Structure and dynamics of balanced supercritical CO2-microemulsions

Balanced scCO 2 -microemulsions contain equal volumes of water and CO 2 and are a novel class of microemulsions of substantial interest for both fundamental research and technical applications. One existing feature of these systems is that the solvent quality of scCO 2 , and hence the overall microemulsion properties, is tuned simply by adjusting pressure, which is not possible with classical microemulsions containing oil instead of CO 2 . Motivated by this, we systematically investigated the phase behavior, the microstructure, and the dynamics of balanced microemulsion systems of the type H 2 OCO 2 Zonyl FSO 100/Zonyl FSN 100. In systematic phase behavior studies, we found that upon increasing pressure, CO 2 and water are more efficiently solubilized. Small angle neutron scattering (SANS) experiments were conducted in order to determine the topology and the length scales of the underlying microstructure. The results obtained strongly suggest the existence of bicontinuously structured microemulsions with an adjustable characteristic length scale of up to 330 . From a quantitative analysis of the SANS data, we found that at a fixed microemulsion composition the stiffness of the surfactant membrane is increased solely by increasing the pressure, whereby the renormalization corrected ( i.e. bare) bending rigidity 0,SANS rises from 0,SANS = 0.88 k B T at 200 bar to 0.93 k B T at 300 bar. These findings were confirmed with high pressure neutron spin echo experiments. We characterized the structural and dynamic properties of balanced supercritical bicontinuous microemulsions with phase diagram measurements, small-angle neutron scattering and neutron spin echo spectroscopy, the latter allowing us to measure directly the pressure dependence of the bending rigidity.