Study of the adsorption properties of the surface of Silochrom S-80 modified with metal alizarinates

Abstract

In the analytical practice of gas chromatography, adsorbents based on silicon dioxide are widely used for the separation and concentration of mixtures of organic compounds of different compositions. To solve specific problems, we need new sorption materials with attached functional groups, layers, or elements. By varying these features, the number and types of adsorption centres of the parent silica can be changed. Chelate complexes of transition metals, including those impregnated in the form of adsorption layers, are considered to be particularly promising modifying additives. In this work, we obtained and studied by physical and chemical methods silica based on Silochrom S-80, which was modified with nickel, copper, and cobalt alizarinates. Thermal analysis showed that copper alizarinate was stable up to 300°C, while nickel and cobalt alizarinates were stable up to 280°C. Using adsorption porosimetry, it was determined that the modification of silica resulted in a reduction of average diameters and total pore volumes. The values of the specific surface area decreased from 84 to 76 m²/g. The fixation of chelates on the surface of SiO₂ was proved by IR and Raman spectroscopy.

Using gas chromatography, we studied the adsorption of organic compounds (n-alkanes, nitromethane, heptene-1, aromatic hydrocarbons, ketones, and alcohols) subjected to various types of intermolecular interactions. The impregnation of SiO₂ with alizarin-containing chelates resulted in changes in the Henry adsorption constants upon varying the metal ion within the complex in the sequence of Ni²⁺˂Co²⁺˂Cu²⁺, as well as the heat and entropy of adsorption in relation to all the sorbates. Applying complex compounds on the surface of Silochrom S-80 contributes to a change in the contributions of specific interactions to the heat of adsorption. The highest values of ∆_{q̅dif,1(spec)} are characteristic of organic compounds prone to donor-acceptor and π-complex formation.

The analysis of the compensation thermodynamic dependences q̅_dif,1 - ∆S̅^S_1,c showed that entropy was the determining thermodynamic parameter for the adsorption of tested compounds on the surface of the chromatographic materials under study.