Percolative phenomena in lecithin reverse micelles: the role of water

The role played by the solvation water molecules on the macroscopically observed sol-gel transition in lecithin/cyclohexane/ water reverse micelles is investigated by quasielastic neutron scattering, dielectric relaxation and conductivi- ty measurements. The experimental results are juxtaposed to those from spherical Aerosol OT reverse micelles. It is shown how the results from lecithin-based system can be interpreted only assuming that, in contrast to Aerosol OT systems, the water molecules are entrapped at the interfaces without coalescing into an inner water pool. It is suggested that, in the case of lecithin, the solvation Introduction It is commonly observed that lecithin dissolved in a number of nonpolar solvents is able to form cylindrical aggregates upon the addition of small quantities of water [1]. Further addition of water determines the growth of the cylinders in giant wormlike micelles, until a maximum R value (R is the number of water molecules per surfactant molecule) is reached, above which the structure of the system changes. Above a critical concentration, the micelles entangle in a transient network [1, 2] and, from a macroscopic point of view, the system appears as a viscous gel, thermostable, thermoreversible and isotropic [3]. On this basis it became usual to describe the structure of the lecithin gels in terms of analogies with semidilute solutions of flexible polymers [4, 5]. Such an approach turns out to be able to describe many of the static properties of the water can induce a change in the surface curvature, in such a way promoting the formation of branch points. Such a hypothesis is sup- ported by the temperature depen- dence of the conductivity which agrees with the hypothesis of an intermicellar bond percolation. The investigation of the structures im- posed by an external electric field is also studied. The observed electro- rheological behaviour seems to con- firm the existence of a percolated transient network in the gel phase.