Thermodynamic study of complexation of Zn(II)/L (L− = acetate, indomethacin and diclofenac anions) by isothermal titration calorimetry

In this work, a thermodynamic study of the Zn(II)/L systems (L− = acetate, AcO− or indomethacin anion, Indo− or diclofenac anion, Dic−) in ethanolic solution was carried out by isothermal titration calorimetry (ITC). Thermodynamic properties such as enthalpy (∆H), entropy (ΔS), Gibbs energy (ΔG) and formation constant (reported as logβi) associated with the complexation reactions were determined; the stoichiometry of the formed species was also found. Zn(II)/AcO− was used as a model system for the complexation of Zn(II) with indomethacin and diclofenac anions, because all of them coordinate through the carboxylate functional group. To determine the thermodynamic properties of each system under study, from the experimental results, a binding model has been devised to calculate the heat Qcalc that is released or absorbed in terms of the molar ratio rL/ZnII\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$r_{{{\text{L/Zn}}\left( {\text{II}} \right)}}$$\end{document} in the complexation process between Zn(II) and L−. Calculated data (Qcalc) are adjusted to the ITC experimental results (Qexp), by means of MicroCal PEAQ-ITC analysis software to determine the enthalpy and formation constants of the MLj(2−j) formed complexes. Distribution diagrams of the fractions of the M and L species were obtained as a function of the molar ratio to discuss the predominance of the species in the systems as the titration was carried out.