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, -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.

Naphthenic acids (NAs) are complex mixtures of carboxylic acids with cyclic structures and aliphatic groups 1,2 . In addition to increasing the acidity of crude oils, the presence of NAs also cause other serious problems such as poisoning catalysis, forming coke, and creating corrosion to equipment and pipelines 3,4 . Ionic liquids (ILs), recognized as "green solvents" and "solvents of the future", provide us an alternative and promising approach to extract undesired products from fossil fuels, considering their outstanding properties such as non-volatility, low flammability, and reusability [5][6][7] . It was reported that ILs are efficient at removing NAs from crude oil [8][9][10] .
Having excellent physicochemical properties and being easily synthesized, imidazolium-based liquids are the most extensively studied ILs 11 . However, the properties (thermal stability and non-volatility) that make ILs attractive result in low biodegradation of ILs 12,13 . 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 . ILs are also reported to be toxic to a broad variety of organisms 16 . The most effective technique to remove organic pollutants from water is biodegradation 17 , hence, a significant challenge associated with the design and application of ILs is to increase the biodegradability of ILs 18 .
To improve the ultimate biodegradation of ILs and minimize the adverse environmental influences, researchers have paid more attention to change the framework of organic cations and anions of ILs. It has been demonstrated that incorporating an ester in the side chain significantly enhanced imidazolium-based ILs biodegradation 19 . Furthermore, oxygenated and hydroxylated imidazolium-based ILs have a better biodegradability 20 . Amino groups are also reported to be able to increase the biodegradability of ILs 21 . Although 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. Five different biodegradable groups, including the hydroxyl group (-OH), amino group (-NH 2 ), formate group (-COOH), methyl ester group (-COOCH 3 ), and methyl ether group (-OCH 3 ) were incorporated to the cation of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]). The structures of the five ILs with these biodegradable groups are displayed in Fig. 1. Cyclohexanecarboxylic acid (CHCA) was selected to be the model NA. The results reported in this study will assist researchers to design ILs with high biodegradability and high extraction efficiency for NAs.

Results and Discussion
Optimized Geometries. The most stable geometries of CHCA and ILs with biodegradable groups are shown in Supplementary Fig. S1 Table S1), which are shorter than the sum of the VDW radii for oxygen and hydrogen. Consequently, it is concluded that hydrogen bonds are formed between the oxygen atom in -OH and hydrogen atoms of [C4OHMIM] [BF 4 ]. In addition, the nitrogen atom in -NH 2 and oxygen atoms in -COOCH 3 , -COOH, and -OCH 3 form hydrogen bonds with hydrogen atoms in the ILs as well. Moreover, hydrogen bonding also occurs between fluorine atoms in [BF 4 ]and hydrogen atoms in the cation for all these five types of ILs (Supplementary Fig. S1 4 ] are shorter than the VDW radius for fluorine and hydrogen, indicating the existence of intramolecular hydrogen bonds between a fluorine atom of [BF 4 ] − and a hydrogen atom of -NH 2, -COOH, and -COOCH 3 for those three type of ILs. On the other hand, the interactions between a fluorine atom in [BF 4 ] − and a carbon atom in the imidazole ring corresponds to the presences of lone pair (LP)-π interactions.
To efficiently acquire the most stable interaction structures for ILs-CHCA, the electrostatic potential was analyzed for ILs and CHCA, shown in Fig. 2. It was deduced that the highly positively charged regions for ILs are located around the imidazole ring, whereas [BF 4 ] − has a strongly negatively electrostatic potential. The addition of -OH and -OCH 3 negatively influences the electrostatic potential, and those regions have stronger negative electrostatic potential compared with that for [BMIM][BF 4 ] 22 . With regards to CHCA, the most negative and positive electrostatic potentials are located around the oxygen atoms in the carboxylic group, and the hydrogen atom in the carboxylic group, separately.
Based on electrostatic potential analysis, CHCA were placed around different regions of ILs to obtain the most stable interaction structures, as shown in Fig. 3 4 ]-CHCA, one is between a fluorine atom of [BF 4 ] − and the hydrogen atom in the carboxylic group of CHCA; the other two are between an oxygen atom in the carboxylic group of CHCA and hydrogen atoms of the cation, presented in Fig. 3A and Table 1. With the formation of complexes, the distance of F25···H14 (displayed in Supplementary Fig. S1 and Table S1) is lengthened from 2.61 Å to 2.79 Å, as displayed in Fig. 3A and Table 1, longer than the sum of VDW radii for the fluorine and hydrogen atoms, indicating that F25 no longer forms a hydrogen bond with H14. In addition, the elongation of distances between H13, H16, and F25 implies weaker intramolecular interactions of [C4OHMIM][BF 4 ] after it adsorbs CHCA. The above phenomena could be explained by the F25···H52 hydrogen bond formation. Therefore, the electronegativity of F25 is decreased and the interaction strengths between F25 and other hydrogen atoms are weaker (see Fig. 3A and Table 1). F23···H13 and F26···C5 distances are also elongated after complex formation, suggesting that the interactions between anion and imidazole ring are weaker as well (Table 1 and Supplementary  Table S1 4 ]-CHCA also leads to the conclusion that the interactions between the anion and imidazole ring are weaker upon complex formation.
Interaction Energies. The interaction energies are important to evaluate the stability of interactions. They are defined as the energy difference between complexes and the sum of isolated ILs and CHCA, and were calculated according to: ILs CHCA I Ls CHCA     4 ]-CHCA. Hence, it is deduced that the interaction energies between CHCA and biodegradable ILs with two electronegative atoms are higher than that between CHCA and biodegradable ILs with one electronegative atom. In addition, the interaction energies between CHCA and five types of biodegradable ILs are higher than that between CHCA and The donor-acceptor interactions of ILs-CHCA and their stabilization energy E(2) were calculated to determine the extent of interaction. The interaction intensity is reflected by the value of E (2). Higher values of E (2) indicate that electrons are more likely to migrate from donor to acceptor orbitals and stronger interaction exists between the donor and acceptor. As displayed in Supplementary Table S4,   Additionally, it is deduced that the incorporation of biodegradable groups with two electronegative atoms to ILs form more intermolecular hydrogen bonds with CHCA than those that have one electronegative atom. As shown in Supplementary Table S5, the value of the Laplacian of the electron density is positive for all ILs, and ILs-CHCA, indicating that the electrons tend to segregate. It also suggests the existence of ionic bonds, hydrogen bonds, and VDW interactions in ILs and ILs-CHCA 22 . Moreover, the strength of interactions can be evaluated by comparing the electron density ρ(r) value of BCPs. Larger values of ρ(r) correspond to stronger interactions 26 . The electron densities of O26···H13 and F31···H28 in [C4COOHMIM] [BF 4 ] are larger than the electron densities of other ILs, which indicates the presence of stronger hydrogen bonds. In addition, the electron density of F25···H52 is greatest for [C4OHMIM][BF 4 ]-CHCA, implying that the hydrogen bonds between fluorine and hydrogen is the strongest hydrogen bond. Further analysis of the largest electron density for other ILs-CHCA also demonstrates that the F···H interaction is the strongest interaction. Compared to the distances of other hydrogen bonds, the F···H distance is shortest for all five ILs-CHCA, therefore, it is inferred that electron density is corre-

