Ion Percolation through a Cubic Lipid/Water Phase

Abstract

X-ray diffraction studies have shown that mixtures of certain lipids with water yield exotic regular phases in which water is arranged in interpenetrating cubic networks of channels.^1,2 Though electrical properties of amorphous lipid-water systems have been extensively studied (see reference 3 for review), regular phases have so far received little attention. A cubic phase formed in a saline solution will conduct electricity if the network of water channels containing dissolved salts spans the sample. By contrast, the unhydrated lipid will have a very low conductance. Starting with pure lipid, the time evolution of conductance as the proportion of water within the lipid increases could provide insight into the kinetics of formation of porous phases otherwise difficult to obtain from structural measurements. Here I report continuous measurements of the ionic conductivity of monoolein samples as water concentration increases from near zero to its equilibrium value in presence of free water. A definite conductance threshold is observed at an intermediate water concentration, after which conductance follows a power law. The results are discussed in the framework of percolation theory.