Phase behavior of salt-free catanionic surfactant aqueous solutions with fullerene C60 solubilized

A salt-free catanionic surfactant system, tetradecyltrimethylammonium laurate (TTAL), was constructed by mixing tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA). The H+ and OH- counterions form water (TTAOH + LA -> TTAL + H2O), leaving the solution salt-free. The phase behaviors at fixing the total surfactant concentration (c(TTAL)) to be 33.0 and 55.0 mmol L-1, respectively, were studied through varying the molar ratio of r = n(LA)/n(TTAOH) from 0.70 to 1.20. With an increasing value of r, one observed an L-1-region, an L-alpha/L-1 two-phase region with a birefringent L-alpha-phase at the top, and finally a single L-alpha-phase. The ability to solubilize a fullerene mixture of C-60 and C-70 of different phases in different regions was tested. The colloidal stability and phase behavior of different phases with embedded fullerenes were investigated as a function of r, c(TTAL), and weight ratio of fullerene to surfactant (W-F/W-TTAL). The 33.0 or 55.0 mmol L-1 zero-charged vesicle-phase at r = 1.00 could solubilize a considerable amount of fullerenes without macroscopic phase separation and obvious vesicular structure breakage. However, these colloidal solutions became unstable at lower concentrations of surfactants, and a precipitate would be observed at the bottom. The micellar (L-1-phase) solubilization at the TTAOH-rich side was less pronounced compared to the vesicular solubilization of the zero-charged vesicle-phase, and the solubilizing ability decreased at higher r values. In the L-alpha/L-1 two-phase region, a brown or dark-brown L-alpha-phase was usually found at the top of a colorless or yellowish L-1-phase, indicating that most of the fullerenes were embedded in the upper L-alpha-phase. The influence of fullerene incorporation on the property of the zero-charged TTAL vesicle-phase was also investigated, and evidence has been found that the system tended to be more fluid after fullerenes were incorporated into the hydrophobic microdomains of aggregates.