Percolation in Concentrated Water-in-Carbon Dioxide Microemulsions

Abstract

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.

Formation of Water-in-Carbon Dioxide Microemulsions with a Cationic Surfactant: A Small-Angle Neutron Scattering Study

“…Conductivity measurements performed on W/C microemulsions formed with a PFPECOO -NH 4 + surfactant have shown that the phase transition which occurs upon lowering the pressure is the result of attractive interactions between droplets, while the phase transition that occurs upon lowering the temperature is the result of the change in the natural curvature at the water-CO 2 interface. 17 The phase transition due to droplet-droplet interactions results from a decrease in the density and, thus, solvent power, of CO 2 as the pressure is lowered at constant temperature. Decreasing the solvent power of CO 2 decreases solvent-tail interactions and thereby increases tail-tail interactions between neighboring microemulsion droplets.…”

Phase behaviors of AOT/longer chain n-alkanes reverse micelles in the presence of compressed ethylene

“…10 −14 −1 cm −1 ), which constitutes the continuous medium of reverse micelles, due to the fact that micelles are able to transport charges. As the volume fraction of water droplets increases, attractive interdroplet interactions increase, leading to droplet clustering and thus results in a sharp increase of electrical conductivity in reverse micellar system [36]. This phenomenon is called percolation.…”

Concentrated CO2-in-Water Emulsions with Nonionic Polymeric Surfactants

“…Emulsions were formed with PDMS 24 -g-EO 22 , which is at the optimum HCB according to a minimum in the plot of γ versus fraction EO in the surfactant. The value of γ is only 0.2 mN/m at 45…”