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.