title: Mixtures of lecithin and bile salt can form highly viscous wormlike micellar solutions in water.
abstract: 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-bile salt mixtures have reported the formation of short, rodlike micelles. Here, we show that lecithin-bile 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 "salt out" the amphiphiles. We postulate a mechanism based on changes in the molecular geometry caused by bile salts and electrolytes to explain the micellar growth.
authors.affiliation: Institute of Polymer Science and Engineering and ‡Instrumentation Center, National Taiwan University , Taipei 10617, Taiwan.
authors.forename: Chih-Yang
authors.initials: CY
authors.lastname: Cheng
authors.forename: Hyuntaek
authors.initials: H
authors.lastname: Oh
authors.forename: Ting-Yu
authors.initials: TY
authors.lastname: Wang
authors.forename: Srinivasa R
authors.initials: SR
authors.lastname: Raghavan
authors.forename: Shih-Huang
authors.initials: SH
authors.lastname: Tung
mesh_terms: D001647:Bile Acids and Salts / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D054709:Lecithins / Q000737:chemistry*; D008823:Micelles; D008855:Microscopy, Electron, Scanning; D012542:Scattering, Radiation; D012996:Solutions; D014783:Viscosity; D014867:Water / Q000737:chemistry
chemical_list: D001647:Bile Acids and Salts; D054709:Lecithins; D008823:Micelles; D012996:Solutions; D014867:Water