The synergistic adsorption of fatty acid and azacrown ether at the toluene–water interface

The interfacial tension of N,N-didecyl-diaza18crown-6 ether (22DD) and lauric acid (LAH) were studied at the toluene–water interface. 22DD was shown to be more surface active than LAH. The two-state reorientation model was applied to explain an unusual shape of the interfacial tension isotherm of 22DD, with areas per molecule of 322 and 42.7 Å2 for states 1 and 2, respectively. The pH of the aqueous phase has no influence on the interfacial tension of LAH below pH 6. The two surfactants were shown to co-adsorb due to the formation of hydrogen-bonded complex at the interface. As a result, the slope of the interfacial tension isotherm of the mixture decreases. The surface elasticity of the mixed monolayer decreases as the bulk molar ratio LAH/22DD increases.