Design and molecular modelling of phenolic-based protic ionic liquids

Design and molecular modelling of phenolic-based protic ionic liquids

Journal of Molecular Liquids 308 (2020) 113062 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier...

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Journal of Molecular Liquids 308 (2020) 113062

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage:

Design and molecular modelling of phenolic-based protic ionic liquids Nur Afiqah Ahmad a, Khairulazhar Jumbri a,b,⁎, Anita Ramli a, Haslina Ahmad c, Mohd Basyaruddin Abdul Rahman c, Roswanira Abdul Wahab d a

Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia Centre of Research in Ionic Liquids (CORIL), Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia d Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM, Johor, Malaysia b c

a r t i c l e

i n f o

Article history: Received 1 January 2020 Received in revised form 26 February 2020 Accepted 1 April 2020 Available online 4 April 2020 Keywords: Ionic liquids Protic ionic liquids Phenolic acid Structure modelling Synthesis Characterisation

a b s t r a c t Five new phenolic-based protic ionic liquids (PILs) were successfully synthesised via neutralisation reaction. The synthesised PILs were characterised using spectral analyses such as 1H NMR, FTIR and thermogravimetric analysis. The effects of alkyl chain length and temperature towards the physical properties of the PILs were comprehensively investigated as well. The alkyl chain length of the cation and temperature significantly influenced both the density and viscosity of the PILs. As the alkyl chain length elongated from 2-hydroxy-Nmethylethanaminium salicylate (2HMES) to 2-hydroxy-N-propylethanaminium salicylate (2HBES) the density decreased from 1158.40 to 1110.60 kg.m−3 (at 293.15 K) and viscosity increased from 541.69 to 1383.00 mPa. s (at 293.15 K). Moreover, the density and viscosity of the PILs declined steadily as the temperature elevated from 293.15 to 363.15 K. Furthermore, the structural conformation of the PILs from the spectral analyses was further validated by the density function theory (DFT) calculation. Based on the optimised structure from the computational study, the most favourable interaction occurred between the –NH and –COO groups of the ion-pairs resulted from the transfer of hydrogen atom from acid to base. © 2020 Elsevier B.V. All rights reserved.

1. Introduction As the interest in synthesising new materials to tackle various industrial problems develop progressively, ionic liquids (ILs) are one of the fascinating materials which have attracted the attention of researchers due to their unique properties. ILs are defined as chemical substances which consist of predominantly ions and exist as liquid or solid at room temperature [1]. Interestingly, they are recognised as “green solvents” since they have low melting point (b100 °C) [2], negligible vapour pressure, high thermal and chemical stability and able to dissolve diverse organic and inorganic compounds [3,4]. ILs are utilised in electrochemistry [5], organic synthesis, biocatalysis [3], pharmaceutical [6] and extraction [7] applications. Ultimately, their adjustable property is the most attractive feature in which their physicochemical properties can be adjusted according to the selection of cation-anion pairing [3,6]. Protic ionic liquids (PILs) are a subset of ILs which can be synthesised by stoichiometric reaction between a Brønsted acid and a Brønsted base [8]. They are simply synthesised by the protonation of the base through the acid-base neutralisation method [9]. The distinctive trait between the protic and aprotic ILs (AIL) is the presence of proton on the cation ⁎ Corresponding author at: Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia. E-mail address: [email protected] (K. Jumbri). 0167-7322/© 2020 Elsevier B.V. All rights reserved.

structure which is able to form hydrogen bonding with the anion [10]. The presence of this additional proton on the cation structure and high liquid range property make PILs exclusively versatile to be utilised in various applications including electrochemistry [11], biomass conversion [12] and pharmaceutical [13]. For example, in electrochemistry, the availability of proton on the cation drives the PILs to naturally become proton conductor and subsequently act as electrolyte [14]. Moreover, PILs can be tuned easily in order to achieve good physical properties that can enhance their performance in certain applications by customising the type of ion pairs. Phenolic acid is a chemical compound which consists of an aromatic ring with a directly attached hydroxyl group (–OH). This compound comprises of two derivatives, particularly hydroxybenzoic and hydroxycinnamic acids. According to Yang et al. [15] phenolic acid exhibits diverse biological properties such as antioxidant, anti-cancer, anti-bacterial, anti-mutagenic, anti-inflammatory and anti-viral. Owing to the multiple biological properties, there is an increasing trend of utilising this compound in pharmaceutical and cosmeceutical industries [16]. Thus, phenolic acid is an excellent source of anion in the synthesis of ILs due to its low toxicity effect and high ability to transfer the hydrogen atom [17,18]. Therefore, the objective of this research is to synthesis new PILs by using phenolic acid as a precursor to meet the relevant criteria in a specific field of applications. Salicylic and vanillic acids were respectively selected as the anion due to their extensive


N.A. Ahmad et al. / Journal of Molecular Liquids 308 (2020) 113062

application as drugs in the pharmaceutical industry which minimise the toxicity of the synthesised PILs [19–22]. In addition, optimised geometry and interaction energy of the synthesised PILs were validated at molecular level to justify the presence of hydrogen transfer and possible interaction between the ions pair.

