Silica structure studies

Polmanteer, K.E. and Lentz, C.W., Reinforcement studies. Effect of silica structure on properties and crosslink density. Rubber Chem. Technol., 48(5), 795-809 (1975). 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

Structural study by solid state NMR and powder X-ray diffraction of the pure silica chabazite through water intrusion-extrusion processes... 

In summary, TEM not only shows direct images of the mesopores, but also reveals a lot of information about the local structures of the individual particles. It is obvious that the future research programs in this field still more or less rely on the structural studies on a scale of nanometer to submicrometer and, as demonstrated above, TEM is one of the most powerful techniques for the studies of mesoporous silicas and related materials. 

The effect of solvent type and aminosilane concentration has been evaluated. The third component in the reaction system is the silica substrate. The surface of the silica gel carries the active sites for adsorption. The concentration of these sites varies with varying silica type, its specific surface area and pretreatment temperature. Additionally, surface adsorbed water has a clear effect on the reaction mechanism. Isotherm data, reported in the previous paragraph, only accounted for fully hydrated or fully dehydrated silica. The effect of the available surface area and silanol number remains to be assessed. Information on these parameters allows the correlation of data from studies in which different silica types have been used. In this part the effect of these parameters in the loading step is discussed. Silica structural effects on the ultimate coating, after curing, are evaluated in the next paragraph. 

Hybrid silica materials were prepared via a sol-gel pathway at pH 9. The influence of anionic surfactant (SDS) was studied by comparing templated materials (TbSn series) with hybrid materials obtained without surfactant (Tbn series). Two mechanisms of mesostructure formation can be considered as represented on Fig. 2. The pka of aminopropyl chain is about 10.6 in the reaction mixture propyl-amines are partially protonated. Electrostatic interactions between dodecylsulfate anion and NH and sodium cation neutralization may then occur, resulting in the condensation of the silica structure around surfactant micelles and aminopropyl groups at the surface of the pores. The other mechanism is SDS chains complex-ation by P-CD cavity, which wonld result in P-CD gronps located at the surface of the pores and aminopropyl less accessible, due to steric hindrance caused by P-CD bulky groups. A complete characterization of the prodncts and adsorption capacities will help nnderstanding the formation mechanism of mesoporons hybrid silica. 

Due to the substitutional disorder of the T-atoms, local geometrical parameters of the Brpnsted sites are not accessible from diffraction studies, the diffraction pattern reflects only an average geometry. If the concentration of the T-atom substitution is small, the average geometry can be quite close to the structure of an ideal all-silica structure and the distortion becomes more and more pronounced with an increasing amount of A1 (in zeolites) or of Si (in SAPOs). The most affected parameters are the T-0 distances and the Si-O-T bond angles. 

Complete descriptions of the particle beam, its operation, its experimental setup, and its utility in protein structural studies have been previously described. (8, 12). Relevant PB dimensions include a 25 pm diameter fused silica capillary for production of the aerosol spray, a 22 cm length desolvation chamber to remove solvent, a single stage momentum separator, and a nozzle-substrate distance of 5 mm. Particle beam deposits ranged in size from 20 pm to 100 pm in diameter, and averaged approximately 50 pm. Deposit were made onto a water insoluble calcium fluoride (CaFj) window (25 mm dia. x 2 mm) from International Crystal Laboratories (Garfield, NJ). 

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