The different phases and their macroscopic properties in ternary surfactant systems of alkyldimethylamine oxides, intermediate chain n-alcohols and water

Abstract

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(α) and L₃ phases, surprisingly three more single-phase regions are observed in the phase diagram and called L*₁, L*₃ and L(α)₁. The L*₁ and L*₃ phases are optically isotropic, slightly turbid and viscous. The L(α)₁ phase is birefringent and highly viscous. It is conceivable that these seemingly single-phase regions are in reality subphases of the normal L(α) phase. With increasing cosurfactant concentration, the sequence of the phases is L₁, L*₁, L(α)₁, L*₃, L(α)_h and L₃. The last five phases all have a bilayer-type structure. The L*₃ and L*₁ phases consist of single and multilamellar vesicles. The L*₃ phase disappears at higher surfactant concentrations and two L(α) 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(α) phases show iridescent colours which are due to interference of white light from the ordered bilayers. The colours of the L(α)_h phase are much brighter than those of the L(α)₁-phase. The L*₁, L(α)₁ and the L*₃ 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(α)_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/L*₁ 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/L*₁ 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 tip 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.