A New Reverse Wormlike Micellar System:  Mixtures of Bile Salt and Lecithin in Organic Liquids

Abstract

Abstract:We report a new route for forming reverse wormlike micelles (i.e., long, flexible micellar chains) in nonpolar organic liquids such as cyclohexane and n-decane. This route involves the addition of a bile salt (e.g., sodium deoxycholate) in trace amounts to solutions of the phospholipid lecithin. Previous recipes for reverse wormlike micelles have usually required the addition of water to induce reverse micellar growth; here, we show that bile salts, due to their unique "facially amphiphilic" structure, can play a role analogous to that of water and promote the longitudinal aggregation of lecithin molecules into reverse micellar chains. The formation of transient entangled networks of these reverse micelles transforms low-viscosity lecithin organosols into strongly viscoelastic fluids. The zero-shear viscosity increases by more than 5 orders of magnitude, and it is the molar ratio of bile salt to lecithin that controls the viscosity enhancement. The growth of reverse wormlike micelles is also confirmed by small-angle neutron scattering (SANS) experiments on these fluids.

Structure of Diglycerol Polyisostearate Nonionic Surfactant Micelles in Nonpolar Oil Hexadecane: A SAXS Study

“…The ternary mixtures of water/Aerosol OT AOT /oils or water/lecithin/oils were the mostly chosen systems for the studies of reverse micelles so far [21][22][23][24][25][26] . In the studies of non-aqueous phase behavior of mono-and diglycerol fatty acid esters in a variety of organic solvents we have found evidences on the formation of various self-assembled structures in the latter systems 19 .…”

New Lecithin Organogels from Lecithin/Polyglycerol/Oil Systems

“…Although the relationship between the reverse structures and the CPP value has not been reported so far, it is thought that the structures of reverse aggregates change in the order reverse vesicle reverse worm-like micelle reverse spherical micelle, as the CPP value increases. Lecithin alone forms a sphere or ellipsoidal reverse micelles in oil, 13 and this means that the CPP value exceeds 1. It is important to clarify here that the CPP value must decrease slightly when a reverse spherical micelle transforms to a reverse worm-like micelle.…”

“…Here, reptation implies the mode of motion by which the entangled polymers gradually disentangle from the chain end under the influence of their own Brownian motion. In contrast, the rheological behavior of reverse worm-like micelles is determined by the balance between reversible breakage and reptation of the micelles, and single Maxwellian behavior is generally observed when the reversible breakage time is suffi ciently shorter than the reptation time 12,13 . If the viscoelastic behavior of reverse worm-like micelles follows the single Maxwell model, Eqs.…”

“…where L is the average length of the reverse worm-like micelle and τ refl ects the length of the reverse worm-like micelle 12,13 . G 0 is the plateau modulus, as calculated by Eq.…”

“…As mentioned above, it is well known that the relationship for expressed in Eq. 7 holds when the experimental data follow a single Maxwell model 12,13 . Here, τ rep increased as the length of the reverse worm-like micelle increased, and τ b increased as the breakage of reverse worm-like micelles became difficult.…”