Shear-Induced Structural Transition and Recovery in the Salt-Free Catanionic Surfactant Systems Containing Deoxycholic Acid

Abstract

Shear-induced structural transition and recovery were studied by freeze-fracture transmission electron microscopy, cryogenic transmission electron microscopy (cryo-TEM), rheological measurements, and the variation of birefringent textures in the system of tetradecyltrimethylammonium hydroxide (TTAOH)/lauric acid (LA)/deoxycholic acid (DeCA)/H(2)O as a function of the molar fraction of DeCA [x = n(DeCA)/(n(DeCA) + n(LA))]. At x = 0.3, giant vesicles and planar lamellar structures were formed before exerting shearing. Shear thickening and large hysteresis loop are observed under shearing, indicating the occurrence of structural transition. Multilamellar and close-packed vesicles were determined by cryo-TEM observations. Exerting further shearing, the elastic properties of the sample were increased due to the strip off of the outer shell of the vesicles and formation of small vesicles. After shearing was stopped, the sample can slowly relax back to the original state. At x = 0.25, lamellar structures, giant vesicles, and small unilamellar vesicles were observed. Cryo-TEM observations show that the multilamellar vesicles can be formed after exerting shearing, and the sample cannot spontaneously recover to the original state. At x = 0.2, vesicles are dominant in the solution, and the aggregates structures are almost the same before and after shearing. Shearing can increase the elastic properties of the sample, which is ascribed to the strip off of the outer shells of the multilamellar vesicles, forming much smaller vesicles in the solution.