced Ce-Ni oxides are first studied by using chemical analysis XRD, FT-Raman, XPS, EPR, TG, surface area measurements, and microreactor test. It is found that silica-supported heteropoly acids have isomerization activity, but are very easy to deactivate by coke deposition. With the promotion of the reduced Ce-Ni oxides, however, the better activity and the higher selectivity to isomers could be obtained, suggesting that the reduced Ce-Ni oxides really have hydrogenating/dehydrogenating functions. Moreover, it is found that the presence of the reduced Ce-Ni oxides is not only beneficial for eliminating the coke deposition, but also effective for maintaining the structure of silica-supported heteropoly acids during reaction. The effect of the composition of the mechanical mixtures of silica-supported heteropoly acids and the Ce-Ni oxides on the catalytic properties has been explored.

Acids↗
PubMed2002

Two independent routes to synthesize identical silicas by grafting ethylenimine or 2-aminoethanethiol, their cation adsorbing abilities, and thermodynamic data.

The same grafted silica gel surface was obtained through two distinct procedures: (i) by immobilization of the compound 2-aminoethanethiol on precursor 3-chloropropylsilylsilica gel in a heterogeneous method, and (ii) by reacting ethylenimine with 3-mercaptopropyltrimethoxysilane to give a new silylating agent, which was anchored on the silica gel in a homogenous method. The first anchored surface gave 0.70 mmol g(-1) of molecules covalently bonded per gram of silica. This value contrasts to that obtained with the homogeneous conditions, with 1.74 mmol of pendant groups per gram of silica. Both organofunctionalized surfaces have the ability to adsorb divalent cations from aqueous nitrate solution. The adsorption was performed using a batchwise process and the maximum adsorption capacity followed the sequence Cu>Ni>Co and Ni>Cu>Co for surfaces prepared by the homogeneous and heterogeneous methods, respectively. Through calorimetric titrations, the thermodynamic values of cation-basic center interactions were obtained. Exothermic enthalpy results were obtained for all interactive processes. The calculated free Gibbs energy is in agreement with the spontaneity of the reactions and the positive entropy values for all reactions are favorable for this system.

Journal Article↗
PubMed2002

Formation and characterization of high surface area thermally stabilized titania/silica composite materials via hydrolysis of titanium(IV) tetra-isopropoxide in sols of spherical silica particles.

A direct synthetic route leading to titania particles dispersed on nonporous spherical silica particles has been investigated; 5, 10, and 20% (w/w) titania/silica sols mixtures were achieved via hydrolyzation of titanium tetra-isopropxide solution in the mother liquor of a freshly prepared sol of spherical silica particles (Stöber particles). Titania/silica materials were produced by subsequent drying and calcination of the xerogels so obtained for 3 h at 400 and 600 degrees C. The materials were investigated by means of thermal analyses (TGA and DSC), FT-IR, N(2) gas adsorption-desorption, powder X-ray diffraction (XRD), and transmission electron microscopy (TEM). In spite of the low surface area (13.1 m(2)/g) of the pure spherical silica particles calcined at 400 degrees C, high surface area and mesoporous texture titania/silica materials were obtained (e.g., S(BET) ca. 293 m(2)/g for the 10% titania/silica calcined at 400 degrees C). Moreover, the materials were shown to be amorphous toward XRD up to 600 degrees C, while reasonable surface areas were preserved. It has been concluded that dispersion of titania particles onto the surface of the nonporous spherical silica particles increase their roughness, therefore leading to composite materials of less firm packing and mesoporosity.

Journal Article↗
PubMed2002

In situ synthesis of polysulfides covalently bonded to silica.

