Structure and composition of mixed surfactant micelles of sodium dodecyl sulfate and hexaethylene glycol monododecyl ether and of hexadecyltrimethylammonium bromide and hexaethylene glycol monododecyl ether

Small-angle neutron scattering has been used to study the structure and composition of mixed ionic-nonionic surfactant micelles. A comparison between two different mixed surfactant systems, sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C₁₂EO₆) and hexadecyltrimethylammonium bromide (C₁₆TAB) and C₁₂EO₆, both in 0.1 M NaCl, has been made. In the latter system, ideal mixing is observed, and in the former, departure from ideality, broadly consistent with regular solution theory, RST, is observed. The deviations from the predictions of RST are attributed to subtle changes in the packing of the two surfactants in the mixed micelles. For the SDS/C₁₂EO₆ mixture, the micellar aggregation number is essentially constant with composition and concentration, whereas for the C₁₆TAB/C₁₂EO₆ mixture there is a marked micellar growth with increasing concentration and mole fraction of C₁₂EO₆ in solution. The SANS results on structure and composition are compared with the results reported for other mixed surfactant systems and are discussed in the context of recent theoretical developments.