Influence of Cholic Acid on Phase Transition, Rheological Behavior, and Microstructures of Salt-Free Catanionic Surfactant Mixtures

The influence of a biosurfactant, cholic acid (CA), on the phase transition, microstructure, and rheological properties of two salt-free catanionic surfactant systems, (i) tetradecyltrimethylammonium laurate (TTAL) vesicular solution and (ii) a mixture solution of tetradecyltrimethylammonium hydroxide (TTAOH)/lauric acid (LA)/H2O, was investigated. The TTAL vesicular system remains homogeneous at 25 degrees C up to 9.8 mmol.L-1 CA. At low concentrations of CA, the addition of CA causes a decrease in the viscosity of TTAL vesicular solutions. Upon further addition, the viscosity increases. When LA is gradually substituted by CA in the TTAOH/LA/H2O system with equimolar TTAOH and LA to form the TTAOH/(CA + LA)/H2O system, the influence of the molar fraction of CA, x = n(CA)/(n(LA) + n(CA)), on the phase transition and rheological behavior was also investigated. With increasing x, the system changes from a birefringent La phase of equimolar TTAOH and LA into an L-1/L-alpha double-phase, and a single L-1 phase. In the birefringent L-alpha phase of x = 0.2, cryo-TEM observations demonstrate that vesicles and tube-like and branched tube structures coexist, indicating that the unique structure of CA has a significant influence on the phase transformations of the mixtures. At higher x value, x = 0.5, a dilute solution was obtained, with a lower viscosity comparable to water. However, at x from 0.25 to 0.5, the system exhibits characteristic worm-like micelles with shearing thinning behavior at low and high shear rates but a Newtonian fluid behavior at intermediate shear rate range. At x = 0.2, the sample separates into two phases. The upper phase also shows shear thinning behavior and a narrow plateau at intermediate shear rate. At x < 0.2, the system exhibits similar rheological behavior to the original TTAL vesicular solution. For comparison, an investigation of the cetyltrimethylammonium hydroxide (CTAOH)/LA/H2O system with long chain cationic surfactant is also reported. The results of these studies may provide a better understanding of biological membranes and the effects of biosurfactants on surfactant phase transitions.