Microstructure analysis at the percolation threshold in reverse microemulsions.


Brian Antalek, Antony J Williams, John Texter, Yuri Feldman, and Nissim. Garti. 1997. “Microstructure analysis at the percolation threshold in reverse microemulsions.” Colloids and Surfaces, A: Physicochemical and Engineering Aspects, 128, 1-3, Pp. 1–11.


Time domain dielec. spectroscopy of reverse water/acrylamide/Aerosol-OT(AOT)/toluene microemulsions shows that percolation induced by increasing cosurfactant concn. (increasing cosurfactant chem. potential) obeys scaling above and below a percolation threshold. This scaling anal. suggests that the obsd. percolation is close to static percolation limits. Self-diffusion measurements derived from NMR pulsed-gradient spin-echo expts. reveal an increase in water proton diffusion above the percolation threshold. This increase is assigned to water transport through fractally chained assemblies of microemulsion droplets. The diffusion of water, cosurfactant, and surfactant (AOT) below threshold is modeled quant. taking into account the chem. partitioning equil. between the microemulsion droplets and the toluene continuous pseudophase. Above threshold, the apparent increasing water and cosurfactant partitioning into the toluene (continuous) pseudophase suggests facilitated transport through fractal aggregates. Ad dynamic partitioning model is used to est. the vol. of percolating fractal cluster, and yields an order parameter for water-in-oil to percolating cluster microstructural transitions. This same order parameter is also illustrated to derive from self-diffusion data wherein percolation and transformation to sponge phase microstructure are driven by increases in temp. and in disperse phase vol. fraction. For microstructural transitions driven by three different field variables, chem. potential, temp., and disperse phase vol. fraction, this order parameter shows that the onset of percolation corresponds to the onset of increasing water proton self-diffusion, and that the onset of increasing surfactant self-diffusion corresponds to the formation of bicontinuous microstructures and the onset of transformation to middle phase microemulsion. [on SciFinder(R)]
Last updated on 06/28/2020