Binding of inorganic and organic cations by p-sulfonatocalix[4]arene in water: a thermodynamic study

We report a microcalorimetry study of the association of various inorganic and organic cations with p-sulfonatocalix[4]arene at 298.15 K. First, we have studied the electrostatic attraction between host 1 and seven rare-earth-metal cations representative of the whole lanthanide series (La3+, Nd3+, Sm3+, Eu3+, Gd3+, Dy3+ and Yb3+) in an acidic solution (pH 2). In order to compare the charge effects in the electrostatic interactions we have also studied the attraction between host 1 and two alkaline-earth-metal cations (Ca2+ and Mg2+). Next, we investigated the binding of a series of quaternary ammonium cations in an acidic solution (pH 2). For each system, both the apparent association constant and enthalpy of complexation have been extracted from the calorimetric data. In all cases, our results are consistent with the formation of 1:1 complexes. Whereas for the inorganic cations (alkaline-earth-metal and lanthanide cations) the association process is enthalpically unfavoured (Delta H-r degrees >0) and entropically favoured (Delta S-r degrees >0), the complexation is driven by a favourable enthalpy change for the organic cations. These thermodynamic properties show that the organic and inorganic cations bind in very different modes.