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®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.