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 concen tration an L-1 phase, two L-alpha phases (a vesicle phase L-alpha 1 and a s tacked bilayer phase L-alpha h), and an L-3 phase, which are separated by t he corresponding two-phase regions L-1/L-alpha and L-alpha/L-3. In this inv estigation, the system was studied where some of the TDMAO was substituted by the protonated TDMAO. Under these conditions, one finds for constant sur factant concentration of 100 mM TDMAO a micellar L1 phase, an L-alpha 1 pha se (consisting of multilamellar vesicles), and an interesting isotropic L-1 * phase in the middle of the L-1/L-alpha two-phase region. The L-1* phase e xists at intermediate degrees of charging of 30-60% and for 40-120 mM TDMAO and 70-140 mM hexanol concentration. At surfactant concentrations less tha n 80 mM the L-1*-phase borders directly on the L-1 phase. The phase transit ion between the L-1 phase and the L-1* phase was detected by electric condu ctivity and rheological measurements. The conductivity values show a sharp drop at the L-1/L-1* transition, and the zero shear viscosity of the L-1* p hase is much lower than in L-1 phase. The form and size of the aggregates i n L-1* were detected with FF-TEM and SANS. This phase contains small unilam ellar vesicles (SUV) of about 10 nm and some large multilamellar vesicles w ith diameters up to 500 nm. The system exhibits another peculiarity. For 10 0 mM surfactant, the clear L-alpha 1-phase exists only at chargings below 3 0%. 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. (C) 2000 Academic Pres s.