Lecithin organogels: Structure, dynamics, and phase behavior

Lecithin self-assembles in non-aqueous media into reversed giant cylindrical micelles when small amounts of water are added. The micellar aggregates entangle forming a three-dimensional network that brings about spectacular viscoelastic properties, and these systems are referred to as organogels. This review considers esults obtained by our group on microstructure, phase behavior and dynamics of organogels. Analysis of pulsed gradient spin-echo(PGSE) NMR data on cyclohexane (C6) based systems reveals a curvilinear motion on timescales up to 1.5 s, indicative of disconnected entangled networks. In contrast, a PGSE-NMR study on the isooctane (iC8) based system suggests the presence of branches connecting wormlike portions of micelles. The dependence of the branch-point density ρB on micellar volume fraction was found to be in qualitative agreement with theories on living networks (ρB ~φ1/2). Water diffusion indicates that in PC/H2O/C6 phase separation is due to a rod-to-sphere transition followed by emulsification failure, while in PC/H2O/iC8 there is shrinking of the interconnected network. Accordingly, the phase diagram of PC/H2O/C6 can be quantitatively described by excluded-volume effects acting on spherical or cylindrical micelles (depending on water content) while the phase behaviour of PC/H2O/iC8 is consistent with predictions for interconnected micellar networks. In both systems the residence time is ~1μs for water and longer than 1s for lecithin. The relaxation from a shear-induced nematic phase suggests a very long micellar lifetime (of the order of minutes). © 2013 The authors mentioned in the table of contents. All rights reserved.