Chapter 4 - Structure and noncovalent interactions in ionic liquids mixtures and deep eutectic solvents

Abstract

The aim of this work is to quantify the evolution of the various noncovalent interparticle interactions in the binary mixtures of imidazolium-based ionic liquids (ILs) with perfluorinated anions, hexafluorophosphate (C4mimPF6), bis(fluorosulfonyl)imide (C4mimNTf2), and tris(pentafluoroethyl)trifluorophosphate (C4mimFAP), and deep eutectic solvents (DESs), composed by choline chloride (ChCl), and ethylene- (EG) and propylene glycol (PG) molecules. For this purpose, by using combined molecular dynamics and density functional theory methodology, we analyze the most important intermolecular, interionic, and ion-molecular hydrogen bonding interactions in ILs and DESs. We found that the addition of FAP− to a mixture enhances the nonpolar domain segregations and weakens the hydrogen-bond network. It was confirmed that H2-site is the most prone to cation–anion interactions, whereas in the binary mixtures of ILs two anions compete for the most acidic hydrogen of imidazolium ring. Nonetheless, most of the hydrogen bonds in the studied IL mixtures are rather weak and can be referred to as “hydrogen-bond-like interactions.” The FAP-anion forms notably less hydrogen-bond-like interactions and only in neat IL or in FAP-rich regions of IL mixtures, stabilizing the ion pair mostly electrostatically. For the DES mixtures, it was established that ethaline-2 (molar ratio of ChCl and EG is 1:2) does not contain any real hydrogen bonds. Propeline (DES-based on ChCl and PG), however, presents weak hydrogen-bond formation between cation and anion and medium hydrogen-bond formation between anion and PG molecule.