Micellar Solutions of Octyl-18-crown-6

Abstract

The solution properties of the novel surfactant octyl-18-crown-6 (C8Φ6) have been investigated with phase diagram, light-scattering, and surface tension measurements. This tenside is made up of an octyl hydrophobic chain attached to a 18-crown-6 ether and cyclic polar head-group that is highly selective for complexing K+ ions. The results show that C8Φ6 produces micellar structures in water and in KCl aqueous solutions, with a radius of about 30 Å. Addition of potassium ions in a molar ratio of 0.4 (KCl/C8Φ6) increases the hydrophilicity of the surfactant, as shown by the increment of the phase separation temperature, and produces a slight shift in the critical micellar concentration value, as it results from surface tension measurements. The intermicellar interaction parameter α, obtained from QELS experiments, has been discussed. Pure C8Φ6 micelles experience attractive intermicellar interactions overimposed to the hard-sphere interactions, while the K+ complexation of the surfactant polar head-groups leads to a decrease of the average attractive component of the interaction. These results show that intermicellar interactions are selectively controlled and modulated by simply adding a proper salt that can be complexed by the polar head-groups of the surfactant.