THE DIFFERENT PHASES AND THEIR MACROSCOPIC PROPERTIES IN TERNARY SURFACTANT SYSTEMS OF ALKYLDIMETHYLAMINE OXIDES, INTERMEDIATE CHAIN N-ALCOHOLS AND WATER

The ternary phase diagram of the dodecyldimethylamine oxide (DDMAO)-hexanol-water system is reported for low volume fractions of the surfactant. Besides the well known L1, L(alpha) and L3 phases surprisingly three more single-phase regions are observed in the phase diagram and called L1*, L3* and L(alpha)1. The L1* and L3* phases are optically isotropic, slightly turbid and viscous. The L(alpha)1 phase is birefringent and highly viscous. It is conceivable that these seemingly single-phase regions are in reality subphases of the normal L-(alpha) phase. With increasing cosurfactant concentration, the sequence of the phases is L1, L1*, L(alpha), L3*, L(alpha)h and L3. The last five phases all have a bilayer-type structure. The L3* and L1* phases consist of single and multilamellar vesicles. The L3* phase disappears at higher surfactant concentrations and two L(alpha) phases then border on each other but seem to be separated by a two-phase region. At low volume fractions of about 1%, both L(alpha) phases show iridescent colours which are due to interference of white light from the ordered bilayers. The colours of the L(alpha)h phase are much brighter than those of the L(alpha)1-phase. The L1*, L(alpha), and the L3* phases are viscoelastic phases with very long structural relaxation times. For all three phases both the storage and the loss moduli are about the same and independent of the oscillating frequency over several decades. The L(alpha)h and the L3 phases have a low viscosity. Some results are also given for the ternary system tetradecyldimethylamine oxide-hexanol-water. Results of electric birefringence and viscosity measurements are reported for the L1 phase. In some of these studies, increasing amounts of tetradecyldimethylamine oxide were substituted by the cationic surfactant tetradecyltrimethylammonium bromide. The L1/L1* phase boundary shifts linearly with increasing mole fraction of the cationic surfactant to a higher cosurfactant/surfactant ratio. The solutions with the mixed surfactants contain globular micelles in the absence of hexanol. With increasing hexanol concentration, a sphere-rod transition is observed and the rods grow in length with the hexanol concentration up to the phase boundary. The lengths of the rods at the L1/L1* phase boundary decrease with increasing molar ratio of the cationic surfactant. The growth of the rods with increasing hexanol concentration in the L1 phase is reflected in the viscosities of the solutions. These increase continuously up to the phase boundary for all surfactant mixtures which contain some cationic surfactants. In contrast, the viscosities and the rotational diffusion time constants of the rods in the L1 phase of the pure alkyldimethylamine oxides pass through a maximum with increasing hexanol concentration.