Solvent effect on crown ether complexation of Na+, Cs+ and Sr2+ions investigated by ion transfer voltammetry at a polarized water/organic solvent interface

Abstract

Ion transfer voltammetry of background electrolytes is used to complete the list of the values of the standard Gibbs energies of transfer of the Na⁺, Cs⁺ and Sr²⁺ cations and the Cl⁻ anion from water to benzonitrile (BN), nitrobenzene (NB), 1,2-dichloroethane (1,2-DCE) and α,α,α-trifluorotoluene (TFT), and the stability constants of the 1:1 or 1:2 crown ether complexes of the studied cations with dibenzo-18-crown-6 (B₂18C6) in the solvent listed. The results agree well with the available literature data or fit in the correlations previously reported. A strong solvent effect on the stability constant is observed, which is proposed to be associated with the solubility of water in the particular solvent, in contrast to current explanations referring to the donor number or the dielectric permittivity of the solvent, or the transfer Gibbs energy of the ion. Standard Gibbs energies of transfer of the B₂18C6 complexes from water to BN, NB, 1,2-DCE or TFT are evaluated with the help of a thermodynamic cycle, and compared with the transfer Gibbs energies of the 18C6 complexes determined by the present voltammetric measurements. Remarkable differences of about 20 kJ mol⁻¹ between the transfer Gibbs energies for the B₂18C6 and 18C6 complexes, which are probably connected with the presence of two aromatic rings on B₂18C6 macrocycle, are surprisingly not very sensitive to the nature of the ion or the solvent.