The thermodynamics of complexation of the ligand KELEX 100 with various metal ions.

Abstract

The experimental work conducted in this thesis was aimed at determining the thermodynamic quantities associated with the formation of complexes formed between various metal ions and KELEX 100, a ligand which is used commercially for liquid - liquid extraction. In order to accomplish this, the beats of protonation of the ligand KELEX 100 were determined calorimetrically at 25°C in a partially aqueous medium of75% (v/v) l,4-dioxane and at an ionic strength of 0.1 mol dm·3• Extraneous heat effects that usually accompany complex formation were accounted for by measuring each heat contribution separately. The complexation of the lead (II), cadmium (II) and nickel (II) ions with KELEX 100 was studied in 75%(v/v) l,4-dioxane medium. The enthalpies of complexation were measured calorimetrically and calculated using the program LETAGROP KALLE. These results were combined with the values of the Gibbs free energies available in the literature for these systems to yield the entropies of complexation. In all calorimetric determinations a constant ionic strength of 0.1 mol dm'3 and a temperature of 25°C was maintained. The calculated enthalpies and entropies are discussed in terms of a number of factors that affect the thermodynamics of the systems. These factors include the structure of the ligand molecule, the nature of the donor atoms, the degree of substitution on the ligand and the properties of the metal atom such as charge and size, and the nature of the solvent. The cumulative enthalpies and entropies of formation of the metal-ligand complexes are favourable, Le. complexation is accompanied by a decrease in enthalpy and an increase in entropy. However, the enthalpy changes contribute more to the stability of the complexes and hence are the driving forces for complex formation. In the case of the Cd(II) ion, the enthalpy and entropy changes are similar. The cumulative enthalpies of formation increase (Le., become more exothermic) in the order: Ni > Pb > Cd while the reverse order is found for the entropies of formation. The smaller enthalpy change for the Cd(II) complex is possibly due to the weak interaction between the 'soft' Cd(II) ion and the 'hard' KELEX 100 ligand while the decrease in entropy for the Ni(II) may be due to the loss of fewer solvent molecules from the Ni(II) hydration sphere on complexation with the ligand. The stepwise entropy and enthalpy changes for the formation of the ML+ complexes are dependent on the ionic radius of the metal ion. However, no linear correlations exist between the cumulative entropy or enthalpy changes of formation of the ML complexes and the size of the metal ion. The stepwise and cumulative enthalpies of formation appear to be largely independent of steric effects of the bulky alkyl substituent on the KELEX 100 ligand. The increased steric hindrance of the substituent decreases the cumulative entropy change for the formation of the Ni(II) complexes. However, in the case of the Pb(II) complexes, steric effects do not appear to affect the cumulative entropies of complex formation.