Density functional theory models are used to examine five biodegradable ionic liquids (ILs) each one consisting of a substitutional group (-OH, -NH2, -COOH, -COOCH3, and -OCH3) incorporated into the cation of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). The results reveal that hydrogen atoms in -NH2, -COOH, and -COOCH3 form intramolecular hydrogen bonds with fluorine atoms in [BF4]−, whereas hydrogen atoms in -OH and -OCH3 do not form hydrogen bonds with [BF4]−. Further analysis of electron density at bond critical points and noncovalent interactions suggest that [BMIM][BF4] 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][BF4] with -COOH and -COOCH3. The interaction energy between model naphthenic acid and ILs with -COOH and -COOCH3 is higher than that with -OH, -NH2, and -OCH3.
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