Silica Gel Samples Research Articles

Influence of high pressure on scattering of light by xerogels

Abstract Scattering of light by dry silica gels has been measured under high hydrostatic pressure. The samples of silica gel were placed in a high-pressure optical gas chamber with argon as the working gas and illuminated by monochromated light of wavelength in the range 400–550 nm. The intensity of the scattered light was measured perpendicular to the incident direction. The range of applied pressure was 0.1–450 MPa. The intensity of scattered light decreases with increasing wavelength and pressure. The Mie theory is employed to explain the experimental results. The estimated mean value of the radius of the scattering centers, a , increases from 37 nm to 62 nm with pressure. At the same time, the concentration of the centers, N 0 , decreases from 2.6 × 10 18 m −3 to 2 × 10 16 m −3 . The agglomeration caused by high pressure is partially non-reversible.

Reactions of electrons generated in photolysis of pyrene on silica gel surface

Photoirradiation of degassed pyrene (Py)/silica gel samples at −196°C induces electron transfer showing Py + . Existence of photogenerated electrons (e tr -−) is confirmed by the ensuing reactions. Warming the sample initiates a recombination of e tr − with Py + showing thermoluminescence, and introduction of O 2 into the photoirradiated sample leads to the formation of O 2 −., observed by EPR. The time-resolved studies show short-lived Py + , indicating that most Py + , after initial generation, recombine with e tr −.

Silica gel analysis by slurry-sampling graphite-furnace atomic absorption spectrometry

Analytical conditions for the determination of cadmium, lead, copper, iron, aluminium and titanium in commercial silica gel samples by slurry-sampling graphite-furnace atomic absorption spectrometry are established. The influence of the silica matrix on the background absorption is described. Stability tests for slurries have been carried out. Results are calculated by the method of standard additions. Statistical evaluations of the results indicate that the slurry method is reproducible and its precision is comparable to other methods used. Advantages of this method are simplicity and short time of analysis.

Method of Discrimination of Surface Fractality

A simple method for the discrimination of surface fractality is proposed. This method is based on the comparison of the nitrogen adsorption isotherm on a given sample with the "standard" isotherm on a smooth surface. The proposed comparative method makes it possible to determine the lower limit of the scale range of surface fractality. A thermodynamic method is recommended as a complement to fractal analysis in the scale range of about 10-1000 Å. The results of the investigation of surface roughness are presented with examples of typical samples of silica gels, porous glasses, apatite, and coal.

Analysis of Porous Texture by Means of Water Vapor Adsorption Isotherm with Particular Attention to Lower Limit of Hysteresis Loop

The hysteresis loops of the adsorption isotherms of water vapor at 25°C have been examined on several porous adsorbents. The adsorbents used are five samples of porous silica gel pressed at different pressures, porous hematite, and porous titania. The lower closure point of the hysteresis loop for water vapor at 25°C was found to be situated at a relative pressure close to 0.27, which is lower than the relative pressure of the closure point (0.42) for nitrogen isotherm at 77 K. The pore radius at the closure point was found to be 0.9-1.1 nm for the water isotherm and 1.7 nm for the nitrogen isotherm. These radii are assumed to be the minimum pore radii determined from the capillary condensation of adsorptive. The water isotherm at 25°C gives more information on the porous texture than can be found by examination of the nitrogen isotherms at 77 K. The pore size distribution curves for the above adsorbents have been calculated from the isotherms of both water vapor and nitrogen gas.

Use of metal liquid-like films to generate nonconductive silver coatings

Metal liquid-like films (MELLFs) have high reflectivity and no conductivity in their liquid state. We report their use to generate coatings on glass slides, silica gels, and porous glass, with analysis by SEM and Raman spectroscopy. The coatings on glass slides are not conductive, but lose their glossiness. In contrast, the silica gel samples are both nonconductive and glossy, like the liquid-phase MELLF. Raman shows that the chemical nature of the film is not changed by drying onto the gels. SEM photos show how the preparatory methods affect the microstructure of the dried films. Finally, we compare Raman spectra from the silica gel samples with spectra of the native MELLF and with spectra of silver island films after exposure to the aqueous phase of the MELLF.

