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)/H(2)O, 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/H(2)O system with equimolar TTAOH and LA to form the TTAOH/(CA + LA)/H(2)O 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 L(alpha) 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/H(2)O 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.
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, PR China.
Changcheng
C
Liu
Jingcheng
J
Hao