Langmuir Monolayers of calix[8]arene derivatives. Conformations and complexation of Alkaline Earth Ions at the Air-Water interface

Calixarenes are cavity shaped molecules made up of n phenolic units linked via alkylidene groups. They are very adaptable compounds: they can be chemically modified through substitutions at the aromatic cavity so that their conformational mobility and affinity towards ions can be easily modulated. In this paper, we report the complexation properties of four new calix[8]arene octa-amide derivatives at the air/water interface as a function of temperature and subphase composition. The studied derivatives are the octa-(N,N-diethylaminocarbonylmethoxy)-p-octa-octyloxy calix[8]arene (AmCAL8Octyl), the octa-(N,N-diethylaminocarbonylmethoxy)-p-octa-benzyloxy calix[8]arene (AmCAL8Bzl), the octa-(N,N-diethylaminocarbonylmethoxy)-p-octa-hydroxy calix[8]arene (AmCAL8H) and the octa-(N,N-diethylaminocarbonylmethoxy)-p-octa-(N,N-diethylaminocarbonylmethoxy) calix[8]arene (AmCAL8Am). All of them have amide groups at the lower rim, and are differently substituted at the upper rim with octyloxy, benzyloxy, hydroxy, and amide groups. Their affinity for some cations belonging to the Group II were investigated by recording surface pressure versus molecular area isotherms at air/water interface in the presence of different 1 M MCl2 subphases. M = Sr2+ and Ba2+. Limiting area values, compared to Corey-Pauling-Koltun (CPK) model and mechanical calculations, are discussed in relation to the orientation of these molecules at the air/water interface. The results support that the macrocyclic ring of calixarenes are oriented flat to the interface. Surface potentials were measured in order to get additional information on the calixarenes orientation and alkali complexation at the air/water interface. The analysis of the surface pressure versus molecular area results clearly indicated the ions complexation by the macrocycles in the sequence AmCAL8Am much greater than AmCAL8H > AmCAL8Bzl > AmCAL8Octyl. All the derivatives present the highest selectivity for the barium ion.