No abstract

The synthesis of organosilicas with a predetermined ratio of trimethylsilyl groups and silanols using trimethyloxysilane and acetic acid as a modifier and catalyst respectively was carried out in this work. It has been established that grafted trimethylsilyl groups influence the structural state of silica. The reducing of the surface tension and, consequently, the Laplace pressure in the SiO2 globules leads to an increase in the valence angle in the siloxane bridges and the length of Si -O-bond. This dimensional effect in the IR-spectra of modified silica is displayed by displacement of absorption bands in the short-wave region associated with asymmetric (a) and asymmetric-deformation (а + O Si O) vibrations of siloxane bridges.

The comparative analysis of atomic structure and morphology of fumed silica nanoparticles prepared under different synthesis conditions are studied using TEM, FTIR, quantum chemistry, and low-temperature nitrogen adsorption methods. It was determined that the structure of amorphous silica nanoparticles is formed by branched chain-like clusters of the length of 0.6-2.4 nm that correspond to proto-particles or nuclei of nanoparticles. A linear part of the smallest clusters is consisted of two tetrahedra SiO4 with common oxygen atom and oppositely directed vertices. The inter-tetrahedral average angle of Si ‒ O ‒ Si bonds is about 180˚. It is shown that textural porosity of fumed silica powder depends on the initial degree of aggregation of nanoparticles. The average mesopores (1 nm < R < 25 nm) volume is in the rage of 0.26-0.60 cm3∙g-1 for materials pretreated at different temperatures. It is found out that the mean mesopores radius decreases from 34 nm to 10 nm with decreasing average size of silica nanoparticles. The micropores (R < 1 nm) contribution to the total pore volumes is small for all materials (0.003-0.029 cm3∙g-1).

It was established that incomplete substitution of free silanols of fumed silica surface on (CH 3 ) 3 Si-groups leads to an increase in the ability of modified SiO 2 to adsorb Ba 2+ cations from an electrolyte solution. Silica with 48.3% of trimethylsilyl (TMS) groups adsorbs 1.8 mmol·g -1 of Ba 2+ cations from 0.01 M BaCl 2 solution, that in 3 times more than the adsorption of these cations by unmodified silica.It was found that the adsorption of cations by basic and modified silica is well described by the pseudosecond order Lagergren equation. The high adsorption activity of silica with chemisorbed TMS-groups is due to the formation of local regions with relatively large values of negative and positive electrostatic potentials in the vicinity of grafted TMS-groups.