The self-assembly of dialkyl chain cationic surfactant dihexadecyldimethyl ammonium bromide, DHDAB, and nonionic surfactants monododecyl hexaethylene glycol, C12E6, and monododecyl dodecaethylene glycol, C12E12, mixtures has been studied using predominantly small-angle neutron scattering, SANS. The scattering data have been used to produce a detailed phase diagram for the two surfactant mixtures and to quantify the microstructure in the different regions of the phase diagram. For cationic-surfactant-rich compositions, the microstructure is in the form of bilamellar, blv, or multilamellar, mlv, vesicles at low surfactant concentrations and is in an Lβ lamellar phase at higher surfactant concentrations. For nonionic-rich compositions, the microstructure is predominantly in the form of relatively small globular mixed surfactant micelles, L1. At intermediate compositions, there is an extensive mixed (blv/mlv) Lβ/L1 region. Although broadly similar, in detail there are significant differences in the phase behavior of DHDAB/C12E6 and DHDAB/C12E12 as a result of the increasing curvature associated with C12E12 aggregates compared to that of C12E6 aggregates. For the DHDAB/C12E12 mixture, the mixed (blv/mlv) L β/L1 phase region is more extensive. Furthermore, C12E12 has a greater impact upon the rigidity of the bilayer in the blv, mlv, and Lβ regions than is the case for C12E6. The general features of the phase behavior are also reminiscent of that observed in phospholipid/surfactant mixtures and other related systems. © 2008 American Chemical Society.
