Complexation properties of calixarenes in Langmuir films at the water-air interface

Calixarene molecules possess very interesting properties as ligands for ions and small organic molecules, and are used as models for host-guest systems. This paper reports the results of a study on the complexation properties of three calixarenes: p-tert-butylcalix[6]arene (CALIX6), p-tert-butylcalix[8]arene (CALIX8) and p-tert-butylcalix[6]arene hexamide (C6AM), in Langmuir films at the water-air interface, in the presence of Na+, K+, Cs+ and [C(NH2)3]+ (guanidinium) and in mixed monolayers with fullerene C60. This study shows that the calixarenes' monolayers are strongly affected by the subphase composition. The analysis of the spreading isotherms shows the high selectivity of CALIX6 and C6AM for caesium and guanidinium cations, respectively. The comparison of the limiting area values with CPK model calculation shows that CALIX6 and C6AM possess a rigid truncated-cone conformation, with different orientation at the water-air interface. The limiting areas found for CALIX8 monomolecular films agree with a perpendicular “pleated loop” conformation at the water-air interface. Electronic spectra on Langmuir-Blodgett films, and also spreading isotherms, show that the CALIX8-C60 1:1 complex is stable at the water-air interface. The complex is also formed after spreading an equimolecular mixture of CALIX8 and C60 on the water surface.