The Rheology, Nanostructure and Formation Mechanism of Lecithin Reverse Wormlike Micelles Induced by Bile Salts
Date Issued
2011
Date
2011
Author(s)
Njauw, Ching-Wei
Abstract
Amphiphilic molecules self-assemble into reverse micelles in low-polar organic solvents. It has been reported that the addition of bile salts into lecithin organosols induces the formation of reverse wormlike micelles and the worms are similar to long polymer chains which entangle to form viscoelastic solutions. In this study, we further investigated the effects of four bile salts on the formation of lecithin reverse wormlike micelles, including sodium deoxycholate (SDC), sodium cholate (SC), sodium taurodeoxycholate (STDC) and sodium taurocholate (STC). We utilized rheological and small-angle x-ray scattering (SAXS) techniques to analyze the properties and structures of the reverse micelles. All the bile salts can transform the originally spherical lecithin reverse micelles into long wormlike micelles and their rheological behaviors can be described by the single-relaxation time Maxwell model. However, the abilities of the four bile salts to induce the worms are different and the order of efficiency is SC > SDC > STC > STDC. In addition to bile salts, we also studied the effect of bile acids and we found that bile acids can only cause the formation of short cylindrical micelles, which are not long enough to impart viscoelasticity. Furthermore, the temperature dependence of structures and rheological behaviors of lecithin/bile salts reverse wormlike micelles were investigated. Upon the increase of temperature, the lengths of the worms decrease dramatically, which in turn causes the decrease of plateau modulus G_p, relaxation time〖 τ〗_(R )and zero shear viscosity〖 η〗_0, while the radii of worms are almost independent of temperature.
The driving force for forming reverse wormlike micelles is the interactions between lecithin and additives. Different bile salts and bile acids have similar chemical structures, i.e. the steroid rings, except one of the functional group and the number of OH groups on the steroid rings. To explain why the abilities to induce reverse worms are different, we used Fourier transform infrared spectrometer (FTIR) to investigate the interactions between lecithin and the additives. We found that the interactions between bile salts and lecithin are the hydrogen bonds formed by the OH groups on steroid ring with the phosphate groups of lecithin molecule, while hydrogen bonds between bile acids and lecithin are mainly formed by the COOH groups of bile acids with the phosphate groups of lecithin. The interactions formed by different groups determine the position of the additives in the headgroups of lecithin. Along with different sizes of additive molecules, the effective critical packing parameter (CPP) of lecithin is altered differently, which we suggested is the reason responsible for the varying efficiencies of bile salts and bile acids to induce the formation of worms.
Subjects
lecithin
bile salt
bile acid
reverse micelles
wormlike micelles
rheology
Type
thesis
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