Abstract
The first part of this work deals with the interactions of amino acids and neutral macrocyclic ligands (18-crown-6 and cryptand 222). Solubilities and Gibbs energies of solution of several amino acids and their complexes with 18-crown-6 in acetonitrile, as well as of some amino acid ester salts in chloroform have been determined at 298.15 K. An increase in the solubility data for the amino acid complexes relative to the free amino acids has been found, as a result of the interactions of the guest molecules with the macrocyclic ligands. Stability constants, Gibbs energies, enthalpies and entropies of complexation of a series of L- and D-amino acids with 18-crown-6 and with cryptand 222 in methanol at 298.15 K were obtained by titration calorimetry. The similar Gibbs energies of complexation found for these systems are the result of an enthalpy-entropy compensation effect. No significant differences have been found between the thermodynamic parameters of complexation of the L- and D-amino acids with these ligands in this solvent with corresponding data previously reported for DL- amino acids and the same macrocycles in methanol. 1H NMR studies on these systems have been carried out to gain a better understanding of the interactions involving amino acids with 18-crown-6 or with cryptand 222 in methanol. These NMR data are the first reported for these systems. The second part of this thesis concerns the interactions of calixarene macrocycles with organic solvents (neutral species). Solubilities and Gibbs energies of solution of p-tert-butylcalix(n)arenes (n = 4,6,8) in various solvents at 298.15 K are reported. Derived transfer Gibbs energy data from acetonitrile to various solvents reflect that the parent calixarenes are better solvated in most reaction media considered than in the reference solvent. Structural studies on calix(4)arene esters in chloroform, acetonitrile, benzonitrile and acetone at 298 K suggest that among these solvents, acetonitrile is likely to be included in the hydrophobic cavity of the calixarene esters. The final part of this thesis concerns conductimetric, thermodynamic and structural studies on the interactions of parent calixarenes and amines in benzonitrile. An important consequence of amine-calixarene interactions in solution is the generation of new electrolytes, as a result of a proton transfer reaction from the calixarene to the amine. Thermodynamic and structural studies reflect the distinctive properties of cryptands relative to cyclic and aliphatic amines; suggesting that cryptand 222 and cryptand 22 are likely to host the proton released from the calixarene in the cavity of the former ligand or in the hole of ciyptand 22. To conclude, the extraction of amines from aqueous solution by calixarenes is presented. It is shown that the transfer of amines from water to benzonitrile is favoured by the presence of calixarenes in the organic phase. Although the ion-pair formation in benzonitrile gives the main contribution to the extraction process, the selective extraction of amines by calixarenes is mainly controlled by the transfer Gibbs energies of these amines from water to the organic medium.