Showing papers on "Silica gel published in 1994"

Interactions of silica surfaces

Abstract: Adhesion, friction, and colloidal forces in air and aqueous salt solutions have been measured between various silica surfaces prepared by depositing amorphous but highly smooth silica films on mica. The results show four interesting and interrelated phenomena: (i) The adhesion of silica surfaces in air increases slowly with contact time, especially in humid air where the "contacting" surfaces become separated by an ∼20-A-thick layer of hydrated silica or silica gel. (ii) The friction of two silica surfaces exhibits large sticking or "stiction" spikes, whose magnitude increases in the presence of water and when the surfaces are kept in contact longer before sliding. (iii) The non-DLVO repulsion commonly seen at short range (<40 A) between silica surfaces immersed in aqueous solutions is monotonically repulsive, with no oscillatory component, and is quite unlike theoretical expectations and previous measurements of forces due to solvent structure. (iv) Dynamic contact angle measurements reveal time-dependent effects which cannot be due to a fixed surface chemical heterogeneity or roughness. The results indicate that silica surfaces undergo slow structural and chemical changes during interactions with water and with each other. More specifically, we propose that the unusual interfacial and colloidal properties of silica are due, not to hydration effects, but to the presence of an ∼10-Athick gel-like layer of protruding silanol and silicilic acid groups that grow on the surfaces in the presence of water. These protruding groups react chemically (sinter) with similar groups located on an opposing surface and give rise to the unusual time-dependent adhesion, friction, and non-DLVO forces observed. Concerning the effects on colloidal interactions, the surface gel-layer effectively shifts the OHP outward and adds a monotonic short-range polymer-like steric repulsion to the DLVO interaction. The mechanism proposed here is quite different from the commonly accepted one, in which modified water structure at the silica surface is believed to give rise to a repulsive "hydration force." The proposed mechanism in terms of a surface layer of silica gel is consistent with the known surface chemistry of silica and accounts for the results reported here and also for many other unusual surface and colloidal properties of silica.

Preparation of high porosity xerogels by chemical surface modification

Abstract: This invention provides an extremely porous xerogel dried at vacuum to below-supercritical pressures but having the properties of aerogels which are typically dried at supercritical pressures. This is done by reacting the internal pore surface of the wet gel (e.g. alkoxide-derived silica gel) with an organic surface modification agent (e.g. trimethylchloro-silane in benzene) in order to change the contact angle of the fluid meniscus in the pores during drying. Shrinkage of the gel (which is normally prevented by the use of high autoclave pressures, such that the pore fluid is at temperature and pressure above its critical value) is avoided even at vacuum or ambient pressures. The figure 4 shows a change in sample weight and sample length during drying for surface modified, ambient pressure gel processed in accordance with the invention, illustrating the initial shrinkage followed by expansion of the gel during the final stages of drying. The extremely low density finely pored gel products have useful insulating and other properties.

A two-component, non-aqueous route to silica gel

Abstract: A new means for generating silica gel has been developed. Simple two component systems comprising tetraalkoxysilanes and strong carboxylic acids such as formic acid can react rapidly to give transparent monolithic gels. The acid serves as solvent, water source, and catalyst for both hydrolysis and condensation. Water need not be present as an initial reactant; it is generated in situ during the reaction. The gelation reaction is at least two orders of magnitude faster than those conducted in conventional acid-catalyzed aqueous systems at comparable pH*. Kinetic evidence indicates a lowering of the activation energy of condensation reactions, believed to be associated with reaction of silyl carboxylates and silanol groups. Physical properties of the dry gels such as bulk density more closely resemble conventional acid-catalyzed gels than those associated with other rapidly gelling systems. Dry gels often exhibit porosity so fine that nitrogen (at 77°K) is not absorbed at significant rates. Independent evidence of porosity arises from comparison of skeletal and bulk densities, sample immersion in water and adsorption isotherms of CO2.

The Colloid chemistry of silica : developed from a symposium sponsored by the Division of Colloid and Surface Chemistry at the 200th National Meeting of the American Chemical Society, Washington, DC, August 26-31, 1990

