ACS Sustainable Chemistry & Engineering, 2018, vol 6, 7, pp. 8983-8991
The design of multifunctional ionic liquids (ILs) that integrate different attractive characteristics, such as strong hydrogen-bonding interactions, good lipophilicity, significant nanoscale organization, and low viscosity, in a molecular structure is of great importance for the application of ILs in extraction, biomass conversion, and catalysis but remains challenging. Here, we synthesized a family of novel phosphonium ILs featuring branched chain carboxylate anions with carbon number up to 18 and systematically characterized the physicochemical properties of prepared ILs. The branched chain carboxylate ILs (BCC-ILs) exhibit very strong hydrogen-bond basicity (β = 1.49–1.72, 30 °C) and excellent lipophilicity (π* = 0.73–0.95, 30 °C), and have obvious nanoscale segregation and moderate viscosity (η = 91.8–200.5 mPa, 40 °C). The d value of BCC-ILs ranged from 10.5 to 15.6 Å when the length of alkyl chain in anions or cation rose. These nanostructured solvents can undergo self-assembly processes with typical bioactive molecules of cholesterol to form highly ordered mesoscopic structures through cooperative hydrogen-bond and van der Waals interactions, which enable unprecedented high solubility of cholesterol in BCC-ILs (molar solubilities 0.31–1.12 at 50 °C). Additionally, we found that n-heptane had the significant synergistic effect on the dissolution of cholesterol molecules, and the introduction of n-heptane promoted the formation of highly ordered lamellar liquid crystals in the BCC-IL/cholesterol system. The dissolving mechanism of BCC-ILs for bioactive molecules was explored by experimental studies. This work indicates that the multifunctional BCC-ILs are promising solvents for extractions, catalysis, and biomass conversion.