Structure and dynamics of balanced supercritical CO2-microemulsions

Abstract

Balanced scCO₂-microemulsions contain equal volumes of water and CO₂ 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 scCO₂, and hence the overall microemulsion properties, is tuned simply by adjusting pressure, which is not possible with “classical” microemulsions containing oil instead of CO₂. Motivated by this, we systematically investigated the phase behavior, the microstructure, and the dynamics of balanced microemulsion systems of the type H₂O–CO₂–Zonyl FSO 100/Zonyl FSN 100. In systematic phase behavior studies, we found that upon increasing pressure, CO₂ 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 Å. 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 κ_0,SANS rises from κ_0,SANS = 0.88 k_BT at 200 bar to 0.93 k_BT at 300 bar. These findings were confirmed with high pressure neutron spin echo experiments.