Abstract: Colloid chemistry of silica - an overview, Horacio E. Bergna. Part 1 Preparation of sols: silica nucleation, polymerization and growth - preparation of monodispersed sols, Akitoshi Yoshida the formation and interfacial structure of silica sols, John D.F. Ramsay et al synthesis and characterization of colloidal model particles made from organoalkoxysilanes, A. van Blaaderen and A. Vrij synthesis of nanometer-sized silica by controlled hydrolysis in reverse micellar systems, F.J. Arriagada and K. Osseo-Asare formation of silica gels composed of micrometer-sized particles by the sol-gel method, Hiromitsu Kozuka and Sumio Sakka. Part 2 Stability of sols: stability of aqueous silica sols, Thomas W. Healy. Part 3 Surface chemistry of silica: the surface structure of amorphous and crystalline porous silicas - status and prospects, K.K. Unger infrared study of chemical and H-D exchange probes for silica surfaces, B.A. Morrow and A.J. McFarlan Fourier transform infrared and Raman spectroscopic study of silica surfaces, A. Burnear et al surveying the silica gel surface with excited states, R. Krasnansky and J.K. Thomas surface chemistry and surface energy of silicas, Alain M. Vidal and Eugene Papirer variable-temperature diffuse reflectance Fourier transform infrared spectroscopic studies of amine desorption from a siliceous surface, Dondald E. Leyden and Kristina G. Proctor multinuclear NMR spectroscopy studies of silica surfaces, Gary E. Maciel et al. Part 4 Particle size and characterization techniques: new separation methods for characterizing the size of silica sols, J.J. Kirkland characterization of colloidal and particulate silica by field-flow fractionation, J. Calvin Giddings et al interpretation of the differences between the pore size distributions of silica measured by mercury intrusion and nitrogen adsorption, A.R. Minihan et al. Part 5 Sol-gel technology. Part 6 Silica gels and powders. Part 7 Silica coatings. Part 8 Uses of colloidal silicas. Part 9 Research in Russia. (Part contents)

Gas-chromatographic determination of phenols in aqueous samples after solid phase extraction

Abstract: 27 phenols including mono-, di-, tri-, tetra- and pentachlorophenols, mono- and dinitrophenols, mono-and dinitrocresols and dimethylnitrophenols have been extracted from aqueous samples by solid phase extraction using both modified silica gel (C18) and XAD resin-adsorbents and determined by gas chromatography (GC) using capillary columns with specially desactivated weakly polar phases. If a 11 sample of spiked water is used, a considerable breakthrough is observed with phenol itself, while all other phenols are almost quantitatively extracted. The recovery of phenol itself can be improved by employing smaller sample volumes.

Raman Spectroscopy of Covalently Bonded Alkylsilane Layers on Thin Silica Films Immobilized on Silver Substrates

Abstract: Results are presented which demonstrate the use of Raman spectroscopy for the characterization of alkylsilane layers covalently bound to silica surfaces. Raman spectra of molecular assemblies of octadecylsilane have been studied by use of a thin silica gel/Ag sandwich structure as the substrate. The Ag surface, used to provide electromagnetic enhancement to the octadecylsilane overlayer, is modified with a monolayer of (3-mercaptopropyl)trimethoxysilane, which is used as a molecular adhesive for anchoring the thin silica gel film. Spin coating a silica sol on the modified Ag surface results in the formation of a thin silica gel layer which contributes minimal fluorescence background to the Raman spectra

Synthesis of a New Trialdehyde Template for Molecular Imprinting

Abstract: A recent approach in the area of molecular recognition involves template-assisted imprinting' of the surface of rigid polymers such as silica gel. Siloxy-Schiff bases of structurally rigid dialdehydes have been used as templates to introduce two appropriately spaced amino groups for substrate binding on the solid surface. The distance between the two amino groups is defined by the structure of the dialdehydes. Moderate selectivity has been reported in rebinding of the dialdehydes to the modified surface.2 Current objectives in this quest involve the development of multidentate template molecules to expand the repertoire of functional groups available a t the binding sites3 We report herein the synthesis of a trialdehyde and its Schiff bases as a new template for molecular imprinting of solid surfaces. The reaction of cyanuric chloride with 3 equiv of p hydroxybenzaldehyde in benzene gave the desired trialdehyde, TRIPOD (1) (Scheme 11, in a single step. Trialdehyde 1 was then reacted at room temperature with (aminopropy1)triethoxysilane (APTES) or p-(methoxydimethylsily1)aniline (MODSA)4 to afford the corresponding silane-TRIPODS, APTES TRIPOD (2), and MODSA TRIPOD 4 (Scheme 2), respectively. Model compounds PROPYL TRIPOD (3) and ANILINE TRIPOD (5) were also synthesized for future mechanistic studies of Schiff base formation. Acid hydrolysis of all the Schiff bases regenerated the intact TRIPOD. Molecular imprinting was carried out with silaneTRIPOD molecules 2 and 4 on silica gel to yield modified silicas 7 and 8, respectively. Porous silica gel, Fractosil 500, was refluxed in dry toluene for 2 days28 with an appropriate amount of the silane-TRIPODS to get a low substitution level on the silica and ensure site isolation (Scheme 3). Initial experiments with APTES TRIPOD (2) indicated that a partial degradation of 2 took place during the imprinting reaction. Refluxing of the model compound 3 in toluene for 2 days produced p-hydroxybenzaldehyde, suggesting that the degradation was caused by the partial hydrolysis of the Schiff base and subsequent attack on the ether linkage by the amine. On the other hand, MODSA TRIPOD (4) did not decompose, presumably due to the greater strength of the aromatic Schiff

