SOL-GEL TRANSITION IN SIMPLE SILICATES II * (original) (raw)
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Sol-gel transition in simple silicates
Journal of Non-Crystalline Solids, 1986
Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.
HYDROLYSIS AND CONDENSATION OF SILICATES: EFFECTS ON STRUCTURE
The hydrolysis and condensation reactions of monomeric alkoxysilanes and organylalkoxysilanes utilized in sol-gel processing are reviewed. Both reactions occur by acid or base-catalyzed bimolecular displacement reactions. The acid-catalyzed mechanisms are preceded by protonation of OH or OR substituents attached to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Many of the observed structural trends are understood on the basis of the pH and [H20 ] dependence of the hydrolysis, condensation, and dissolution reactions.
Mediterranean Journal of Chemistry, 2020
The synthesis of geopolymers requires the formation of an irreversible gel. This gel is produced by the sol-gel process using sodium silicate and hydrochloric acid reagents. On the one hand, the experimental study of the gelling reaction shows that for concentrations of hydrochloric acid varying between 0.5-2.0 mol/L and concentration of sodium silicate varying between 1.0-6.0 mol/L, three reaction products are obtained. The first product is a clear solution, the second product is a reversible gel, and the last product is an irreversible gel. The products obtained are characterized by SEM, X-Ray Diffraction, FTIR spectroscopy and EDS Spectroscopy techniques. The microstructural analyzes of the two gels revealed that these obtained solids are almost similar whereas the structural and elementary analyzes show that the silica content in the irreversible gel is higher than the silica content in the reversible gel. On the other hand, the theoretical study of the reactivity of gelling, de...
Hydrolysis catalyst effect on sol–gel silica structure
Materials Letters, 1998
Silica samples were prepared by the sol-gel process varying the hydrolysis catalyst. The samples were characterized by Ž. X-ray diffraction and the radial distribution function was determined R.D.F.. Fourier transform infrared spectroscopy and thermal analysis were also used. It was found that OH-retention capacity as well as order in the material may be modified with hydrolysis catalysts. When the hydrolysis catalyst was NH OH the sample did not dehydroxylate even at 10008C.
Sol–Gel Synthesis of Microporous Amorphous Silica from Purely Inorganic Precursors
Journal of Colloid and Interface Science, 1997
the microporous material is deposited as a thin layer on the Aqueous sodium silicate solutions and hydrochloric acid were top of a porous support which consists generally of a similar used to prepare silica gels without any additives. In order to estabmaterial with a coarser pore structure. lish the optimum conditions for the preparation of microporous Amorphous silica, which can be obtained with one of the and monodisperse gels, the sol-gel reaction has been studied as highest known degrees of dispersion (700 m 2 /g, (1)), is a a function of pH, concentration of silicate, time, and temperature. promising candidate to be used as a microporous top layer. From 29 Si NMR spectroscopy it was found that polycondensation First studies in this direction have recently been reported sets on immediately after mixing the reagents, with a continuous and show that a silica top layer could be formed either by increase of the fraction of 4-connected silicate tetrahedra (Q 4) at deposition from the gaseous phase (2-4) or by sol-gel the expense of Q 1 and Q 2. Polycondensation continues beyond the gelation point for 1 £ pH £ 7, but not for 7 £ pH £ 10. Two techniques, the latter using either organic (5, 6) or inorganic mechanisms were also observed for the gelation kinetics which is (7-9) precursors for the polycondensation. However, the about second order with respect to [Si] for pH £ 7, whereas for resulting membranes had so far a poor definition with regard pH ú 7, the order increases rapidly with pH. Gels formed at pH to cracks, thickness, and pore size distribution, and almost ú 7 had an electrostatic nature and redissolved in water, whereas nothing is known about their temperature resistance. In the for pH £ 7, gelation was irreversible as expected for a gel where sol-gel techniques, these problems are obviously related individual colloid particles are linked by chemical (GSi{O{ to organic compounds (unavoidably present in the case of SiG) bonds. From nitrogen adsorption, it is found that the pore organosiloxane precursors, and so far always employed as size depends mainly upon pH, the finest pores being obtained for additives in the case of inorganic precursors) which can pH 2-3 (BET surface area 650-700 m 2 /g, porosity 35%, pore be expected to introduce important structural uncertainties radius 5-11 Å). ᭧ 1997 Academic Press during the drying and eventual calcination of the gel.
