The Influence of Water-Acetonitrile Solvent Composition on Thermodynamic Characteristics of Adsorption of Aromatic Heterocycles on Octadecyl-Bonded Silica Gels

The thermodynamics of adsorption of some substituted 1,3,4-oxadiazoles and 1,2,4,5-tetrazines from water-acetonitrile solutions with acetonitrile volume fraction varying within a range of 0.35-0.90 on chemically modified Luna C-18 and Discovery C-18 silica gels has been studied within a temperature range of 313.15-333.15 K. The thermodynamic characteristics of the adsorption of these compounds on Luna C-18 and Discovery C-18 silica gels have been determined as depending on solution composition. In the case of Discovery C-18, the character of the dependences of different thermodynamic characteristics of the adsorption on liquid phase composition is similar to that of the corresponding dependences previously observed for modified Ascentis (R) Phenyl silica gel. A similarity has also been found for the dependences of the thermodynamic characteristics of the adsorption on Luna C-18 and modified Diasfer-110-C-18 silica gel studied previously. The mechanism of the liquid-phase adsorption of 1,3,4-oxadiazoles and 1,2,4,5-tetrazines on Luna C-18 is changed within a range of acetonitrile volume fractions of 0.60-0.75, as it is in the case of Diasfer-110-C-18. At acetonitrile volume fractions of higher than 0.75, the studied heterocycles are adsorbed on Luna C-18 surface dynamically modified with acetonitrile without a direct interaction with the grafted layer of the chemically modified silica gel. At the same time, the mechanism of their adsorption on Discovery C-18 remains unchanged while acetonitrile concentration is increased in the liquid phase.