Saxs study of silica gel formation from aqueous silicate solutions

Abstract

In situ SAXS measurements of silica gel formation in the" presence of different cations are presented as a function of reaction time. Conditions used are similar to those applied to prepare catalyst supports. The data can be interpreted in terms of aggregation of elementary silica particles with sizes of approximately 1 nm, thus finally forming a continuous silica gel network. The aggregates are fractal with respect to their masses, in the range 2-10 nm. The dimensionality of the fractals can be determined from the SAXS curves; it increases from 1.8 to 2.5 during gelformation, corresponding with a diffusion limited aggregation process (DLA) of gel formation followed by densification of the silica gel network. The finding that the rate of change in fractal dimensionality does not change as a function of cation, indicates strongly that gelformation is a diffusion limited aggregation process.

Molecular Dynamics Simulations of the Polymerization of Aqueous Silicic Acid and Analysis of the Effects of Concentration on Silica Polymorph Distributions, Growth Mechanisms, and Reaction Kinetics

“…Sol-gel processing has been extensively studied experimentally [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and with theoretical methods. These studies have provided valuable knowledge about the structures of sol-gel materials.…”

“…Sol−gel processing has been extensively studied experimentally − and with theoretical methods. − These studies have provided valuable knowledge about the structures of sol−gel materials. In this work, we will focus on the oligomerization of aqueous silicic acid, which proceeds via a water-producing condensation reaction: The interactions between silicic acid and water have been studied using quantum-mechanical calculations at various level of theory. ,,,− ,− ,, It is widely believed that the condensation mechanism involves over-coordinated silicon intermediates for both acid-catalyzed and base-catalyzed reactions .…”

Aggregation and Gel Formation in Basic Silico−Calco−Alkaline Solutions Studied: A SAXS, SANS, and ELS Study

“…To understand the role of reaction conditions (temperature, pH, concentrations, salt addition) on the structure of the precursor sols and final gels, a number of investigations were performed. The mechanism of aggregation and the structure of the final gels were studied for pH values ranging from 2 to 10 which leads to a three-dimensional silica tetraedra network. , …”

“…The mechanism of aggregation and the structure of the final gels were studied for pH values ranging from 2 to 10 which leads to a three-dimensional silica tetraedra network. 3,4 Silica gels with high pH values, above 10, can be obtained by adding calcium ions to water glass solutions. 5,6 Provided silica concentration is high enough, a gel is obtained even under this strongly basic condition.…”

Volume fraction dependence and reorganization in cluster–cluster aggregation processes

“…[1][2][3][4][5][6][7][8][9] Scattering techniques such as small angle scattering ͑SAS͒ of neutrons or x rays, light scattering ͑LS͒, and microscopic experiments ͑In this paper chemical or physical experiments will be referred to as ''experimental.'' Computer models will be referred to as ''simulated'' or ''model.…”