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 ha ve studied the electrostatic attraction between host 1 and seven rare-earth -metal cations representative of the whole lanthanide series (La3+, Nd3+, S m3+, Eu3+, Gd3+, Dy3+ and Yb3+) in an acidic solution (pH 2). In order to c ompare the charge effects in the electrostatic interactions we have also st udied the attraction between host 1 and two alkaline-earth-metal cations (C a2+ 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 ap parent association constant and enthalpy of complexation have been extracte d from the calorimetric data. In all cases, our results are consistent with the formation of 1:1 complexes. Whereas for the inorganic cations (alkalin e-earth-metal and lanthanide cations) the association process is enthalpica lly 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.