Control of pore size of porous silica by means of pyrolysis of an organic–inorganic polymer hybrid

Abstract: Pyrolysis of a silica gel hybrid with a starburst dendrimer gives porous silica, in which the pore-size is successfully controlled by the size of the organic component used.

Direct Reaction of Phosphorus Acids with Hydroxy of a Silanol and on the Silica Gel Surface

Abstract: Several phosphorus acids (methyl- and phenylphosphonic, diphenylphosphinic, and ethyl phosphoric acids) were round to give inters on reaction with tert-butyldimethylsilanol. The reactions occurred simply on beating the reactants in an inert solvent (hexane or toluene). The reaction is reversible, and yields or inters were improved by continuous water removal. The dibasic acids gave either mono- or disilyl inters; the latter were more easily purified and characterized. The OH groups on the surface or silica gel were also round to react with various phosphorus acids under similar conditions to give surface-bonded esters. Products were characterized by CP/MAS 31 P and 28 Si NMR

Spectroscopic characterization of doped sol-gel silica gels and glasses: evidence of inner-sphere complexation of europium(III)

Abstract: Eu3+ luminescence, excitation and lifetime spectroscopy measurements have been made during the gelation and densification of Eu3+-doped silica gels prepared by the hydrolysis and condensation of tetraethoxysilane (TEOS). Particular attention was paid to the effect of counter-ion complexation of Eu3+ in gels prepared with different europium precursors. Precursor-dependent spectroscopic variations have been observed. The Eu3+ probe ion was introduced in the initial stage of the sol-gel process by dissolving a hydrated europium salt in the sol. Europium precursors studied include: Eu(NO3)3·6H2O, EuCl3·6H2O, Eu(ClO4)3·6H2O. Sol, gel point and wet gel spectra and lifetimes are interpreted by analogy with spectroscopic studies of the europium salts in mixed ethanol-water solutions. In particular, the energy of the 7 F 0 → 5 D 0 excitation spectrum is used to identify complexes containing counter-ions in the inner coordination sphere of Eu3+. Results are presented showing that the NO3− ions have an affinity for the inner coordination sphere of Eu3+ which is greater than the affinity of Cl−. ClO4− ions retain an outer-sphere interaction with Eu3+ throughout the sol-gel process. Thermal densification of the gels destroys the inner-sphere complexation of the counter-ions and, above 300°C, the counter-ion-dependent effects were removed. The results show that the method by which the Eu3+ probe ion is introduced to the system significantly affects the Eu3+ spectra.

Recording sheet and process for its production

Abstract: A recoding sheet comprising a substrate, a porous layer of pseudo-boehmite having a thickness of from 10 to 100 μm formed on the substrate and a layer of silica gel having a thickness of from 0.1 to 30 μm formed on the porous layer of pseudo-boehmite.

Process for the isolation and purification of taxol and taxanes from Taxus using methanol or acetone

Abstract: A process for the separation of taxanes, particularly taxol, cephalomannine, baccatin III and deacetylbaccatin III, from ornamental yew tissue is described. The process involves a specific solvent mixture of water and 50 to 95 percent by volume methanol, ethanol or acetone and treatment of the resulting extract using activated carbon, preferably charcoal. The taxanes are preferably separated from the crude extract by a normal phase chromatographic step which preferably is through vacuum and then medium pressure column chromatographic separation, using inexpensive silica gel as an absorbent. The silica gel is regenerated by heating in a furnace above about 500° C. to remove adsorbed organic materials.

Pore structure evolution in silica gel during aging/drying. IV. Varying pore fluid pH☆

