Influence of the molecular structure on physicochemical properties of choline carboxylate ionic liquids: A combined experimental and theoretical study

Choline carboxylate ionic liquids (ILs), which act as a type of bio-based protic ionic liquid (PIL), are excellent solvents for insoluble natural products. In this work, four choline carboxylate ILs were synthesized by a one-step method; physicochemical properties such as density, surface tension, and ionic conductivity were measured as a function of temperature. Additionally, physico-chemical parameters that are difficult to obtain through experiments, including thermal expansion coefficients, molecular volumes, standard entropy, lattice energy and polarity of the ILs were further calculated. To better understand intermolecular interactions of choline carboxylate ILs, the sigma profile of choline cation and carboxylate anion was determined using COSMO-RS. The results showed that the choline cation has potential as an H-bond donor, while the carboxylate anion demonstrates significant capability as a hydrogen bond acceptor. By DFT calculations, the possible interactions between the choline cation and carboxylate anions were investigated. All calculations indicate that the choline cation prefers to connect to the carboxylate anion through O-H···O interactions. Non-covalent interactions in choline-based ILs were further analyzed using reduced density gradient (RDG) isosurfaces. The results show that the hydrogen bond interaction between the O-atom of butyric acid anion and OH of choline cation is the strongest.