Interfacial Water and Microheterogeneity in Aqueous Solutions of Ionic Liquids

In this work, aqueous solutions of two prototypical ionic liquids (ILs), [BMIM] [BF4] and [BMIM][TfO], were investigated by UV Raman spectroscopy and small-angle neutron scattering (SANS) in the water-rich domain, where strong heterogeneities at mesoscopic length scales (microheterogeneity) were expected. Analyzing Raman data by a differential method, the solute-correlated (SC) spectrum was extracted from the OH stretching profiles, emphasizing specific hydration features of the anions. SC-UV Raman spectra pointed out the molecular structuring of the interfacial water in these microheterogeneous IL/water mixtures, in which IL aggregates coexist with bulk water domains. The organization of the interfacial water differs for the [BMIM] [BF4] and [BMIM] [TfO] solutions, being affected by specific anion-water interactions. In particular, in the case of [BMIM] [BF4], which forms weaker H-bonds with water, the aggregation properties dearly depend on concentration, as reflected by local changes in the interfacial water. On the other hand, stronger water-anion hydrogen bonds and more persistent hydration layers were observed for [BMIM] [TfO] which likely prevent changes in IL aggregates. The modeling of SANS profiles, extended to [BPy] [BF4] and [BPy] [TfO], evidences the occurrence of significant concentration fluctuations for all of the systems: this appears as a rather general phenomenon that can be ascribed to the presence of IL aggregation, mainly induced by (cation-driven) hydrophobic interactions. Nevertheless, larger concentration fluctuations were observed for [BMIM] [BF4], suggesting that anion-water interactions are relevant in modulating the microheterogeneity of the mixture.

Automated summary

In this study, aqueous solutions of two ionic liquids were investigated using UV Raman spectroscopy and small-angle neutron scattering. The results revealed the presence of microheterogeneity in the IL/water mixtures, where IL aggregates coexisted with bulk water domains. The interfacial water organization differed between the two IL solutions, with specific anion-water interactions playing a role. Concentration fluctuations were observed for all systems, indicating the presence of IL aggregation induced by hydrophobic interactions. The findings provide important insights into the interactions between ionic liquids and water.