Thermodynamics of the complexation of cerium-, europium- and erbium-(III) with 1,4,10-trioxa-7,13-diazapentadecane-N,N′-diacetic acid and 1,4,10,13-tetraoxa-7,16-diazaoxa-cyclooctadecane-N,N′-diacetic acid

Abstract

Enthalpy changes for the complexation of Ce³⁺, Eu³⁺ and Er³⁺ with two cyclic diaza diacetates 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid (H₂L¹) and 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-N,N′-diacetic acid (H₂L²) have been obtained by isoperibol titration calorimetry. From these values and corresponding Gibbs energy data, the entropy changes for these reactions were derived. The results show that complex formation is stabilized in each case by entropy changes. There is a favourable enthalpy contribution for interactions of Ce³⁺ and Eu³⁺ with H₂L². There is a direct correlation between the entropy contributions and enthalpy terms for the complexes of both the ligands. The results are consistent with an encapsulated model for complexes of H₂L² and a cis-carboxylate model for complexes of H₂L¹.