THERMODYNAMICS OF THE COMPLEXATION OF CERIUM-(III), EUROPIUM-(III) AND ERBIUM-(III) WITH 1,4,10-TRIOXA-7,13-DIAZAPENTADECANE-N,N'-DIACETIC ACID AND ETRAOXA-7,16-DIAZAOXACYCLOOCTADECANE-N,N'-DIACETIC ACID

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 (H(2)L(1)) and -tetraoxa-7,16-diazacyclo-octadecane-N,N '-diacetic acid (H(2)L(2)) have been obtained by isoperibol titration calorimetry. From these values and corresponding Gibbs energy data, th e entropy changes for these reactions were derived. The results show t hat complex formation is stabilized in each case by entropy changes. T here is a favourable enthalpy contribution for interactions of Ce3+ an d Eu3+ with H(2)L(2). There is a direct correlation between the entrop y contributions and enthalpy terms for the complexes of both the ligan ds. The results are consistent with an encapsulated model for complexe s of H(2)L(2) and a cis-carboxylate model for complexes of H(2)L(1).