Abstract
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.
Highlights
Having excellent physicochemical properties and being synthesized, imidazolium-based liquids are the most extensively studied ILs11
There are numerous studies to investigate approaches to increase the biodegradability of ionic liquids (ILs), to the best of our knowledge, few theoretical studies are available to compare intramolecular and intermolecular interaction differences, and extraction mechanism variations of ILs with different biodegradable substitutional groups, especially for the removal of NAs from liquid oil
The most stable geometries of CHCA and ILs with biodegradable groups are shown in Supplementary Fig. S1, with bond lengths listed in Supplementary Table S1
Summary
Having excellent physicochemical properties and being synthesized, imidazolium-based liquids are the most extensively studied ILs11. The properties (thermal stability and non-volatility) that make ILs attractive result in low biodegradation of ILs12,13 Due to their high solubility, high stability, and low biodegradability, imidazolium-based ILs are persistent pollutants that cause serious contamination after being released to aqueous media[14,15]. There are numerous studies to investigate approaches to increase the biodegradability of ILs, to the best of our knowledge, few theoretical studies are available to compare intramolecular and intermolecular interaction differences, and extraction mechanism variations of ILs with different biodegradable substitutional groups, especially for the removal of NAs from liquid oil. The objective of the research was to fill the knowledge gap by exploring the influence of biodegradable substitutional groups on ILs intramolecular interactions, examining ILs extraction mechanisms of model NAs, and investigating the nature of the molecular interactions by using density functional theory (DFT) calculation.
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