Characterization of Mixed Micelles of Sodium Dodecyl Sulfate and Tetraoxyethylene Dodecyl Ether in Aqueous Solution

The mixed micelles of sodium dodecyl sulfate (SDS) and tetraoxyethylene dodecyl ether (C₁₂E4 or Brij 30) were studied by surface tension, ζ potential, and fluorescence spectroscopy measurements. In the range of 0-0.5 mole fraction (x) of added Brij 30 to a 30mM SDS solution, the mixed micellar aggregation number (N_agg) changed from 65 to 171, and the ζ potential of the micelles varied from -60 to -15 mV. The change in size was acompanhied by a 2-fold increase in the internal micellar viscosity, indicating that addition of the nonionic surfactant with similar alkyl chain enhances the surfactant packing in the mixed aggregate. The reduction of the electrical surface potential causes a proportional decrease of the binding constant of the cationic dye acridine orange. The change in temperature from 15 to 45 °C of a solution with x = 0.5 results in a decrease in the N_agg from 175 to 128, but the SDS to Brij 30 monomers ratio remains constant. This fact is typical of ionic micelles, opposite to the usual behavior of pure nonionic micellar aggregates with an oxyethylene surfactant headgroup.