Effect of electrolytes on the aggregation of sodium dodecyl sulphate: the interactions of different counterions with formed micelles

The aggregation numbers (N) of sodium dodecyl sulphate (SDS) micelles in solutions of different electrolytes have been determined by the fluorescence quenching method. The micropolarity of SDS micelles in the presence of the electrolytes has been obtained from the intensity ratio of the third to the first peak in the fluorescence spectrum of pyrene. The results have been discussed against the experimental data obtained from electrospray ionization mass spectrometry. As far as we know, this method has not been used hitherto for interpretation of micelle-ion interactions. So, the electrostatic interactions of SDS monomers with different cations have been studied by electrospray ionization mass spectrometry (ESI-MS). It has been found that in the presence of both NaCl and NH4CIO4, the ions mainly localised close to the surface of SDS micelles are the sodium ones, while in the presence of KCI or Mg(CIO4)(2), the ions mainly accumulating at the surface of SDS micelles and influencing the micelle size are K+ or Mg2+, respectively. Moreover, in the presence of KCI, a large number of the sodium counterions is also present at the micelle surface.