Self-assembled Structures and Rheological Properties of Lecithin/Bile Salt Mixtures in Aqueous Solution
Date Issued
2015
Date
2015
Author(s)
Cheng, Chih-Yang
Abstract
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. Lecithin and bile salts are well known for their diversity of structure and ability to self-assemble in both polar and non-polar solvents. In this thesis, we focus on controlling micellar structures and rheological properties of the mixtures in water. We seek to obtain methods to control self-assembled structure and then to influence rheological properties. Two controlling methods are studied: (a) increasing electrolyte concentration in solution; (b) increasing concentration of lecithin in pure water. While our focus is self-assembly in water, we note that controlling rheological property of aqueous solution can largely enhance possibility for applications such as drug delivery and controlled release. In first part, we report a route for forming lecithin/bile salts wormlike micelles in water that are long enough to enhance the viscosity by more than three orders of magnitude. The wormlike micelles can even entangle into transient network and transform the solutions into viscoelastic fluids. In water, highly soluble bile salt molecules bind the headgroup of lecithin and stabilize the low-water-soluble lecithin. The molar ratio of bile salt to lecithin plays a key role in determining the shape and size of micellar structures. At a specific molar ratio and sufficient ionic strength that is tuned by the addition of electrolytes, such as NaCl and CaCl2, the mixed micelles grow longitudinally into long, flexible chains. We utilize rheology, cryogenic transmission electron microscopy and small-angle neutron and X-ray scattering technologies to study the rheological properties and self-assembly structures, and we propose a mechanism based on the change of molecular geometry caused by the insertion of bile salts and the addition of electrolytes to explain the micellar self-assembly. In the second part, we discuss the effect of lecithin concentration on self-assembly of the mixtures of lecithin/bile salts in water. We reveal an unusual biological hydrogel formed by mixing bile salts and lecithin at low bile salt/lecithin molar ratios in water. The gel can be prepared at a total lipid concentration as low as ~ 15 wt%. The solid-like property of the solutions was confirmed by dynamic rheological measurements. We used cryo-TEM and SAXS/SANS techniques to probe the self-assembled structure and clearly evidence that the gel is made up of jammed swollen multilamellar vesicles (liposomes), instead of typical fibrous networks found in conventional gels. A mechanism based on the strong repulsion between bilayers due to the incorporation of negatively charged bile salts is proposed to explain the swelling of the liposomes. In addition to gel, a series of phases, including viscoelastic, gel-like, and low-viscosity fluids, can be created by increasing the bile salt/lecithin molar ratio. Such a variety of phase behaviors are caused by the transformation of bilayers to cylindrical and spheroidal micelles upon the change of the effective molecular geometry with the bile salt/lecithin molar ratio.
Subjects
Lecithin
Bile Salt
Self-assembly
Viscoelastic Fluid
Gel
Type
thesis
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