MONOLAYER AND BILAYER STABILITY OF AMMONIUM AMPHIPHILES CARRYING AN AZOBENZENE UNIT - EFFECTS OF SUBSTITUENTS AT THE AZOBENZENE UNIT

Novel double-chained ammonium amphiphiles carrying one azobenzene moie ty (I) have been synthesized. The bilayer-stabilizing effect of differ ent azobenzenes (ABs) has been investigated by measuring the ''gel-to- liquid crystalline'' phase transition; temperatures of the formed bila yers in water using differential scanning calorimetry. It is found tha t the stabilizing effect of the azobenzenes strongly depends on the su bstituents at the aromatic ring. The stabilizing effect increases in t he sequence F < H < NO2 < CN < OCH3 < N=N-Ph, which cannot be correlat ed with the electron-withdrawing or -donating properties of the substi tuents. It is concluded that dipole-dipole interactions between the AB s are of minor importance for the overall stabilization of the bilayer . Instead, other factors determine the strength of the van der Waals i nteractions between the ABs. In the bilayers the ABs form H-aggregates as is observed by the blue shift of the UV absorption maximum, This b lue shift is not affected by the ''gel-to-liquid crystalline'' phase t ransition. Bilayers of I-N=N-Ph do not exchange monomers with vesicles of didodecyldimethylammonium bromide, whereas all other investigated bilayers do. Compound I-N=N-Ph forms monolayers at the water-air inter face, which are much more stable than the monolayers of the other inve stigated compounds.