Self-Assembly in Mixed Dialkyl Chain Cationic−Nonionic Surfactant Mixtures: Dihexadecyldimethyl Ammonium Bromide−Monododecyl Hexaethylene Glycol (Monododecyl Dodecaethylene Glycol) Mixtures

Abstract

The self-assembly of dialkyl chain cationic surfactant dihexadecyldimethyl ammonium bromide, DHDAB, and nonionic surfactants monododecyl hexaethylene glycol, C(12)E(6), and monododecyl dodecaethylene glycol, C(12)E(12), 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 L(beta) lamellar phase at higher surfactant concentrations. For nonionic-rich compositions, the microstructure is predominantly in the form of relatively small globular mixed surfactant micelles, L(1). At intermediate compositions, there is an extensive mixed (blv/mlv) L(beta)/L(1) region. Although broadly similar, in detail there are significant differences in the phase behavior of DHDAB/C(12)E(6) and DHDAB/C(12)E(12) as a result of the increasing curvature associated with C(12)E(12) aggregates compared to that of C 12E 6 aggregates. For the DHDAB/C(12)E(12) mixture, the mixed (blv/mlv) L(beta)/L(1) phase region is more extensive. Furthermore, C(12)E(12) has a greater impact upon the rigidity of the bilayer in the blv, mlv, and L(beta) regions than is the case for C(12)E(6). The general features of the phase behavior are also reminiscent of that observed in phospholipid/surfactant mixtures and other related systems.