Phase Behavior of Lecithin at the Oil/Water Interface

The viscoelastic properties of lecithin organogels formed by the addition of trace amounts of water in a n-decane solution were studied by means of oscillatory rheology. The viscoelasticity of jelly-like phases of this sort is caused by a transient three-dimensional network consisting of entangled cylindrical reverse (polymer-like) micelles. It is shown that although the organogel properties depend on the lecithin concentration, the phase and rheological behavior is mainly regulated by the polar additive. The homogeneous jelly-like phase exists for molar ratios (nw) of water to lecithin from 1.6-1.7 to 3.2-3.4. At nw ratios below 2.7-2.8, the scaling exponents of the main rheological parameterssthe zero shear viscosity, plateau modulus, and terminal relaxation timesare rather close to the theoretical predictions that follow from a model by Cates. This means that the lecithin polymer-like micelles are linear and flexible. At larger molar ratios the scaling behavior 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 the growth of cylindrical micelles changes with increasing water amount; at the initial stages there is uniaxial growth of linear micellar aggregates, and then the polar additive induces their branching. First results on an jelly-like phase that separates from the homogeneous organogel when the molar ratio nw is over 3.2 are presented. It is established that the phase separation results in a change in the rheological behavior. An intermediate region is found in which the loss and storage moduli scale with frequency with an exponent of 1/2. Similar scaling was previously observed only with polymers. Conceivable reasons for the square root frequency dependence of the dynamic moduli are considered. It is suggested that this scaling is caused by partial or local ordering of polymer-like micelles due to the significant decrease of the organogel volume with the phase separation.