Effects of regeneration conditions on the characteristics of water vapor adsorption on silica gel

Equilibrium absorption and desorption isotherms of water vapor on a sillica gel were measured at 288, 298, and 308 K. Isotherms were of type I according to Brunauer's classification and provided a single characteristic curve when correlated according to Polanyl's potential theory. Literature data on different samples of silica gel exhibited different uptake capacity from each other and from the present data. The uptake capacity at 298 K did not change significantly when regeneration temperatures were varied in the range of 373-442 K, nor when the regenration time was increased from 4 to 16 h

Surface Alkoxylation of Silicas by Mild Reactions with Alcohols

Abstract The surface alkoxylation reaction of silicas was investigated under mild reaction conditions with some low-molecular-weight saturated aliphatic alcohols. Silica gels and porous silica glass were treated with liquid alcohols using a Soxhlet extractor below their boiling points. The number of formed alkoxyl groups was examined by both thermal analyses and infrared spectra. It was shown that some of the surface hydroxyl groups were easily esterified to alkoxyl groups, even by contact with the alcohols under mild reaction conditions. The degree of alkoxylation was higher for the silica gels than for porous silica glass. The adsorption isotherms of methanol vapor at room temperature were measured on silica gel samples which were previously activated at various temperatures. The adsorption equilibrium strongly depends on the pretreated temperature. The surface methoxylation reaction took place not only on the surface hydroxyl groups, but also on the siloxane bridges, even at room temperature.

Amorphous AlPO 4 as catalyst support 3. CO FTIR study of AlPO 4 impregnated with chromium

CO FTIR spectra of chromium/AlP0 4 samples give IR bands at 2192 (strong), 2212 (weak), and 2207 (weak) cm −1 at room temperature and after oxidation (O 2) and reduction (CO) at 500°C. After oxidation at 800°C and reduction at 350°C the main CO band at 2192 cm −1 decreases in intensity at low temperature by 20% and several groups of new CO bands at lower wavenumbers are observed [(i) 2124 and 2071 cm −1; (ii) 2153, 2095, and 2034 cm −1; (iii) 2121, 2109, and 2051 cm −1). These bands are interpreted as due to CO ligands bridging two chromium(II) ions of dinuclear chromium surface compounds bonded to the AlPO 4 surface through PO- groups. The ratio of dinuclear chromium(II) surface species to mononuclear ones is estimated from the decrease of the band around 2185 and 2192 cm −1, respectively (on cooling to low temperatures), as 3 for silica gel and 0.25 for AlPO 4. A weak CO band at 2202 cm −1 on chromium(III)/AlPO 4 samples is interpreted as caused by CO adsorbed on a chromium(III) surface species attached to AlPO 4 by AlO- and PO- groups. Converting this chromium(III) surface species to the chromium(II) one by oxidation at 800°C and reduction at 350°C gives two new CO bands at 2177 and 2167 cm −1 in addition to the one at 2192 cm −1. On the basis of an increase in the ratio of the integrated absorbance of the POH and AlOH groups and the opposite shift compared with CO bands on chromium(II)/silica gel samples, these IR bands are assigned to CO adsorbed on chromium(II) surface species attached to the AlPO 4 by AlO- groups.

Adsorption of CsOH on controlled porous glasses

Adsorption of CsOH has been studied for 18 samples of porous glasses prepared from different raw glass materials by means of different thermal and chemical treatment. For all the samples studied and for 3 samples of chromatographic silica gels the adsorption capacity in relation to CsOH is proportional to the BET surface area and equals 4 μmol of cesium per 1 m2 of BET surface.

FTIR-PAS analysis of silica gel modified with amines for the treatment of natural gases contaminated with sulphur compounds

Silica gel samples modified with different amounts of diethanolamine and monoethanolamine molecules are capable to adsorb H2S, CS2, SO2 and CH3SH. Once a complete coverage of the silica surface with amines through interaction with the surface hydroxyl groups has been reached an inactivation of the filter towards mercaptans occurs. FTIR-PAS spectra of the different silica samples are in agreement with these observations.

Nuclear magnetic relaxation dispersion measurement of water mobility at a silica surface

Nuclear magnetic relaxation rates are measured as a function of magnetic field strength corresponding to proton Larmor frequencies ranging from 0.01 to 42 MHz for silica gel samples with a nitroxide free radical covalently attached at the surface. The field dependence of the relaxation rate is interpreted using a translational model for the relaxation equation to yield a translational diffusion coefficient for the water, in the immediate vicinity of the radical attached to the surface, of 2.1 × 10 −6 cm 2 s −1 at 278 K for Si-4000 silica.

Quantitative Photoacoustic Spectroscopy: Determination of Fluorescein and Bromocresol Green Adsorbed on Modified Silica Gel Samples

A series of standards was prepared by mixing the fluorescein or bromocresol green adsorbed on silica gel modified with N,N-diethylaminopro-pyltriethoxysilane with varying amounts of unmodified silica. Photoacoustic magnitude and phase values were obtained at their respective absorption maxima, 470 and 600 nm. Calibration graphs were prepared by combining the magnitude and phase into the response function. Two-component samples were prepared by mixing the different ratios of the standard samples. The concentration of the dyes in the two-component samples was determined by referring to the respective calibration graphs and solving a pair of simultaneous equations. The relative error in the determination ranged from −5.6% to 5.1% for fluorescein and −16.8% to 38% for bromocresol green when compared to their concentration determined by spectrophotometry.