Silanol groups, triple bond SiOH, on the surface of silica were esterified with unsaturated alcohols and long-chain alcohols bearing thiol groups. The modified silicas obtained were used as substrates for a vulcanization-analogous reaction with sulfur catalyzed by zinc dimethyldithiocarbamate. Surface-esterified thiols could be smoothly converted to bridged polysulfides bonded to the silica surface, whereas the use of surface-esterified unsaturated alcohols led to removal of the surface-esterified alcohol from the silica surface. The materials were characterized by solid-state NMR and thermal and microanalytical analysis. The linking of surface-esterified alkenols and thiols by sulfide bridges was investigated by a numerical model for a flat surface. This showed that for a typical density of 3-4 micromol/m(2) surface groups, a statistical maximum of 70-75% of groups could be linked by S(n) bridges (n=2-4).

Journal Article↗
PubMed2002

Preparation and flow cytometry of uniform silica-fluorescent dye microspheres.

Uniform fluorescent silica-dye microspheres have been prepared by coating preformed monodispersed silica particles with silica layers containing rhodamine 6G or acridine orange. The resulting dispersions exhibit intense fluorescent emission between 500 and 600 nm, over a broad excitation wavelength range of 460 to 550 nm, even with exceedingly small amounts of dyes incorporated into the silica particles (10-30 ppm, expressed as weight of dye relative to weight of dry particles). The fluorescent particles can be prepared in micrometer diameters suitable for analyses using flow cytometry with 488-nm laser excitation.

Journal Article↗
PubMed1994

Characterization and toxicological behavior of synthetic amorphous hydrophobic silica.

During almost three decades of experience with hydrophobic silicas, no adverse health effects have been observed in manufacturing and applications with appropriate handling of the materials. The oral LD50 for rodents is > 7.9 g/kg body wt. Fumed or precipitated hydrophobic silicas do not produce inflammation of the skin or mucous membranes. Likewise, acute and chronic oral tests yielded no adverse systemic effects. A limited carcinogenesis study in rats did not induce tumors and the Ames test of a toluene extract was negative. Reproductive or developmental toxicity was not observed. In general, hydrophobic silicas provide a toxicological profile essentially the same as common silicas.

Animals↗
PubMed1991

A comparison of the properties of vicinal water in silica, clays, wood, cellulose, and other polymeric materials.

The properties of water are known to be significantly modified by propinquity to solid surfaces. This author has previously measured some thermodynamic properties of water in silica gel pores and offered a statistical thermodynamic model for vicinal water that is apparently able to account successfully for the observed properties of water in silica pores. This model suggests that hydrogen bonding between water molecules is enhanced by propinquity to solid surfaces. The experimental work suggests that the structural modification extends at least to distances approaching 5 nm from the surface. In addition to the work of the present author, Low has conducted an extensive study of the thermodynamic properties of water in clays. Low's results are remarkably similar to those of the present author, suggesting that the "state" of vicinal water in both systems is similar. In this paper, the properties of water adjacent to several surfaces are compared. The properties of water near cellulosic surfaces, including wood, are compared to those in silica. It appears that the properties of vicinal water adjacent to dissimilar surfaces are nearly identical to those in silica gel. Significantly, it appears that the notion of "structure breaking" surfaces suggested earlier by Goring and the notion of dense water near surfaces discussed, for instance, by Stamm are, in fact, inconsistent with the available thermodynamic data, despite their rather frequent mention in the wood and paper literature.

Aluminum Silicates↗
PubMed1980

Inhibition of adjuvant arthritis by intraperitoneal administration of low doses of silica.

Daily intraperitoneal administration from day 3 to day 10 post-adjuvant of crystalline silica in doses 1.5-3.1 mg/kg per day or of amorphous silica in doses of 12.5 mg/kg per day inhibited the development of adjuvant arthritis mostly suppressing the swelling of the non-injected paw. These doses of silica did not impair the carbon clearance. Higher doses of silica (25 mg/kg per day of the crystalline form and 50 mg/kg of the amorphous form) administered from day 17 to day 24 post-adjuvant had no effect on the already established disease.

Animals↗
PubMed1988

Induction of a silica air-pouch granuloma in rat.

