Bicontinuous structure zones in microemulsions

Winsor IV systems¹, or microemulsions², are macroscopically monophasic fluid transparent compounds made up by mixing water and hydrocarbon in the presence of suitable surface active agents. Depending on composition factors, either water-in-oil (w/o), or oil-in-water (o/w), type systems can be formed. The so-called microemulsions have been intensively investigated^3-8 because of their potential applications as liquid agents in technical processes (for example, enhanced oil recovery, photochemistry and biotechnology). As they belong to the class of short-range organized fluids, microemulsions also concern condensed matter physicochemists interested in elucidating their structural properties^9-12. That critical phenomena can be observed in microemulsions is confirmed by reports that the electrical conductivity variations with water content increasing in certain w/o microemulsions can be accounted for on the basis of the Percolation and Effective Medium theories^13-16. It has been suggested^15,16that percolative conduction reflects disperse aqueous droplet clustering and interlinking processes predictive of a w/o to o/w transition (phase inversion), involving the formation of equilibrium bicontinuous structures described by Scriven¹⁷ as being ``related to ordinary liquids as porous media are to homogeneous solids'', and modelled by Talmon and Prager¹⁸ as a random geometry of interspersed water and hydrocarbon domains generated by a Voronoi tesselation. This suggestion is supported by the results reported here, showing that the existence of bicontinuous structures in Winsor IV systems could be identified with the limited composition range separating the w/o and o/w microemulsion regions and over which a post-percolation anomalous conductive behaviour is observed.