GROWTH, BRANCHING, AND LOCAL ORDERING OF LECITHIN POLYMER-LIKE MICELLES

The viscoelastic properties of lecithin organogels formed by the addit ion of trace amounts of water in a n-decane solution were studied by m eans of oscillatory rheology. The viscoelasticity of jelly-like phases of this sort is caused by a transient three-dimensional network consi sting of entangled cylindrical reverse (polymer-like) micelles. It is shown that although the organogel properties depend on the lecithin co ncentration, the phase and rheological behavior is mainly regulated by the polar additive. The homogeneous jelly-like phase exists for molar ratios (n(w)) of water to lecithin from 1.6-1.7 to 3.2-3.4. At n(w) r atios below 2.7-2.8, the scaling exponents of the main rheological par ameters - the zero shear viscosity, plateau modulus, and terminal rela xation time - are rather close to the theoretical predictions that fol low from a model by Cates. This means that the lecithin polymer-like m icelles are linear and flexible. At larger molar ratios the scaling be havior with the lecithin concentration is changed. The observed power law exponents for the main rheological parameters are in satisfactory agreement with those expected from a model of the branched (connected) cylindrical micelles. These findings suggest that the mechanism for t he growth of cylindrical micelles changes with increasing water amount ; at the initial stages there is uniaxial growth of linear micellar ag gregates, and then the polar additive induces their branching. First r esults on an jelly-like phase that separates from the homogeneous orga nogel when the molar ratio n(w) is over 3.2 are presented. It is estab lished that the phase separation results in a change in the rheologica l behavior. An intermediate region is found in which the loss and stor age moduli scale with frequency with an exponent of 1/2. Similar scali ng was previously observed only with polymers. Conceivable reasons for the square root frequency dependence of the dynamic moduli are consid ered. It is suggested that this scaling is caused by partial or local ordering of polymer-like micelles due to the significant decrease of t he organogel volume with the phase separation.