Showing papers on "Silica gel published in 1984"

The nature of the silica cage as reflected by spectral changes and enhanced photostability of trapped Rhodamine 6G

Abstract: Rhodamine 6G was embedded in a matrix of silica gel glass by the sol to gel technique. The special features of an inorganic oxide glass as a carrier of an organic dye are discussed in comparison with other solid environments, such as adsorption on powders and within porous glasses, thin films, and doping of plastic matrices. Among the advantages mentioned are photostability of the glass matrix, trapping of the dye molecule, and its total isolation from undesired interactions with its neighboring dye molecules, impurities, and photodecomposition products; nonleachability of the dye; the ability to reach stable very high dye concentrations; reduction of translational, rotational, and vibrational degrees of freedom of the trapped dye; good transparency down to the UV. Embedding R6G in the silica glass enables one to reach high concentrations without undesirable dye aggregation. Stokes shift is larger in the glass than in water. Photostability of the dye is higher in the glass than in water. A remarkable front-face fluorescence stability is observed. These observations, and a critical review of the literature, are used to elucidate the nature of the silica glass cage: it is suggested that it is a hydroxylic polar environment, though somewhat less polar than water. The rigidity of the cage is discussed in terms of required reorientation of the environment around an excited state.

Useful chiral packing materials for high-performance liquid chromatographic resolution. cellulose triacetate and tribenzoate coated on macroporous silica gel

Abstract: Cellulose triacetate and tribenzoate coated on macroporous silica gel showed excellent capabilities of chiral recognition as packing materials for high-performance liquid chromatographic resolution of various enantiomers. Chiral recognition by the cellulose triacetate was different from that by the microcrystalline cellulose triacetate.

The hydration of tricalcium silicate in the presence of colloidal silica

Abstract: Reactions of colloidal silica fumes with calcium hydroxide or hydrating tricalcium silicate (C3S) have been studied using calorimetry, chemical analyses, and scanning electron microscopy. Silica fume reacts immediately with calcium hydroxide forming a colloidal calcium silicate hydrate (C-S-H) similar to that formed by the hydration of C3S. When excess silica is present it reacts with C-S-H already formed to produce a new, highly polymerized C-S-H, having a very low C/S ratio (1.0). Silica fume accelerates the hydration of C3S, reduces the amount of calcium hydroxide formed by reacting with it, and slightly lowers the C-S-H ratio of the C-S-H formed by hydration. When large amounts of silica fume are present the formation of calcium hydroxide may be entirely suppressed and a highly polymerized C-S-H is formed. Silica fume is considered a good model for reactive pozzolans used in concrete.

Further investigations on Raman spectra of silica gel evolving toward glass

Abstract: In a previous investigation a continuous evolution of silica gel structure toward that of v-SiO 2 was verified by Raman spectroscopy. Modifications on the intensity of some Raman bands usually associated in the literature to “defects” of the structure were observed. In the present work the study has been extended to the transformation with temperature of silica gels obtained from tetraethoxysilane (TEOS) with different H 2 O/TEOS molar ratios. Raman spectra as well as thermogravimetric data are discussed in terms of the effect of this ratio on the local structure and the sintering process of silica gel.

Densification of monolithic silica gels below 1000°C.

Abstract: Changes in the microstructure of an acid-catalyzed silica gel have been studied through the gel to glass conversion temperature. The gel composition selected was known to produce a dense silica glass at 1000°C that was not distorted or cracked. Surface area, porosity and pore size distribution were measured. Shrinkage during constant rate heating was observed. Effects of heating rate and atmosphere on densification were noted. An optimum heating rate of 2.5°C/min, which is a balance between the temperature dependence of the viscosity and the temperature dependence of the dehydroxylated surface tension, was found to give the greatest shrinkage below 1000°C.

Surface photochemistry: the effect of surface modification on the photophysics of naphthalene and pyrene adsorbed on silica gel'

Abstract: The influence of heat treatment and of added coadsorbates on the surface photophysics of pyrene and naphthalene adsorbed on silica gel is reported When the silica gel surface is partially dehydroxylated, the distribution of pyrene or naphthalene is inhomogeneous, as indicated by multiexponential fluorescence decay and broad emission spectra Addition of polar coadsorbates renders the surface more homogeneous with singlet lifetimes approaching solution values The pretreatment and addition of coadsorbates, in the case of pyrene, also influence the relative importance of dynamic excimer formation vis-a-vis ground state association

Silica gel as an effective catalyst for the alkylation of phenols and some heterocyclic aromatic compounds

Abstract: t-Butylation du phenol, benzenediols, biphenylediol-4,4', furanne, thiophene et methyl-1 indole par le bromure de t-butyle en presence de gel de silice

Synthesis of sinterable SiC powders by carbothermic reduction of gel-derived precursors; and pyrolysis of polycarbosilane

Abstract: Silicon carbide powders were synthesized by 1) carbothermic reduction of carbon-doped silica gel derived from colloidal silica or methyltrimethoxysilane and 2) pyrolysis of polymeric SiC precursors. Powder characteristics, including sinterability, depended on the type and composition of the precursor. The colloidal silica-sucrose and -pitch SiC powders doped with boron and carbon were sintered to 95% to 96% of theoretical density at 2100C in argon. The microstructures of these sintered SiC bodies consisted of mixtures of elongated grains (up to 50 m) and equiaxed grains (approx. = 10 m) with residual pores (up to 8 m) at grain boundaries or inside grains. 12 references, 7 figures, 2 tables.

