Showing papers on "Silica gel published in 1990"

Structural investigation of silica gel films by infrared spectroscopy

Abstract: Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c‐Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c‐Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v‐SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a st...

Calcium phosphate formation at the surface of bioactive glass in vivo

Abstract: Bioactive glasses are glasses which can adhere to bone through chemical bonding. This paper discusses the bonding to bone of phosphate-free glasses, and the formation of calcium phosphate at the surface of these glasses in vivo. The bone bonding was proved by a push-out test. In the bonding to bone, the glass surface initially transforms into a silica gel. By SEM/EDX analysis it was shown that phosphate ions from the physiological solution penetrate far into the gel. Calcium accumulation occurs only in the presence of phosphate. The results suggest that the initial stage of the calcium phosphate build-up is complexation of phosphate by the silica gel.

Quantitative analysis of phospholipids by HPLC with a light scattering evaporating detector – application to raw materials for cosmetic use

Abstract: A method is described for separation and quantitative analysis of various phospholipids and sphingolipids by high performance liquid chromatography on a silica gel column with a mobile phase consisting of chloroform, methanol, and ammonium hydroxide. Gradient elution is necessary. A study of the light scattering evaporative detector response is presented. The method has been applied to the analysis of phospholipids of a commercial soja lecithin and a pig brain extract.

An intrinsic mechanism for the activity of alumino‐silicate based electrorheological materials

Abstract: The presence of significant amounts of adsorbed water on the particulate phase of most electrorheological (ER) materials is essential to their performance. This paper presents evidence showing that alumino‐silicate particles, when used as the dispersed component, produce ER active fluids without the apparent need for any water, adsorbed or structured. It is suggested that these materials by virtue of their unique chemistry and morphology, contain an intrinsic mechanism for producing their ER activity which does not require water. The ER activity and bulk conductivity of alumino‐silicate based and silica gel based ER fluids are examined as a function of water content. Whereas the silica gel based fluid loses its ER activity between 1.5% and 6% H2O, the alumino‐silicate based fluid continues to function at levels of H2O below FTIR detectability. Further, bulk current densities reach a constant value at around 0.5% H2O and below, but the fluids continue to operate without change. This suggests that models re...

Pre-concentration of copper, cobalt and nickel with 3-methyl-1-phenyl-4-stearoyl-5-pyrazolone loaded on silica gel

Abstract: 3-Methyl-1-phenyl-4-stearoyl-5-pyrazolone loaded on to silica gel was used as a pre-concentration reagent for copper, cobalt and nickel prior to atomic absorption spectrometric determination. Both batch and column methods were used for the concentration of the above metals. These metals are quantitatively retained on the proposed adsorbent above pH 4. The adsorption-elution cycle can be repeated ten times with no observable decline in the efficiency of the adsorbent. As there are no observable effects due to changes in the volume of sample solution up to a volume of 1 l, a concentration factor of 40 can be achieved. The method has been applied to samples of sodium chloride solution and tap water.

Rapid analysis of fatty acids in plasma lipids.

Abstract: A rapid and convenient procedure for the quantitative determination of the fatty acid composition of plasma lipids is described. Human plasma was applied directly to the preadsorbent zones of thin-layer silica gel plates with added antioxidant, internal standards and carriers. The thin-layer chromatography (TLC) plates were partially developed with methanol followed by chloroform/methanol (1∶1, v/v), and then they were fully developed in hexane/diethyl ether/acetic acid (80∶20∶1, v/v/v) to separate the major classes of lipids. Silica gel from regions containing the separated lipids was scraped into screw-capped tubes and treated with boron trifluoride-methanol prior to gas chromatography. The method of direct application to TLC plates gave yields and compositions of fatty acids very similar to the method of applying extracted plasma lipids. This relatively simple method is suitable for analyzing the fatty acids in plasma lipids from a 50 microliter finger-tip blood samples from an individual, and it may be useful in wide-scale screening of different individuals to estimate the relative amounts of ingested polyunsaturated fatty acids.

Enantioselective addition of dialkylzincs to aldehydes using heterogeneous chiral catalysts immobilized on alumina and silica gel

Abstract: The first example of the use of alumina and silica gel as insoluble supports for the chiral catalyst in the enantioselective addition of dialkylzincs to aldehydes is described. Chiral N-alkylnorephedrines were immobilized on (3-chloropropyl)silyl-functionalized alumina or silica gel. Ephedrine was also immobilized on silica gel coated with chloromethylated polystyrene. Using these chiral catalysts, optically active secondary alcohols were obtained in 24-59% enantiomeric excess. The catalyst immobilized on silica gel coated with polystyrene was recycled without the loss of its enantioselectivity

Surface structure and chemistry of high surface area silica gels

Abstract: Combined Raman and {sup 29}Si NMR investigations of high surface area silica gels indicate that dehydroxylation of the a-SiO{sub 2} surface results preferentially in the formation of cyclic trisiloxanes (3-membered rings). Estimates of the maximum experimentally observed concentration of 3-membered rings correspond to a surface coverage of about 28-58{percent}. Two consequences of 3-membered rings on the a-SiO{sub 2} surface are enhanced hydrolysis rates and increased skeletal densities. 29 refs., 6 figs.

