Percolative conduction in microemulsion type systems

In the presence of a suitable combination of surfactants (an alkaline metal soap and a medium chain length alcohol, for example), water and oil type organic liquids can form transparent compounds of low viscosity that have been labelled “microemulsions”, with little or no mechanical agitation (spontaneous emulsification) (refs 1–3). Depending on the chemical nature of the surface active agents and the relative constituent proportions, water-in-oil (w/o) or oil-in-water (o/w) systems can be obtained. (Because of the similarity between tertiary solutions of inverted micelles and w/o microemulsions, the term ‘inverted micellar solutions’ has also been suggested4–7.) Microemulsions have been investigated by many scientists interested in liquid state and surface physicochemistry and by many technologists foreseeing numerous applications in industry8,9. It has been suggested9–13 that conductivity and permittivity studies could provide, along with other techniques, valuable information about the structure and phase behaviour of microemulsions which are considered to consist of dispersions between a few tens and a few hundreds of angströms in diameter globules made up of an inner spherical core surrounded by a concentric shell of mixed surfactant and cosurfactant1–10. The experiments reported here show that the conductive behaviour of certain microemulsion systems can be accounted for using the percolation and effective-medium theories14–16 that have depicted transport properties and continuous metal–non-metal transitions in disordered materials with microscopic inhomogeneities associated with, for example, density, composition15 or bonding configuration17 fluctuations. This result, which could help in understanding the structural behaviour of micro-emulsions, is considered in connection with the postulated existence in liquid systems of equilibrium bicontinuous structures, a state described by Scriven18 as ‘related to ordinary liquids as porous media are to homogeneous solids’.