The effect of different interfaces and confinement on the structure of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide entrapped in cationic and anionic reverse micelles

Abstract

The behavior of the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf₂N]) entrapped in two reverse micelles (RMs) formed in an aromatic solvent as dispersant pseudophase: [bmim][Tf₂N]/benzyl-n-hexadecyldimethylammonium chloride (BHDC)/chlorobenzene and [bmim][Tf₂N]/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/chlorobenzene, was investigated using dynamic light scattering (DLS), FT-IR and ¹H NMR spectroscopies. DLS results reveal the formation of RMs containing [bmim][Tf₂N] as a polar component since the droplet size values increase as the W_s (W_s = [[bmim][Tf₂N]]/[surfactant]) increases. Furthermore, it shows that the RMs consist of discrete spherical and non-interacting droplets of [bmim][Tf₂N] stabilized by the surfactants. Important differences in the structure of [bmim][Tf₂N] entrapped inside BHDC RMs, in comparison with the neat IL, are observed from the FT-IR and ¹H NMR measurements. The electrostatic interactions between anions and cations from [bmim][Tf₂N] and BHDC determine the solvent structure encapsulated inside the nano-droplets. It seems that the IL structure is disrupted due to the electrostatic interaction between the [Tf₂N]⁻ and the cationic BHDC polar head (BHD⁺) giving a high ion pair degree between BHD⁺ and [Tf₂N]⁻ at a low IL content. On the other hand, for the AOT RMs there is no evidence of strong IL–surfactant interaction. The electrostatic interaction between the SO₃⁻ group and the Na⁺ counterion in AOT seems to be stronger than the possible [bmim]⁺–SO₃⁻ interaction at the interface. Thus, the structure of [bmim][Tf₂N] encapsulated is not particularly disrupted by the anionic surfactant at all W_s studied, in contrast to the BHDC RM results. Nevertheless, there is evidence of confinement in the AOT RMs because the [bmim]⁺–[Tf₂N]⁻ interaction is stronger than in bulk solution. Thus, the IL is more associated upon confinement. Our results reveal that the [bmim][Tf₂N] structure can be modified in a different manner inside RMs by varying the kind of surfactant used to create the RMs and the IL content (W_s). These facts can be very important if these media are used as nanoreactors because unique microenvironments can be easily created by simply changing the RM components and W_s.