Mixtures of Lecithin and Bile Salt Can Form Highly Viscous Wormlike Micellar Solutions in Water

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• 作者：Chih-Yang Cheng ; Hyuntaek Oh ; Ting-Yu Wang ; Srinivasa R. Raghavan ; Shih-Huang Tung
• 刊名：Langmuir
• 出版年：2014
• 出版时间：September 2, 2014
• 年：2014
• 卷：30
• 期：34
• 页码：10221-10230
• 全文大小：532K
• ISSN：1520-5827

文摘

The self-assembly of biological surfactants in water is an important topic for study because of its relevance to physiological processes. Two common types of biosurfactants are lecithin (phosphatidylcholine) and bile salts, which are both present in bile and involved in digestion. Previous studies on lecithin鈥揵ile salt mixtures have reported the formation of short, rodlike micelles. Here, we show that lecithin鈥揵ile salt micelles can be further induced to grow into long, flexible wormlike structures. The formation of long worms and their resultant entanglement into transient networks is reflected in the rheology: the fluids become viscoelastic and exhibit Maxwellian behavior, and their zero-shear viscosity can be up to a 1000-fold higher than that of water. The presence of worms is further confirmed by data from small-angle neutron and X-ray scattering and from cryo-transmission electron microscopy (cryo-TEM). We find that micellar growth peaks at a specific molar ratio (near equimolar) of bile salt:lecithin, which suggests a strong binding interaction between the two species. In addition, micellar growth also requires a sufficient concentration of background electrolyte such as NaCl or sodium citrate that serves to screen the electrostatic repulsion of the amphiphiles and to 鈥渟alt out鈥?the amphiphiles. We postulate a mechanism based on changes in the molecular geometry caused by bile salts and electrolytes to explain the micellar growth.