Polyethylenimide bound chromatographic packing

Abstract: The reaction product of silica gel or controlled pore glass and polyethyleniminopropyl trimethoxy silane suitable for use as chromatographic column packing.

Rapid quantitation and confirmation of aflatoxins in corn and peanut butter, using a disposable silica gel column, thin layer chromatography, and gas chromatography/mass spectrometry.

Abstract: A simple, rapid, and solvent-efficient method for determining aflatoxins in corn and peanut butter is described. Aflatoxins B1, B2, G1, and G2 were extracted from 50 g sample with 200 mL methanol-water (85 + 15). A portion of the extract was diluted with 10% NaCl solution to a final concentration of 50% methanol, and then defatted with hexane. The aflatoxins were partitioned into chloroform. The chloroform solution was evaporated, and the residue was placed on a 0.5 g disposable silica gel column. The column was washed with 3 mL each of hexane, ethyl ether, and methylene chloride. Aflatoxins were eluted with 6 mL chloroform-acetone (9 + 1). The solvent was removed by evaporation on a steam bath, and the aflatoxins were determined using thin layer chromatography (TLC) with silica gel plates and a chloroform-acetone (9 + 1) developing solvent. Overall average recovery of aflatoxin B1 from corn was 82%, and the limit of determination was 2 ng/g. For mass spectrometric (MS) confirmation, aflatoxin B1 in the extract from 3 g sample (20 ng/g) was purified by TLC and applied by direct on-column injection at 40 degrees C into a 6 m fused silica capillary gas chromatographic column. The column was connected directly to the ion source. After injection, the temperature was rapidly raised to 250 degrees C, and the purified extract was analyzed by negative ion chemical ionization MS.

Direct separation of 2-hydroxy acid enantiomers by high-performance liquid chromatography on chemically bonded chiral phases

Abstract: The resolution of 2-hydroxy acid enantiomers by the principle of ligand exchange chromatography on chemically modified silica gel is described. The phases were synthezised by fixing L-amino acids via 2-glycidoxypropyl-trimethoxysilane to silica gel. The highest enantioselectivity for 2-hydroxy acids was obtained by using L-hydroxyproline as fixed ligand and Cu(II) as complexing agent. A number of aromatic as well as aliphatic 2-hydroxy acids could be resolved on this sorbent.

The Processing and Characterization of DCCA Modified Gel-Derived Silica

Abstract: The production of small silica gel monoliths by the hydrolysis and polycondensation of tetramethyl orthosilicate (TMS) in a methanol solution is now a common procedure. Drying is generally done slowly in a methanolic atmosphere [1] or under hypercritical conditions [2]. The potential application of sol-gel technology in the production of materials for large structures requires a rapid processing time, which causes a problem due to cracking. For large scale space structures the materials produced also require a low molecular weight, a low densification temperature, control of devitrification and a wide range of physical properties and gel densities. These requirements are potentially met by multicomponent silica based gels [3]. To decrease the drying time, which is the longest part of gel processing, the gel strength needs to be increased to resist cracking and the drying stress reduced. The gel strength can be improved by optimizing the variables in the gel manufacturing process.

Effects of Different Stationary Phases and Surfactant or Cyclodextrin Spray Reagents on the Fluorescence Densitometry of Polycyclic Aromatic Hydrocarbons and Dansylated Amino Acids

Abstract: The detectable luminescence of twelve dansy1 amino acids and four polycyclic aromatic hydrocarbons (PAH's) spotted on five common TLC stationary phases was evaluated. The detectable luminescence varied appreciable for compounds associated with different stationary phases. The use of surfactant and cyclodextrin spray reagents caused luminescence enhancements on some stationary phases but not others. The reagents did not affect all compounds to the same degree indicating that qualitative information could be obtained in some cases. The largest luminescence increase for a compound spotted on silica gel was for pyrene (i.e., 47-fold) sprayed with sodium cholate. The degree to which the plates were dried also affected the luminescence intensity. Possible reasons for the observed effects are discussed.

Fixation of dissolved metal species with a complexing agent

Abstract: A process for removing metal species from solution comprising passing the liquid over a composition comprising a support such as a porous silicate glass or silica gel or charcoal having interconnected pores and containing water soluble amine complexing agents absorbed on the support capable of forming a stable complex with the metal species. The preferred amine complexing agent is triethylenetetramine. The process is especially useful for removing radioactive cobalt from liquid waste streams.

Application of thermal analysis for determination of film pressure and film surface energy on silica gel surface

Abstract: The results of dynamic and quasi-isothermal thermodesorption of water from a silica gel surface at low furnace heating rates in the temperature range 20–125° are presented. From the experimental results, the water film pressure π on silica gel surface, the activation energy ΔE and the evaporation heat ΔH were calculated. An interpretation of π changes in relation to the film thickness and wetting process has been proposed. It is concluded that the characteristic film pressure values correspond to the work of spreading, and immersional, adhesional and adhesional-cohesional wetting. From the determined film pressure values, the average value of the silica gel polar component, γ s P , was calculated to be 114.67 mJ/m2.