Viscoelastic lecithin reverse micelles: A simple model system for equilibrium polymers?

Abstract

We report on a detailed study of lecithin reverse micellar solutions with novel polymer-like properties using dynamic (QLS) and static (SLS) light scattering and rheological measurements. The QLS and SLS experiments show a pronounced water-induced micellar growth from small and almost spherical reverse micelles at w ₀=2.0 to giant polymer-like reverse micelles at w ₀ ≥ 8.0 for lecithin/cyclohexane w/o-microemulsions. An excellent agreement between polymer theory for dilute and semidilute solutions and the light scattering results was found. At volume fractions Φ < < Φ*, where Φ* is the overlap threshold, the scattering data is in good agreement with the worm-like chain model. At Φ > Φ* a viscoelastic network is formed, and properties such as the static (ξ_s) and hydrodynamic (ξ_h) correlation length or the osmotic compressibility exhibit universal behavior which is in agreement with scaling theory and experimental results from polymer literature.

While the dependence of the zero shear viscosity η_s on Φ can be explained by the ‘living polymer’ model, the frequency-dependent measurements of storage and loss moduli G′ and G″ are in clear disagreement with the single exponential stress relaxation predicted by this model. The frequency-dependent measurements could indicate the formation of crosslinks between the entangled micelles, a hypothesis further supported by the very large values for the “scission energy” found in a preliminary study of the temperature dependence of the micellar size distribution at low values of Φ.