COMPLEXATION PROPERTIES OF P-TERT-BUTYLCALIX[6]ARENE HEXAMIDE IN MONOLAYERS AT THE WATER-AIR INTERFACE

p-tert-Butylcalix[6] arene, CAL6, has been functionalized by the intro duction of amide moieties (OCH2-CONEt(2)) on the six phenolic OH group s to give the corresponding hexamide, HXAM, which shows an interesting selectivity toward the guanidinium cation. The present study reports the behavior of HXAM monolayers at the water-air interface as a functi on of temperature and subphase composition. The parameters obtained fr om the spreading isotherms (limiting areas, A(0), compressibility modu li, C-s(-1) = (IIA)(partial derivative pi/partial derivative A), and c hanges in the Gibbs free energy upon passing from water to salt subpha se, Delta G(salt-water)) and the surface potential data show that the HXAM assumes a ''cone-shaped'' conformation. The molecules are in a pa rallel orientation at the water-air interface with the six amide group s in the water phase. Changes in the subphase from water to salt solut ions (NaCl, KCl, CsCl, guanidinium thiocyanate) affect the spreading i sotherms but not the surface potential behavior. The results are discu ssed in terms of the two-dimensional phases of the monomolecular films , the stoichiometry of the complexes with the cations, and the conform ations of uncomplexed and complexed HXAM molecules. The comparison of these results with those of the unsubstituted CAL6 behavior shows the high selectivity of HXAM monolayers toward the guanidinium cation.