Dielectric relaxation spectroscopic studies of ionic surfactants in aqueous solutions

The aggregation of micelles is governed by a delicate balance between the reduction of the hydrocarbon-water interface and the coulomb interactions of the polar/ionic head groups with the counterions and the solvent. When polar organic solvents are introduced as cosolvents, the micelle structure is modified. We report here the frequency dependence of the permittivities and the relaxation time of the micellar solutions of sodium dodecyl sulphate (SDS), cetyl trimethyl ammonium bromide (CTAB) and dodecyl pyridinium chloride (DPC) in which acetophenone (ACP), pyridine (PYR), ethyl methyl ketone (EMK) and triethylamine (TEA) were introduced as cosolvents. Time domain dielectric spectra were obtained in the reflection mode in the frequency range 10 MHz to 20 GHz using a HP 54750A sampling oscilloscope and a HP 54754A TDR Plug-in module. The volume of the hydrated micelles was obtained through a measurement of viscosity. The relaxation time of bound water was calculated using the Cole-Cole method. The relaxation time gradually increased with increase of surfactant concentration. The hydrodynamical volume of the micelle increased with increase of concentration of the cosolvents and is almost twice that of the micelles without additives. On increasing the concentration of the cosolvents, the micelles become more anisotropic giving space for the further solubilization. The hydrated micellar volume increases on addition of cosolvents in the order ACP < PYR < EMK < TEA.