Quantitative photoacoustic spectrometry : Determination of copper(II) and iron(III) complexed on modified silica gel samples

Calibration curves for copper(II) and iron(III) complexed with N-2-aminoethyl-3-aminoprophyltrimethoxysilane (AEAPS) immobilized on silica gel were prepared separately by photoacoustic spectroscopy (p.a.s.) based on a response function that combines the amplitude and phase values. The concentration of copper and iron in two-component samples are determined by obtaining the response at 600 and 400 nm, respectively, and referring to the calibration graphs. The relative errors in the concentrations determined by p.a.s. compared to atomic absorption spectrometry for copper range from −9.3 to 3.2%, while for iron the range is −52.7 to 4.5%. The large error for iron was due to its low concentration and absorptivity. With suitable chromophores and matrix, the response function extends the linear range of the calibration graph and can be used for nondestructive determination of two components.

Thermal behaviour of silicagel investigated by emanation thermal analysis

The thermal behaviour of two different silicagel samples (PORASIL A, S = 300 m 2 g −1, Waters, U.S.A., and PRAGOSIL, S = 600 m 2 g −1, Nuclear Research Institute, Czechoslovakia) have been characterized by means of the emanation thermal analysis during heating in air at the constant heating rate. The temperature interval of the silicagel stability (950 – 1000 °C corresponds to the relaxation of the silicagel) was established.

Carbon-13 NMR and infrared spectroscopic investigations of acetone adsorbed on silica gel surfaces

Carbon-13 NMR and IR investigations were carried out for adsorption of acetone on partially dehydroxylated and on methylated silica gel samples. The appreciable downfield shift for the carbon-13 resonance line of the CO group on partially dehydroxylated silica gel (ca. 9 ppm with respect to adsorption on methylated silica gel at higher coverages) and the shift of the IR stretching vibration band of the surface hydroxyl groups ( Δ v ̄ OH = 370 cm −1 ) are due to a strong interaction of the acetone molecules with the surface hydroxyl groups. The four IR bands observed for the CO stretching vibration of acetone adsorbed on partially dehydroxylated silica gel are ascribed to interactions with geminal OH groups, with isolated (free) OH groups, with nonhydroxylic adsorption sites, and to the gaseous state. The number of interacting sites evaluated from the NMR shifts was 1.4/nm 2 and 0.3/nm 2 for partially dehydroxylated and methylated silica gel, respectively.

An improved procedure for sampling and analysis of dinitrotoluene vapor concentrations in workplace air.

Allied Chemical Corporation, Specialty Chemicals Division industrial hygienists required an accurate, convenient method for the sampling and analysis of dinitrotoluene vapor concentrations in the workplace air. A method has been developed which combines collection of dinitrotoluene vapor onto silica gel sample tubes, desorption in chloroform, and gas chromatographic determination of the various isomers of dinitrotoluene. Concentrations of less than 10 percent of the TLV can be determined in a 50 liter air sample.

Room temperature phosphorescene of nitrogen heterocycles adsorbed on silica gel

The room temperature phosphorescence (RTP) of benzo(f)-quinoline adsorbed on several silica gel samples was investigated by luminescence, reflectance, and infrared spectroscopy to obtain a better understanding of the analytical conditions needed for strong RTP. The results showed that silica gel chromatoplates containing a polymeric binder with carboxyl groups were the best samples for inducing strong RTP from benzo(f)quinoline. The polymer itself was essential for inducing strong RTP. It was postulated that benzo(f)quinoline was adsorbed flatly on the surface with the carboxyl groups anchoring benzo(f)quinoline via hydrogen bonding with ..pi.. electrons. 5 figures, 4 tables.

Industrial silica sorbents for the gel chromatography of polymers

A collection of non-swelling sorbents, i.e. macromolecular sieves, has been compiled from amongst the industrial silica gel samples and aerosils (silchromes); these have pore sizes ranging from 30 to 200 Å and are suitable for the gel chromatography of polymers. A study has been made of their pore structure and chromatographic properties. The possibility of using these sorbents for the fractionation and separation of polymers with a molecular weight ranging from 1 × 10 3 to 2 × 10 6 is being demonstrated.

Physico-chemical and gas-solid chromatographic evaluation of surface-modified silica gels

Silica gel samples coated with copper(II) tetra-ammonium sulphate, copper(II) bisethylenediamine sulphate and copper(II) bistriethanolamine sulphate were investigated by differential thermal analysis and thermogravimetric analysis. Surface area measurements were made in order to determine the influence of loading on the surface area. Gas-solid chromatographic studies were then carried out in order to evaluate silica gel and surface-modified silica gels as potential adsorbents in gas chromatography. Graphs of retention time against percentage loading of copper were plotted at different temperatures for a large number of gases and vapours. Some possible gas chromatographic separations that can be achieved with the use of surface-modified silica gels as stationary phase are also indicated.