Intermolecular complexation thermodynamics between water-soluble calix[4]arenes and diazacycloalkanes

Calorimetric titration experiments have been performed in pH 2.0 and 7.2 phosphate buffer solutions at 298.15 K to calculate the complex stability constants (KS) and thermodynamic parameters (ΔG°, ΔH°, and TΔS°) for the stoichiometric 1:1 inclusion complexation of water-soluble calix[4]arene tetrasulfonate (CAS) and thiacalix[4]arene tetrasulfonate (TCAS) with some diazacycloalkane guests, i.e. piperazine (1), homopiperazine (2) and 1,5-diazacyclooctane (3). The results indicated that complexes of CAS and TCAS with diazacycloalkane guests were enthalpy-stabilized, and an acidic environment was more favorable to host-guest complexation than a neutral one. CAS forms more stable complexes with guest molecules than TCAS due to the more favorable enthalpic gain.