Phase Behavior, Structure, and Physical Properties of the Quaternary System Tetradecyldimethylamine Oxide, HCl, 1-Hexanol, and Water

The phase diagram of the ternary surfactant system tetradecyldimethylamine oxide (TDMAO)/HCl/1-hexanol/water shows with increasing cosurfactant concentration an L1 phase, two Lα phases (a vesicle phase Lα1 and a stacked bilayer phase Lαh), and an L3 phase, which are separated by the corresponding two-phase regions L1/Lα and Lα/L3. In this investigation, the system was studied where some of the TDMAO was substituted by the protonated TDMAO. Under these conditions, one finds for constant surfactant concentration of 100 mM TDMAO a micellar L1 phase, an Lα1 phase (consisting of multilamellar vesicles), and an interesting isotropic L1* phase in the middle of the L1/Lα two-phase region. The L1* phase exists at intermediate degrees of charging of 30–60% and for 40–120 mM TDMAO and 70–140 mM hexanol concentration. At surfactant concentrations less than 80 mM the L1*-phase borders directly on the L1 phase. The phase transition between the L1 phase and the L1* phase was detected by electric conductivity and rheological measurements. The conductivity values show a sharp drop at the L1/L1* transition, and the zero shear viscosity of the L1* phase is much lower than in L1 phase. The form and size of the aggregates in L1* were detected with FF-TEM and SANS. This phase contains small unilamellar vesicles (SUV) of about 10 nm and some large multilamellar vesicles with diameters up to 500 nm. The system exhibits another peculiarity. For 100 mM surfactant, the clear Lα1-phase exists only at chargings below 30%. With oscillating rheological measurements a parallel development of the storage modulus G′ and the loss modulus G″ was observed. Both moduli are frequency independent and the system possesses a yield stress. The storage modulus is a magnitude larger than the loss modulus.