Noncovalent interaction (NCI) analyses.
Through performing NCI analyses, which is based on the reduced density gradient (RDG) 27 , the intramolecular and intermolecular interaction types and strengths can be evaluated 28 . In the plots of RDG versus sign(λ 2 )ρ, the peaks in the sign(λ 2 )ρ < 0, sign(λ 2 )ρ = 0, and sign(λ 2 )ρ > 0 region suggest attractive interactions, VDW interactions, and steric effects, respectively. Furthermore, the interaction types and strength can be identified through analyzing the color and area in the gradient isosurface diagram; red indicates steric repulsions, green means weak interactions such as VDW interactions, and blue represents strong attractive interactions such as hydrogen bonds 29,30 . To investigate intramolecular and intermolecular interaction types and strength, the plots of RDG versus sign(λ 2 )ρ and the gradient isosurface (s = 0.6 a.u.) for ILs are shown in Supplementary Fig. S4. In addition, the plots of RDG versus sign(λ 2 )ρ and gradient isosurface (s = 0.7 a.u.) for ILs-CHCA are displayed in Fig. 4.
Electron Density Difference Analysis. When ILs interact with CHCA, there is a transfer of electron density during the interaction process 31 . The electron density change was determined by subtracting the electron density of complexes from the sum of electron density of isolated ILs and CHCA:

ILs CHCA I Ls CHCA
To evaluate electron density redistribution caused by the interaction between ILs and CHCA, electron density distribution maps were plotted. As depicted in Supplementary Fig. S5, the obvious electron density change mostly locates around the interaction region between ILs and CHCA. The formation of F25···H52 hydrogen bonds in [C4OHMIM] [BF 4 ]-CHCA, shown in Supplementary Fig. S5A, increases the electron density of F25 and decreases that of H52 because of electron transfer from H52 to F25. In addition, due to O50···H13 and O50···H14 interactions, the electron density of O50 increases whereas the electron densities of H13 and H14 decrease. The formation of hydrogen bonds between electronegative and hydrogen atoms provoke the increase of density for the electronegative atom such as F and O, and induce the decrease of electron density for hydrogen atom.
In summary, the interaction energy between biodegradable ILs and CHCA is higher than that between [BMIM][BF 4 ] and CHCA. Moreover, biodegradable ILs with two electronegative atoms have higher interaction energy with CHCA than that having one electronegative atom. Compared with the extraction mechanism for [BMIM] [BF 4 ], the main interaction is still hydrogen bonding. However, biodegradable ILs form more hydrogen bonds with CHCA than [BMIM] [BF 4 ]. Therefore, it is deduced that the design of biodegradable ILs promote the extraction of CHCA. Additionally, the greater the number of electronegative atoms in biodegradable group of ILs, the easier it is to extract CHCA.

Computational Methods
The density functional computations were carried out using Gaussian 09 program packages 32 . The M06-2X functional is suitable for calculation with nonmetals and is recommended to calculate main-group thermochemistry, kinetics, NCI, and electronic excitation energies to valence and Rydberg states 33 . It also has better performance than B3LYP and PW91 for systems with dispersion and ionic hydrogen-bonding interactions 34 and is compatible to CCSD(T) and MP2 in describing NCI 35 . The geometries of CHCA and ILs with biodegradable groups were fully optimized by the M06-2X method in combination with the empirical dispersion-correction (DFT-D3) 36 method and the 6-311 + + G(d,p) basis set. The ILs-CHCA interaction structures were also optimized by employing the same method and basis set. Vibrational analyses were performed to confirm that the structures are at minimal energy without imaginary frequencies. The interaction energies were calculated with the correction by the counterpoise method for basis set superposition error 37 . The second-order perturbation energy E(2) in NBO was determined by using the Gaussian 09 program packages with the M06-2X/6-311 + + G(d,p) level of theory. The Multiwfn software package was adopted to analyze the wave functions of the optimized structures to obtain NCI, and electron density differences 38,39 . The interaction regions of NCI analysis were visualized and colored with the Visual Molecular Dynamics (VMD) software package 40 . Topological properties were analyzed by using AIM theory 24 .