Interfacial tension studies of crown ethers at air/water and hexane/water interfaces

The adsorption of phase transfer catalysts, 18-crown-6 and dicyclohexano-18-crown-6, at the air/water and the hexane/water interfaces were investigated. Interfacial tension sigma decreased by increasing concentrations of these compounds and therefore both of these crown ethers are accumulated at interfaces. The variation of sigma with concentration for both compounds follows the Szyszkowski equation very well, from which the values of saturated surface densities and interaction parameters have been evaluated. On the basis of occupied surface area of each molecule, the orientation of each of molecules at the air/water and the hexane/water interfaces have been proposed. The present results show that dicyclohexano-18-crown-6 has the higher tendency not only to dissolve into the hexane phase but also to adsorb at the hexane/water interface than 18-crown-6 and that the Starks extraction mechanism was suggested for the present phase transfer catalysis systems.