Thermodynamic characteristics of adsorption of some 1,3,4-oxadiazoles and 1,2,4,5-tetrazines from water-acetonitrile solutions on phenyl-bonded silica gel

Standard differential molar changes of the Gibbs energy, enthalpy, and entropy during adsorption in the Henry region of 17 derivatives of 1,3,4-oxadiazole and 1,2,4,5-tetrazine from water-acetonitrile solutions with acetonitrile concentrations of 20 to 80 vol % and a variation step of 5 vol % on silica gel with grafted phenyl groups at column temperatures of 313.15 to 333.15 K with at a step of 5 K are determined under dynamic conditions by high-performance liquid chromatography. The dependence of the thermodynamic characteristics of adsorption on the molecular structure of adsorbates is discussed. It is established that the absolute values of the changes in the Gibbs energy and enthalpy during adsorption as a whole increase with increasing Van der Waals surface area and volume of the adsorbate molecules, while the adsorption of most 1,3,4-oxadiazoles is characterized by a dependence close to isentropic. It is assumed that the determining role in the change of entropy during adsorption from solutions is played by the polarity of the adsorbate molecule and the conformational flexibility of its substituents.