Ionic Liquids in Soft Confinement

Abstract

The majority of studies on reverse micelles (RMs) use water as the polar component. However, RMs with ionic liquids (ILs) as the polar phase, nonaqueous RMs, present unique properties and potential applications (such as reaction, separation, or extraction media) for the combined features of ILs and RMs. This chapter highlights our contribution to the development and characterization of RMs formed with ILs. We have prepared RMs using sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT), benzyl-n-hexadecyldimethylammonium chloride (BHDC), and 4-(1,1,3,3-tetramethylbutyl)phenyl polyethylene glycol (9–10) and Triton X-100 (TX-100) surfactants dissolved in nonpolar solvents such as benzene and chlorobenzene with the following IL polar phases: 1-butyl-3-methylimidazoliumtetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim][TfO]), and 1-butyl-3-methylimidazolium trifluoroacetate ([bmim][TfA]). We monitored the formation and properties of these novel organized systems using dynamic light scattering (DLS), as well as absorption, multinuclear NMR, and FT-IR spectroscopy. Our observation of increasing droplet size with increasing IL content in all these systems demonstrates that the RMs consist of discrete spherical and noninteracting droplets of IL stabilized by the surfactants. The properties of the ILs encapsulated strongly depend on the nature of the interface present in the organized system. UV–Vis absorption data of a molecular probe and FT-IR and NMR spectra show important structural differences for ILs entrapped inside BHDC RMs, compared with the neat ILs or the ILs entrapped in TX-100 or AOT RMs. Our results suggest that confinement substantially modifies the ionic interactions (with the surfactant polar head groups, with surfactant counterions, or with the IL counterions). These interactions produce segregation of IL ions altering the composition of the RM interfaces. Taken together, these results show the versatility of these organized systems to alter the ionic organization, information that can be very important if these media are used as nanoreactors because unique microenvironments can be easily created simply changing the RM components and the IL content.