Counterion condensation on charged micelles in an aqueous electrolyte solution as studied with combined small-angle neutron scattering and small-angle X-ray scattering

Combined studies of small-angle neutron scattering and small-angle x-ray scattering have been carried out for the direct observation of counterion condensation on charged micelles of two cationic surfactants, cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium chloride (CTACl) in aqueous electrolyte solutions. The addition of the electrolytes KCl and KBr in the CTABr and CTACl micellar solutions, respectively, yields an increase in the counterion condensation as well as the exchange of the counterions. It is found that the counterions in CTABr/KCl and CTACl/KBr micellar solutions are exchanged to the extent of maintaining the same concentration gradient for both the counterions (Br- and Cl-) around the charged micelles with respect to the solution.