Complexation of macrocyclic ligands in ionic SDS micellar solutions: A dielectric spectroscopy investigation

The influence of two different macrocyclic ligands, [2.2.2] cryptand (C222) and 18-crown-6 (18C6) ether on the structural properties of sodium dodecyl sulfate (SDS) micellar solutions has been investigated by means of dielectric spectroscopy, in a frequency range from 1 kHz to 50 GHz. The dielectric spectra of SDS micellar solutions present three relaxation processes. In the MHz region, two different relaxations centered at about 30 MHz and 300 MHz, can be attributed to the micelle contribution, while, close to 20 GHz, the contribution of the bulk water orientational polarization is observed. The addition of ligands causes relevant changes in the overall dielectric relaxation spectrum. Significantly, the relaxation processes corresponding to the micelle component are largely modified by the presence of the different ligands investigated both in the relaxation frequency and in the dielectric strength, thus indicating a reorganization of the charged interface of ligand-decorated micelles. These changes are briefly discussed in the light of the phenomenology observed in attractive colloids, where the presence of a long-range electrostatic repulsive interaction and a short-range attractive interactions results in the presence of a well-defined cluster phase.