Abstract: In an effort to understand the various phenomena which occur during aging of silica gels at different pH, silica particles prepared by three routes distinguished prinipally by their silicon coordination (Qn distributions) Stober silica spheres, Ludox and Cab-O-Sil) were employed. In situ techniques were used including proton spin-lattice relaxation, SAXS and zeta potential for the wet state and 29Si MAS-NMR, SEM, TEM and N2 sorption for dry samples. In solution, the surface area increased in the following order after aging at pH 10 as compared with that at pH 7 and the initial dry surface area: Cab-O-Sil < Ludox ⪡ Stober which is inversely proportional to the degree of silicon coordination. This same trend was observed for the surface charge measurements. Surface areas calculated from 29Si MAS-NMR for four different Stober sphere sizes were of order 1000 m2/g as compared with nitrogen/BET surface areas which varied from 9 to 370 m2/g, indicating significant internal surface which is inaccessible to nitrogen. The 29Si NMR results for the Stober spheres were consistent with a close packed aggregate of ∼3 nm primary particles. In addition to particulate silica, aging at varying pH of a two-step acid/base-catalyzed silica gel was studied. At all pH values, the surface area was higher in the wet gel as compared with the xerogel. Aging at higher pH was found to yield lower surface area, larger pore volume and a narrower pore size distribution depending upon the pH, aging time and surface tension of the final pore fluid.

Apatite crystallization from metastable calcium phosphate solution on sol-gel-prepared silica

Abstract: Rehydration of silica gels sintered at 900–1100°C can stimulate apatite crystallization from a metastable calcium phosphate solution on their surfaces. This precipitation reaction is related to the thermal history of the silica. Increasing sintering temperature can retard apatite formation on the silica gel. Sintering at 1200°C can make the silica gel insoluble and thereby unable to induce apatite crystallization. It is believed that apatite crystallization on the silica can be modulated by the rate of hydrolysis on its surface. Certain coverage of silanol group is required to trigger heterogeneous nucleation of apatite from the metastable calcium phosphate solution.

Magnetic polymeric silicon dioxide

Abstract: Process for the preparation of magnetic polymeric silicon dioxide compounds in the form of silica gel, silica sol, precipitated or pyrogenic silicon dioxide, by incorporation of magnetic materials into their matrix. The many possible applications in the sectors of process technology, chemical technology, analysis, biotechnology and pharmacy result from the possibility of influencing such compounds magnetically.

Better bioactive ceramics through sol-gel process

Abstract: Bioactive glasses and glass ceramics need to be capable of growing a calcium phosphate layer at their surfaces in physiological environment in order to bond with living bone. Sol-gel prepared silica (silica gel) and titania (titania gel) are efficient calcium phosphate absorbents. Both gels extract calcium and phosphate from surrounding physiological and other calcium phosphate solutions and form a calcium phosphate at their surfaces in return. Thus, they can integrate with bone. Under the same condition, however, silica and titania, as both prepared through a conventional high temperature process, are unable to transfer calcium and phosphate from the solutions to obtain a calcium phosphate at their surfaces. Therefore, it is concluded that using inorganic or metal organic precursors, sol-gel process can yield bioactive materials with a high bioactivity.

Purification and Characterization of Lipid Bioflocculant Produced by Rhodococcus erythropolis

Abstract: The bioflocculant produced by Rhodococcus erythropolis S-1 was found to exist as huge assemblies, the molecular mass of which is over one million daltons, composed of many polypeptides and lipids in aqueous solution. We have isolated and purified this lipid bioflocculant by ultracentrifugation, extracting with 90% acetone, and two successive silica gel chromatographies from the culture broth. It was homogeneous on silica gel thin-layer chromatography. 1H-NMR and HPLC studies showed that it was a kind of glycolipid that contained a C16 methylene chain on the average and glucose in its chemical structure. The flocculating activity against kaolin clay suspension was dependent on the Ca2+ concentration.

Characteristics of composites based on PMMA modified gel silica glasses

Abstract: Gel silica glass prepared by the sol-gel process can be modified by incorporating an organic phase into the intrinsically porous inorganic gel matrix, which results in a composite material with much improved mechanical and optical properties. Characterisation of PMMA modified gel silica glass prepared by the in situ polymerisation method using FT-Raman spectroscopy, gel permeation chromatography and the nitrogen adsorption technique are reported. Some essential problems encountered in the preparation are discussed.

CP/MAS NMR investigations of silica gel surfaces modified with aminopropylsilane

Abstract: Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.29Si and13C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.

Isotherms of chemical adsorption of bases on solid catalysts for acidity measurement

Abstract: The measurement of the adsorption equilibrium constants of the bases and the total mass of base adsorbed at 200°C are proposed as reproducible indices of average acidity strength and of total acidity of the solid, respectively. The adsorption of different bases (ammonia, n-butylamine, tert-butylamine, pyridine) in gas phase on solid acidic catalysts (silica–alumina and zeolite catalysts and a silica gel) has been studied, by applying the calorimetric method in flow regime through an isothermal bed of solid. Both the conditions under which the adsorption is exclusively chemical as well as those under which the adsorption is not limited by the internal diffusion of the base have been determined. For these acidity measurements tert-butylamine behaves well and is an alternative base to ammonia.