Structural Transitions at the Water/Oil Interface by Ionic-Liquid-like Surfactant, 1-Butyl-3-methylimidazolium Dioctyl Sulfosuccinate: Measurements and Mechanism.

Abstract

Reverse micelle (RM) aggregates have a wide range of applications in various areas of science and technology. A continuous demand exists to replace interfacial surfactant molecules with various nonconventional amphiphiles. Ionic liquid (IL)-like surfactants (IL-surf's) constitute a class of such molecules that are being researched extensively. Here, we have formulated several water/IL-surf/oil microemulsions by optimizing the core droplet size with varying oil phases. The best composition of water/[BMIM][AOT]/IPM ([BMIM][AOT]: 1-butyl-3-methylimidazolium dioctyl sulfosuccinate; IPM: isopropyl myristate) was then analyzed in detail through experimental and computer simulations. Our results from DLS measurements suggest a structural transition from spherical aggregates in the parent water/[Na][AOT]/IPM solution to cylindrical droplets in the IL-surf-based system. The Raman and ATR-FTIR spectral analysis suggest a variation in the microstructure of the water/oil interface due to the differential interaction of the counterions with AOT headgroups and water. Molecular dynamics simulation results provided the direct image of the interface showing a structured versus uneven water/oil interface in [Na][AOT] versus [BMIM][AOT] RMs, where the larger [BMIM] cations weakly bind with the AOT headgroups due to their low charge density. Finally, an application of this IL-surf-based formulation was tested by carrying out a Heck cross-coupling reaction that showed significantly higher yield under milder reaction conditions.