Role of counterion concentration in determining micelle aggregation: Evaluation of the combination of constraints from small-angle neutron scattering, electron paramagnetic resonance, and time-resolved fluorescence quenching

Small-angle neutron scattering (SANS) has been used to study a series of aqueous solutions of (i) the anionic surfactant sodium dodecyl sulfate and the simple electrolyte sodium chloride and (ii) the cationic surfactants dodecyltrimethylammonium bromide and dodecyltrimethylammonium chloride, in the presence of sodium bromide and sodium chloride, respectively. For all three systems, the surfactant spans a wide concentration range, and by suitable choices of electrolyte concentration, it is possible to engineer these solutions to have the same concentration of counterions in the aqueous phase. According to a recent hypothesis by Bales et al. (J. Phys. Chem. B 2001, 105, 6798), such a series is expected to produce micelles having the same aggregation number (N-agg), a point verified by time-resolved fluorescence quenching (TRFQ) and electron paramagnetic resonance (EPR). The SANS results presented here are in good agreement with the TRFQ and EPR studies. We therefore show unequivocally that the sizes and shapes of these surfactant micelles depend only on the free counterion concentration, a conclusion supported by conventional theories of micellization. Subsequently, for the SDS data, a comparison is made between fitting the SANS data to a model in which the aggregation number and degree of hydration are constrained and fitting them to a model in which these features are fittable parameters. In essence, both approaches yield the same conclusion. Such an approach is not possible with the cationic surfactants, as the contrast between the hydrated shell and the core is minimal.