Probing the Interactions of Calix[4]arene-Based Amphiphiles and Cyclodextrins in Water

The surfactant behavior of the water-soluble calix[4]arene 1, which has carboxylate groups at the upper rim and two phenyl groups at the lower rim, and its complexation to γ-cyclodextrin were studied. The critical micelle concentration (cmc) of 1 and of tetrapropoxycalix[4]arenetetracarboxylate 2 are 35 and 650 μM, respectively. The stabilities of the 1:1 inclusion complexes of γ-cyclodextrin with 1 and 2 are 1.3 × 10 4 and 1.5 × 10 3 M -1, respectively. The stability of the former complex was derived from a calorimetric titration under conditions where 1 is largely micellized. A minimization routine is described for determining the heat contributions of demicellization and complexation. The association of 1 to γ-cyclodextrin is enthalpy-driven, which, together with the high K value and the 1:1 binding stoichiometry, indicates that both pendant phenyl rings of 1 are included in the γ-cyclodextrin cavity.