An inflammatory reaction was induced by the injection of crystalline silica into a subcutaneous air pouch formed on the dorsa of rats. The degree of inflammation depended on the quantity of silica injected and on the time interval between air pouch formation and silica injection. The silica provoked exudation of fluid, accumulation of polymorphonuclear and mononuclear leucocytes and the formation of a granuloma. These parameters reached peak values by day 4 whilst the serum acute phase protein levels peaked on day 1. Cyclophosphamide, indomethacin and prednisolone inhibited fluid accumulation but only cyclophosphamide inhibited cell infiltration. The increase in wet and dry granuloma weight was suppressed only by steroid treatment. The suitability of this model as a screen for anti-inflammatory compounds is discussed.

Acute-Phase Proteins↗
PubMed1980

Effect of silica on the proliferative behavior of human lung fibroblasts.

We show that silica enhances the proliferation of some but not all lung derived fibroblastlike cells (FLC) of fetal and adult origin. Cultures were obtained from lung biopsy and lavage of normal individuals and patients with interstitial lung disease. The responses of these cell lines were heterogeneous; some cultures were stimulated to proliferate in the presence of silica and others were not stimulated. There was no apparent correlation between source of the cell lines and the effects of silica. The proliferative capacity of the cells appeared to be an important determinant of the effect of silica on the growth of the cultures.

Adult↗
PubMed2000

Formation of model lipid bilayers at the silica-water interface by co-adsorption with non-ionic dodecyl maltoside surfactant.

This present article describes a new and simple method for preparing model lipid bilayers. Stable and reproducible surface layers were produced at silica surfaces by co- adsorbing lipid with surfactant at the silica surface from mixed micellar solutions. The adsorption was followed in situ by use of ellipsometry. The mixed micellar solution consisted of a lipid (L-alpha-dioleoyllecithin) and a non-ionic sugar-based surfactant (n-dodecyl-beta-maltoside). The latter showed, by itself, no affinity for the surface and could, therefore, easily be rinsed off the surface after the adsorption step. By first adsorbing from solutions with high lipid and surfactant concentrations and then, in succession, rinsing and re-adsorbing from solutions with lower lipid-surfactant concentrations, a dense-packed lipid bilayer was produced at the silica surface. The same result can be achieved in a one-step process where the rinsing, after adsorption from the concentrated solution, is performed very slowly. The thickness of the adsorbed lecithin bilayer after this treatment found was to be about 44 +/- 3 A, having a mean refractive index of 1.480 +/- 0.004. The calculated surface excess of lipids on silica was about 4.2 mg m(-2), giving an average area per lipid molecule in the two layers of 62 +/- 3 A2. The physical characteristic of the adsorbed bilayer is in good agreement with previously reported data on bulk and surface supported lipid bilayers. However, in contrast to previous investigations, we found no support for the presence of a thicker multi-molecular water layer located between the lipid layer and the solid substrate.

Adsorption↗
PubMed2002

A new attempt to study biomineralised silica bodies in Dactylis glomerata L.

The accumulation process of inorganic compounds in animals and plants by biomineralisation is not well understood nowadays, though it may be the key to an environmental-compatible production of modern materials in future. In this paper a new attempt will be made on the investigation of silica accumulation in grasses (especially Dactylis glomerata L.). The silicic acid agglomerates in Dactylis glomerata L. were studied by means of scanning electron microscopy in combination with energy dispersive X-ray spectrometry as well as infrared and micro-RAMAN spectrometry.In particular blades were prepared by critical point drying or shock-freezing for anatomical studies of silica cells and bristles in the plant tissue. SEM imaging and EDX microanalysis for elemental composition were done in the cryostage as well as under variable pressure. The localized silica bodies were examined for their structural properties by means of IR and micro RAMAN spectroscopy. The results are comparable to SiO(2) polytypes such as high disperse silica and opal.

Dactylis↗