Kinetics, Thermodynamics, and Modeling of Complex Formation between Calix[4]biscrowns and Cesium

Complex formations between calix[4]arene-bis(crown-6-ether) calix-COU2 (A1) and the tetrasulfonated species calix-COUSULF (A2) with Cs+ are investigated in water and ethanol, and in 9:1 (M1) and 1:9 (M2) H2O/EtOH v:v mixtures, by chemical relaxation and molecular modeling. In ethanol and M2, two Cs+ are included in A1 in two kinetic steps, whereas complex formation in M1 becomes controlled by a slow first-order kinetic process, which is accompanied by very fast Cs+ inclusions, second-order rate constant: k′1 = (3.4 ± 0.8) × 107 M−1 s−1. In water and M1, A2 forms 1:1 and 1:2 cesium complexes in a single kinetic step, whereas in M2, two Cs+ are included in two kinetic steps. The rate and thermodynamic constants involved are reported. They show that the second-order rate constants increase with the ethanol-to-water ratio, e.g., A2, second-order rate constant for the first Cs+ in water: k 1A2water = (9.7 ± 0.3) × 104 M−1 s−1 and in M2: k 1A2M2 = (6.3 ± 0.4) × 109 M−1 s−1. The affinities of both A1 and A2 for Cs+ also increase with the ethanol-to-water ratio, e.g., first inclusion of A1 in M1: K 1A1M1 = (5 ± 1.3) × 103 and in ethanol: K 1A1EtOH = (7 ± 3) × 106. The deviation from the expected mechanism of complex formation with alkali is attributed to the comparatively more difficult access of Cs+ to the inclusion cavity of the capped calixarene. An analysis of calix-COU2 and calix-COUSULF and their Cs+ complexes with only one rim capped by the crown ether confirms the thermodynamic and kinetic results, by showing that the inclusion cavity of calix-COUSULF is more adapted to Cs+ than that of calix-COU2. This added to the presence of the shielding effect of the negative sulfonates can explain that the affinity of calix-COUSULF for Cs+ is higher than that of calix-COU2. These results can be of interest in the search of an efficient Cs+ decontaminant.