Medium-property relationships and compensation effect in the adsorption of heteroaromatic compounds from aqueous acetonitrile solutions

Abstract

The thermodynamics of the adsorption of heteroaromatic compounds of the 1,3,4 oxadi azole and 1,2,4,5 tetrazine series from aqueous acetonitrile solutions was studied in Henry´s region under the conditions close to equilibrium using the dynamic method. The applicability of the adsorption retention models to the description of medium-property relationships at different temperatures was evaluated. The effect of the composition of the solution on the thermodynamic characteristics of adsorption was studied. The effect of the acetonitrile concen tration on the enthalpy and entropy of adsorption cannot be described by simple plots, since it is determined by the nature of the adsorbate. At the same time, the Gibbs free energy change of adsorption is characterized by a monotonic dependence on the acetonitrile concentration. Linear relationships between changes in the enthalpy and entropy of adsorption of the same substance from water-acetonitrile media of different compositions were established. The char acter of these relationships is determined by the adsorbate nature and explains the dependence of its thermodynamic characteristics of adsorption on the solution composition. The compen sation of the values of enthalpy and entropy changes in a series of the studied adsorbates measured for the constant composition of the solution was discovered. The concentration of acetonitrile significantly affects the character of this compensation.High performance liquid chromatography (HPLC) enjoys wide use today in both the practical analytical chemistry and physicochemical studies aimed at investi gating the chemistry of adsorbent surface and processes at interfaces. 1-3 Success in the separation of mixtures of compounds was achieved due to the application of highly selective adsorbents as stationary phases for HPLC. Among these are chemically modified silicas, porous graphitic carbon, hypercrosslinked polystyrenes, and metal organic frameworks. 4-12 A further development of analytical pos sibilities of HPLC and physicochemical foundations of its application for study of adsorption equilibrium in solu tions is impossible without the progress in the theory of adsorption selectivity found in liquid phase adsorption in Henry´s region. It is also necessary to understand how the structure and properties of all components of a complicat ed heterogeneous system solution-solid affect the ther modynamic characteristics of adsorption (TCA) from so lutions. 13- 16 It is known that among the most important parameters determining the values of TCA from solutions are the com position of the liquid phase and nature of its components. 13 Many works were published in which the influence of the mobile phase composition on retention in HPLC at the same temperature of the column was studied. 1,17 At the same time, the knowledge of dependence of the TCA on the solution composition is needed to predict the position of the adsorption equilibrium and to control adsorption selectivity under variable experimental conditions. It is known that linear relation...