Imidazolium-based ionic liquids with inorganic anions in the extraction of salidroside and tyrosol from Rhodiola: The role of cations and anions on the extraction mechanism

The role of cations and anions of imidazolium ionic liquid-based microwave-assisted extraction system for salidroside and tyrosol of Rhodiola was investigated. To evaluate eight kinds of imidazolium-based ionic liquid with different cations and anions on molecule level, the geometries of related IL molecules and target extracts were optimized by density functional theory at the B3LYP/6-311++G(d, p) level. Electrostatic potential (ESP) distributions of these molecules above are identified in the water modeled by the polarizable continuum model. The theoretical and experimental analysis shows there is some correlation between the characteristic values of ESP distribution and extraction yield. The more similar the ESP distribution of ILs are to salidroside and tyrosol, the higher the extraction yields are. Wider variability of ESP is beneficial to extraction. The main role of cations is interaction with extract molecules by its positive ESP regions. So the cations influence the solubility of extracts by the molecular interaction. Unlike the cations, the types of anions can affect the structure of the cell wall. The ILs with Br anion have greater ability to dissolve cellulose and break the cell wall, which made the target components within the cell could effectively dissolve. Due to the differing function of cations and anions of ILs, the ionic liquid-based MAE could provide higher extraction yields and take much shorter extraction time than conventional organic solvents.