Physical-chemistry of sodium silicate gelation in an alkaline medium
Journal of Sol-Gel Science and Technology, 2011
The behavior of sodium silicate solutions in an alkaline medium has been studied in the 11.56-9 pH range by adding different amount of hydrochloric acid into a concentrated commercial solution ([Si] = 7 mol/L, Si/Na = 1.71, pH = 11.56). The formed products and their evolution during long ripening (up to 150 days) have been characterized by cryo-SEM, elementary analysis (ICP-AES), X-ray diffraction and surface area and relative density measurements. In the studied narrow ranges of pH (11.56-9) and silicon concentration (7-0.2 mol/L) four different situations have been observed: (i) a stable and clear solution, (ii) a reversible and transparent physical gel; (iii) a soluble white gel characterized by a significant contraction during ripening and (iv) an irreversible gel which presents a slow syneresis leading to a consolidate solid. The characterizations of the different solids, liquids and gels have shown that the observed behaviors were the results of the formation of nanometric soluble NaSi 1.87 O 4.24 particles and/or insoluble silica-like (NaSi 12.66 O 25.82 ) grains and of the contribution of a dissolution/precipitation mechanism.
In Situ SAXS Analysis of Silica Gel Formation with an Additive
The Journal of Physical Chemistry B, 2004
ABSTRACT Time-resolved small-angle X-ray scattering (SAXS) measurements performed during the formation of tetraethyl orthosilicate (TEOS) based silica gels in alcohol with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) as an additive are reported. The measurements reveal no discontinuity of the nanostructure at the gel point. A chemically induced spinodal phase separation is found to give a coherent picture of the collected data. Increasing the amount of EDAS induces the phase separation on a smaller length scale, which finally leads to a modified gel morphology. The SAXS measurements and the electron micrographs associated with the dry gels could be interpreted in terms of the suggested wet gel formation mechanism.
Aggregation profiles of silica sols in a sol-gel process
Materials Letters, 1993
The kinetics of aggregation of sols in a sol-gel system was followed by measuring the particle size profile of sols from the start of the chemical reaction. The particle size was measured using dynamic light scattering. The particle size profiles show three different regions. The first one corresponds to an unstable regime characterized by reproducible oscillations in the size of the ~01s. This is due to a wide variety of different dissipative structures formed during the early stage of the reaction because of the autocatalytic character of the polymerization reaction. The second region is a steady-state regime which is characterized by the fact that the particle size remains practically constant for a long period of time. In this regime, the relative scattered intensity grows in an approximately linear way with the reaction time, i.e. the number of sols is increasing with time. In the last region, the particle size profile shows a rapid increase of the size of sols, and this corresponds to gelation. The number concentration of sols is so large that they begin to touch each other to form the gel. This is the mechanism by which the system reaches gelation.
Synthesis and characterizations of silica nanoparticles by a new sol-gel method
Silica nanoparticles were synthesized by chemical methods from tetraethylorthosilicate (TEOS), polyethylene glycol 5% and hydrochloric acid 0.001 N. The sol-gel process was applied for the preparation of nano silica gel. This method is hydrolysis and condensation reactions of TEOS as precursor of silica. The optimal synthesis conditions for the preparation of silica nanoparticles were obtained and the produced silica nanoparticles were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that the silica nanoparticles were successfully formed. The prepared samples change from amorphous to α-crystallite phase. The XRD analysis indicated the amorphous structure of the synthesized silica nanoparticles while the SEM and TEM images exhibited monodispersedNano sized silica particles with a size about 34 nm. In this study, the soft process of sol-gel reaction is favourable from a view point of energy conservation. Additionally, the advantages of this technique were the purity of products and ability to control nanometer sized internal structure.