Adsorption and diffusion of a Lennard‐Jones vapor in microporous silica

Abstract: The properties of a dilute Lennard‐Jones vapor in contact with an adsorbing microporous medium are investigated using grand canonical ensemble Monte Carlo and molecular dynamics techniques. The bulk structure of the microporous system is modeled as an assembly of randomly distributed interconnected solid spheres, and vapor/surface interactions are treated in two ways: (i) using a smooth continuous interaction potential and (ii) using a molecular model for the surface structure of the solid. The microporous solid representation employed in these simulations is chosen to conform in realistic manner with the bulk and surface properties of silica gel. The results obtained from the simulations include equilibrium partition coefficients, diffusivities, and related microscopic properties. By comparing these results with available experimental data it is shown that the properties of simple nonpolar gases in microporous silica may be predicted with reasonable accuracy. This is particularly true when the molecular ...

Interfacial aggregation of nonionic surfactants onto silica gel: Calorimetric evidence

Abstract: The search for the origins of the macroscopic interfacial phenomena, the adsorption layer structure, and the nature of interactions between solid and surfactant molecules and between molecules at adsorbed states has led us to a calorimetric investigation of adsorption. The adsorption of nonionic surfactants onto the hydrophilic surfaces of silica gel has been studied by batch microcalorimetry for a wide range of coverages. Differential molar enthalpies of adsorption corresponding to the adsorption processes have been obtained. The microcalorimetric method for the investigation of adsorption from solution is shown to be a very sensitive tool for the elucidation of the adsorption mechanism. The calorimetric results indicate that there are at least two kinds of interactions between nonionic surfactant molecules and the silica surface, the first one being due to a direct interaction between the polar parts of molecules and the surface (exothermic effect) and the second resulting from lateral interactions between the hydrophobic chains, leading to the formation of interfacial aggregates (endothermic effect). The principal conclusion resulting from the present investigation is that after the adsorption of individual “gaseous” surfactant molecules which probably constitute nucleation sites, there is the formation of interfacial micelles. The main driving forces for the formation of aggregates are of the same nature as those for the formation of micelles in the bulk solution.

Dependence of the elastic moduli of porous silica gel prepared by the sol-gel method on heat-treatment

Abstract: The elastic moduli of silica gel monolith prepared by the sol-gel method from a tetramethoxysilane solution have been measured before and after heat treatment. The elastic moduli showed a drastic change with heat treatment; for example, Young's modulus changed from 0.95 GPa for the gel before heating to 72.5 GPa for the densified product after heating to 1050 ° C. The change in the Young's modulus of the gel with heating temperature is discussed on the basis of changes of porosity and strength of the silica skeleton.

Extraction and seperation method and apparatus using supercritical fluid

Abstract: Extraction and separation method and apparatus using a supercritical fluid for extracting a specified component from a sample by a supercritical fluid and introducing the supercritical fluid containing the extracted component into a silica gel column coated with silver nitrate so as to separate the extracted component By the combination of the extracting operation by a supercritical fluid under mild conditions and chromatography taking the advantage of the selectivity of the silica gel column coated with silver nitrate to olefins, the extract is obtained at a high concentration without producing denaturation

Estimation of the distribution of surface hydroxyl groups on silica gel, using chemical modification with trichlorosilane

Abstract: An estimate of the relative distribution of the free and bridged surface hydroxyl groups is made, using a surface modification with trichlorosilane according to the following main reaction Si—OH + HSiCl3→Si—O—SiHCl2+ HCl. The amount of chemisorbed chloride groups on the substrates is compared with data on the total amount of surface hydroxyl groups and with hydroxyl group information derived from infrared spectra. The interpretation of these data gives an indication of the reactivity, distribution and concentration of the different types of hydroxyl groups on the surface of silica gel.Trichlorosilane reacts mainly with bridged hydroxyl groups on silica gel, pretreated at low temperatures and exclusively with free hydroxyls at high pretreatment temperatures.Extrapolation of the reactivity of trichlorosilane towards the surface hydroxyl groups demonstrates that bridged hydroxyl groups are present up to pretretment temperatures of 973 K.

Gaussian approximation to the unique heterogeneous Langmuir-Hinshelwood type fluorescence quenching at the silica gel gas/solid interface: pyrene and 9,10-diphenylanthracene singlet quenching by oxygen

Abstract: The temperature dependency of the oxygen quenching of the singlet excited states of both pyrene and 9,10-diphenylanthracene on nonporous silica gel has been studied by both laser-induced time-resolved and steady-state fluorescence spectroscopy. A Gaussian distribution of rate constants is presented to account for the heterogeneous kinetics. Oxygen adsorption isotherms on the silica gel surface above the gas/liquid critical temperature are determined; oxygen assimilation times of 23.2, 44.2, 99.5, and 238 ps have been calculated at {minus}47, {minus}64, {minus}80, and {minus}98{degree}C, respectively. A dynamic Langmuir-Hinshelwood model for the bimolecular quenching reaction is presented.

