Microstructure of ionic liquid (EAN)-rich and oil-rich microemulsions studied by SANS.

In a previous study we investigated the phase behavior of microemulsions consisting of the ionic liquid ethylammonium nitrate (EAN), an n-alkane and a nonionic alkyl polyglycolether (CiEj). We found the same general trends as for the aqueous counterparts, i.e. a transition from an oil-in-EAN microemulsion via a bicontinuous microemulsion to an EAN-in-oil microemulsion with increasing temperature. However, unlike what happens in the corresponding aqueous systems, in EAN-in-oil microemulsions only a very small amount of EAN was detected by NMR-measurements. This is why we investigated the phase behavior and microstructure of EAN-rich n-dodecane-in-EAN microemulsions and oil-rich EAN-in-n-octane microemulsions. We found that the ionic liquid emulsification failure boundary has an extraordinarily small slope, which suggests that the amphiphilic film loses its ability to solubilize EAN with an increase in temperature by only a few degrees. The analysis of the small angle neutron scattering (SANS) curves unambiguously shows that this behavior is due to the fact that the EAN molecules form a substructure with a characteristic length scale of Λ ≈ 8 Å inside the EAN-in-oil droplets. In more detail, the analysis of the SANS data with the GIFT method revealed a transition from spherical to cylindrical structures approaching the respective critical endpoint temperatures. By using the respective form factors and combining them with a Gaussian spatial intensity distribution to account for the EAN sub-structure we were able to describe the scattering curves nearly quantitatively.

Formation Kinetics of Oil-Rich, Nonionic Microemulsions.

The formation kinetics of oil-rich, nonionic microemulsions were investigated along different mixing pathways using a fast stopped-flow device in combination with the new high-flux small-angle neutron spectrometer D33 (ILL, Grenoble, France). While the kinetics along most pathways were too fast to be resolved, two processes could be detected mixing brine and the binary cyclohexane/C10E5 solution. Here, too, the formation of large water-in-oil droplets was found to be faster than 20 ms and therewith faster than the accessible dead time. However, subsequently, both the disintegration of the large water-in-oil droplets (600 Å) and the uptake of water by swollen micelles (50-60 Å) could be resolved. Both processes occur on the time scale of a second. Strikingly, the total internal interface forms faster than 20 ms and does not change over time.

A Journey toward Sulfolane Microemulsions Suggested as Inert, Nonaqueous Reaction Media.

Recently, it turned out that nanostructured reaction media containing highly inert solvents as tetrahydrothiophen-1,1-dioxide (sulfolane) are beneficial for strongly oxidizing or reductive reactions. Because of their ability of solubilizing polar and nonpolar solvents with a large nanostructured interface in particular microemulsions provide such interesting reaction media. Starting from the pseudoternary microemulsion H2O-n-octane-C12E4/C12E5 (polyoxyethylene n-alkyl ether), water was successively replaced by the highly inert tetrahydrothiophen-1,1-dioxide (sulfolane). We found that an increasing sulfolane content drives the system beyond the tricritical point. Replacing the already long chain surfactants C12E4 and C12E5 by a mixture of the even longer chain surfactants C18E6 and C18E8, we were able to prepare nonaqueous sulfolane microemulsions for the first time. We also teach how in a second step the phase behavior of the hydrophilic sulfolane-n-octane-C18E8 system can be tuned at constant temperature (as required by the reaction conditions) by addition of the hydrophobic cosurfactant 1-octanol (C8E0). The change in curvature that occurs by adding 1-octanol is demonstrated measuring the size of reverse micelles by DLS. We found that the radius varies from at least 8 to 16 nm, a suitable sizes for inverse nanoreaction vessels.