Effect of various cations on the acidity of p-sulfonatocalixarenes

Photometric and pH-metric titration curves of p-sulfonatocalixarenes, Cn]ASO₃H (n=4, 6, 8), were measured in the presence of electrolytes of various cations. These titration curves revealed that the presence of tetramethylammonium (TMA⁺) and tetraethylammonium (TEA⁺) ions largely decreased pK_a values for Cn]ASO₃H (n=4, 6, 8), while alkali and alkaline-earth metal cations had small effects. Comparison of the pH dependence of absorption spectra for Cn]ASO₃H (n=4, 6, 8) with that for corresponding monomer, p-hydroxybenzenesulfonate, indicated that the small values of pK_a1 and pK_a2 observed for C8]ASO₃H were attributable to dissociation of its OH groups in this compound. The dependence of pK_a values for C4]ASO₃H and p-hydroxybenzenesulfonate on the concentration of NaCl was due to the difference in their activity coefficients before and after their deprotonation steps estimated on the basis of Debye-Hückel theory. These results suggested that Cn]ASO₃H (n=4, 6, 8) hardly formed stable complexes with Na⁺ or other alkali metal cations in aqueous solutions while Cn]ASO₃H (n=4, 6, 8) formed stable complexes with tetraalkylammonium cations. It was also shown that the p-sulfonatophenol or p-sulfonatophenoxide units in the calixarene interacted independently with ionic atmospheres formed around the phenol units.