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 car boxylate groups at the upper rim and two phenyl groups at the lower rim, an d its complexation to gamma-cyclodextrin were studied. The critical micelle concentration (cmc) of 1 and of tetrapropoxycalix[4]arenetetracarboxylate 2 are 35 and 650 mu M, respectively. The stabilities of the 1:1 inclusion c omplexes of gamma-cyclodextrin with 1 and 2 are 1.3 x 10(4) and 1.5 x 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 mi nimization routine is described for determining the heat contributions of d emicellization and complexation. The association of 1 to gamma-cyclodextrin is enthalpy-driven, which, together with the high K value and the 1:1 bind ing stoichiometry, indicates that both pendant phenyl rings of 1 are includ ed in the gamma-cyclodextrin cavity.