Counterion-mediated micellar growth for cetyltrimethylammonium 2,6-dichlorobenzoate alone and together with cetyltrimethylammonium chloride has been investigated using small-angle scattering techniques. Variations in counterion composition at constant ionic strength cause large changes in micelle size. The selectivity coefficient for adsorption of the aromatic counterion over chloride ions at CTA+ surfaces has been determined. The effects of the two counterions on micellar surface potentials, together with 1H NMR chemical shift data, allow inferences to be drawn about the penetration of the aromatic counterion into the head group region of the micelles. The aromatic ion increases the surfactant-packing parameter by both increasing the average volume per surfactant monomer, a cosurfactant-like effect, and decreasing the area per head group by screening electrostatic repulsions. Counterions such as chloride, which show only surface adsorption, affect only area and are much less effective at driving micellar growth. Some comparisons are also made with literature data for CTABr.

Aqueous salt solutions containing elongated micelles of sodium dodecyl sulfate (SDS) have been studied at 45 °C using small-angle neutron scattering. At NaCl concentrations in the 1-2 M range, the scattering data are consistent with semiflexible rather than rigid rodlike micelles, and this conclusion is consistent with previous studies of SDS micelles using small-angle scattering. However, until now quantitative determinations of micellar flexibility, that is, of persistence lengths, especially as a function of ionic strength, have not been available. By making measurements in the range of scattering vectors appropriate for this length scale and by incorporating the effect of intermicellar interactions into the fitting protocol, it has been possible to determine persistence lengths accurately. Partitioning of the values into intrinsic and electrostatic components is discussed, and the results are compared with data for the highly flexible anionic polyelectrolyte sodium polystyrenesulfonate. The one-dimensional bending moduli for the flexible SDS micelles are compared to the analogous two-dimensional bending moduli for SDS bilayers.