Interactions of Biodegradable Ionic Liquids with a Model Naphthenic Acid

Density functional theory models are used to examine five biodegradable ionic liquids (ILs) each one consisting of a substitutional group (-OH, -NH(2,) -COOH, -COOCH(3), and -OCH(3)) incorporated into the cation of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF(4)]). The results reveal that hydrogen atoms in -NH(2,) -COOH, and -COOCH(3) form intramolecular hydrogen bonds with fluorine atoms in [BF(4)](−), whereas hydrogen atoms in -OH and -OCH(3) do not form hydrogen bonds with [BF(4)](−). Further analysis of electron density at bond critical points and noncovalent interactions suggest that [BMIM][BF(4)] with -COOH has stronger intramolecular hydrogen bonds than other ILs. The extraction mechanism for a model naphthenic acid is hydrogen bonding, with F···H being the strongest hydrogen bond and O···H ranking second. More intermolecular hydrogen bonds occur when model naphthenic acid is adsorbed by [BMIM][BF(4)] with -COOH and -COOCH(3). The interaction energy between model naphthenic acid and ILs with -COOH and -COOCH(3) is higher than that with -OH, -NH(2), and -OCH(3).