Characterization of silicas by inverse gas chromatography at finite concentration: Determination of the adsorption energy distribution function

Abstract: Amorphous and crystalline silicas, eventually modified by heat treatment or by surface grafting of alkyl chains, were investigated by inverse gas chromatography, a method which easily provides adsorption isotherms of various linear alkanes. The isotherms were analyzed in order to obtain the adsorption energy distribution of alkane probes. The distributions were calculated using a local solution of the adsorption integral equation which is evaluated and discussed. Silicas show typical energy distribution patterns. Furthermore, the grafting of hexadecanol chains on the silica surface brings the greatest changes in the energy distributions.

Iron magnetic moments in iron/silica gel nanocomposites

Abstract: Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low‐temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X‐ray diffraction, electron microscopy, Mossbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer‐sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie‐Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as‐cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After ...

Rotational and vibrational relaxation of small molecules in porous silica gels

Abstract: Samples of transparent silica gels of controlled porosities are produced stabilized at 800{degree}C. Raman spectra of samples impregnated with CS{sub 2}, CHCl{sub 3}, CH{sub 3}CN, or acetone are recorded in order to obtain rotational and vibrational correlation functions and correlation times for those liquids. The effect of pore diameters on vibrational dephasing and rotational diffusion is discussed. It is shown that surface interactions, in particular, hydrogen bonding between the imbedded molecules and silanol groups, are responsible for slowing down the rotational relaxation within small pores. A simple model for reorientational motion of molecules hydrogen bonded to the silica surface is proposed. The vibrational modulation times are obtained from the Kubo theoretical function and used to analyze molecular interactions near the silica surface. The number of silanol groups on the surface is estimated from the {double bond} band of acetone.

Control of pore size distribution of silica gel through sol-gel process using water soluble polymers as additives

Abstract: To control the pore size distribution of silica gel, several kinds of water soluble polymers were added in the sol-gel process starting from tetraethyl orthosilicate. The polymers employed here were classified into three groups. The first group comprised uncharged polymers such as polyvinyl alcohol and polyethylene glycol, which affected only the amount of micropores and small mesopores ( 20nm). The third group comprised proteins such as lipase and albumin and decreased the number of micropore and mesopores and greatly increased the number of macropores. The results could almost be explained in terms of the principal effects of the polymers on a process of sol particle growth.

Packing material for reversed phase chromatography and process for its preparation

Abstract: A packing material for reversed phase chromatography, which comprises silica gel with its entire surface substantially coated with repeating units of a polycarbosilane of the formula (I): ##STR1## wherein R1 is an octyl group, an octadecyl group or a phenyl group, R2 is a methyl group or a methylene group, and n is a positive integer.

Carbon dioxide adsorbent and method for producing the adsorbent

Abstract: A reversible adsorbent for carbon dioxide, which can be used as a decontaminant of closed environments, has been devised. It consists of a novel material formed by a surface modified silica gel with active sites for the chemisorption of CO 2 at room temperature combined with a clay which acts as an agglomerant in a preferred weight ratio of 80/20. The carbon dioxide is liberated under heating at 110° C. The surface area of the material ranges between about 120 and about 240 m 2 /g with a total pore volume between about 0.4 and about 0.8 cc/g where about 80% of the total pore volume contains pores of a diameter between about 60 Å and about 180 Å. The material is capable of adsorbing between about 7.5 and 11.7 cc of dry CO 2 per gram and can be regenerated between about 70° and about 150° C. without losing any adsorptive capacity. Dry carbon dioxide is chemisorbed as a surface ammonium carbamate which becomes a surface bicarbonate in the presence of water.

An organic/inorganic hybrid polymer

Abstract: Poly(N-acetylethylenimine) (polyoxazoline) (POZO) with a terminal triethoxysilyl group was successfully synthesized by the ring-opening polymerization of 2-methyl-2-oxazoline followed by termination with 3-aminopropyltriethoxysilane. Triethoxysilyl-terminated telechelic POZO was prepared by using a bifunctional initiator. These silane coupling POZOs were subjected to acid-catalyzed cohydrolysis polymerization with tetraethoxysilane by the so-called “sol-gel” method to produce a novel organic/inorganic hybrid polymer (block copolymer), which was a homogeneous transparent/glassy composite material. The obtained hybrid showed higher hydrophilic properties compared with silica gel without POZO segments. On the other hand, a hybrid polymer consisting of poly(2-ethyl-2-oxazoline) and silica gel, which absorbed both water and organic solvents, showed amphiphilic properties. POZO segments were eliminated by pyrolysis of the present hybrid polymer to produce a silica with micropores.

Preparation of inorganic/organic hybrid gels by the sol-gel process

Abstract: Preparation of inorganic/organic hybrid gels was carried out using tetraethoxysilane and organic polymers via the sol-gel process. Hydroxyl terminated siloxane polymers were reactive enough to be incorporated into the silica networks. However hydroxyl terminated organic polyethers failed to react; thus most of them were extracted by organic solvents. Induction of triethoxysilyl groups were found very effective to prepare truly hybrid gels by the sol-gel process.