The influence of surface tension of interstitial fluid on the structural and surface properties of silicon dioxide obtained from nepheline-containing raw materials
Authors: Maiorov Dmitriy V. 
Published: August 2024

Silicon dioxide (silica gel, amorphous silica, SiO2), which has a developed specific surface, is widely used in various industries - in the production of paper, elastomers, as a filler, as an active component in the production of building materials, in fibre optic technology, in the production of catalysts as their carrier, sorbents and desiccants, as a component of thermal insulation materials, etc. Also, it is used as a basis for special chromatographic columns, which make it possible to analyse objects that were previously inaccessible for research.
In sorption and catalytic processes, an important role belongs to the structural and surface properties of the materials used – the specific surface area (Ssp), pore volume (Vpore), etc., which, in turn, depend on the method of obtaining these materials.
The traditional method for obtaining such materials is the sol-gel process, which consists in mixing aqueous solutions of the initial reagents with the formation of framework-forming substances (particles) in a colloidal state, followed by the formation of a gel from them by the polycondensation reaction, which is then subjected to drying. At the same time, the surface tension of the liquid inside the pores possess a significant effect on the structural and surface properties of the resulting xerogels, which causes deformation and destruction of the structure (framework) of the xerogel, which, in turn, leads to a decrease in the values of both Ssp and Vpore.
One of the methods for preventing this phenomenon and obtaining materials with developed structural and surface properties is drying the obtained gels at a temperature equal to or higher than the critical one, at which there is no surface tension of the interstitial fluid. This allows eliminating the deformation of the initial structure of the gel framework, formed during the polycondensation of colloidal gel particles, during the drying process. However, this method requires increased energy and material costs associated with the use of high temperatures and special equipment for its implementation, which leads to a high cost of the obtained materials. The study presents the results of investigation of the effect of interstitial fluid surface tension on the structural and surface properties of silicon oxides (SiO2) synthesized using the method of acid decomposition of mineral raw materials (nepheline). The obtained samples were studied by chemical analysis, X-ray diffraction, BET and BJH. It was shown that the replacement of the aqueous medium in the pore space with an organic one before drying the synthesized SiO2 allows not only significantly (by ~2 times) to increase the capacity of the adsorption monolayer of products, but also affects the distribution of pore volume over their diameter. Based on the obtained values of the specific capacitance of the surface of SiO2 samples and changes in the Gibbs energy (ΔGo) in the sorption process, it was concluded that the method of obtaining samples did not provide significant effect on the physicochemical properties of their surfaces (type and number of sorption centres per unit surface) and mechanism of nitrogen sorption.