Percolation in Concentrated Water-in-Carbon Dioxide Microemulsions

The phase behavior and electrical conductivity of water-in-carbon dioxide (W/C) microemulsions are reported over a range of temperatures (5−65 °C), pressures (100−450 bar), and droplet volume fractions ( φ = 0.0347−0.483) at a constant water-to-surfactant molar ratio ( W o ) of 12.5. A φ of 0.483 is a 5-fold increase over those reported previously. A critical point is observed at a droplet volume fraction of approximately 0.12, at which the single-phase microemulsion splits into two microemulsion phases of similar volume upon lowering the pressure (upper critical solution pressure). At low temperatures, a lower critical solution pressure is also observed upon increasing the pressure. Both of the critical solution pressures result from an increase in the attractive interdroplet interactions; consequently, pressure has little effect on the conductivity in the one-phase region. The conductivity increases nearly 3 orders of magnitude with changes in the droplet concentration or temperature. Scaling analysis of the conductivity data supports a dynamic percolation model, whereby the attractive interdroplet interactions form clusters of discrete droplets with rapid charge transport.