2. Experimental 2.1. Materials 2-(Methylamino)ethanol (0.98) and other chemicals such as 2(propylamino)ethanol (0.98), 2-(butylamino)ethanol (0.98), methanol (0.97), chloroform (0.96), salicylic acid (0.98) and diethyl ether (0.98) were purchased from Merck. Vanillic acid with purity 0.98 was supplied by Acros. These chemicals were used as received without further purification. Table 1 summarises all sources of the compounds used to synthesise the PILs.

2.4. Characterisation The 1H NMR spectrum was recorded using Bruker Avance 500 spectrometer in which deuterated chloroform (d–CDCl3) and methanol (d– MeOD) were used as solvents, respectively. Fourier Transform Infrared (FTIR) measurement was performed using Shimadzu Fourier Transform Infrared Spectrometry. The attenuated total reflection (ATR) method was conducted and the spectra were collected at 4000 to 500 cm−1. The OMIC spectra software was used to analyse the presence of the functional group. 2.5. Thermal analysis Thermogravimetric analysis (Perkin-Elmer, TGA-4000) was carried out to determine the thermal decomposition temperature (Td) of the synthesised PILs. The samples were heated from 323.15 to 723.15 K with a heating rate of 4.72 K s−1 under nitrogen gas flow. 2.6. Water content

2.2. Synthesis of protic ionic liquids (PILs) The PILs were synthesised based on the method discussed by Chennuri et al. [23] with some modifications. Basically, an equimolar amount of acid (salicylic and vanillic acid) and base (2-(methylamino) ethanol, 2-(propylamino)ethanol, and 2-(butylamino)ethanol) were mixed through neutralisation reaction. Initially, 0.05 mol of the base was transferred into a two-neck round-bottomed flask equipped with a reflux condenser and immersed in an ice bath. The reaction was performed in a reflux condenser to minimise the evaporation of the volatile base. Subsequently, 0.05 mol of acid was dissolved in 50 mL methanol and added dropwise into the base under vigorous stirring. The temperature was maintained around 0 to 5 °C while adding the acid since the reaction is exothermic. After that, the methanol was removed under vacuum using rotary evaporator at 50 °C for 5 to 6 h. The name, abbreviation and chemical structure of the synthesised PILs are summarised in Table 2.

2.3. Purification of PILs The liquid-liquid extraction method was used to purify each synthesised PILs. The synthesised PIL was first transferred into a 100 mL separating funnel. Afterwards, 20 mL diethyl ether (solvent) was added into the separating funnel and the mixture (solvent-PIL) was shaken vigorously for a few minutes. The mixture was allowed to settle down until a two-layer solution was formed. The upper layer is the mixture of diethyl ether with the excess of parent phenolic acid and the bottom layer is the pure PIL. The upper layer was discarded. These steps were repeated three times to ensure all the impurities were completely removed from the PIL. The remaining solvent was further removed with a rotary evaporator for 30 min at 40 °C (150 mbar). The synthesised PILs were kept in a desiccator to prevent vapour absorption.

Table 1 Name, source and purity of the chemicals used in the synthesis of PILs. Chemical name


Mass fraction purity

Purification method

2-(Methylamino)ethanol 2-(Propylamino)ethanol 2-(Butylamino)ethanol Methanol Chloroform Salicylic acid Diethyl ether Vanillic acid

Merck Merck Merck Merck Merck Merck Merck Across

0.98 0.98 0.98 0.97 0.96 0.98 0.98 0.98

None None None None None None None None

The water content of the synthesised PILs was measured using coulometric Karl Fisher titrator, DL 39 (Mettler Toledo). The drift of instrument was calibrated below 20 mg/min before analysing the sample. The sample was then added dropwise into the titration flask. The water content of PILs was recorded in percentage (%). 2.7. Density and viscosity The dynamic viscosity and density of PILs measurements were carried out using Anton Paar Stabinger Viscometer (SVM3000) in the temperature range of 293.15 to 363.15 K. The uncertainties of the instrument for all parameters are u(T) = ±0.01 K, u(η) = ±0.35%, and u(ρ) = ±0.1 kg m−3. 2.8. Computational simulation The structure of the PILs was optimised using density functional theory (DFT) method implemented in Turbomole 4.11 software package known as Tmolex. The geometry optimisation structure was performed at basic set B3-LYP/6-311G***. The quantum chemical calculation was executed by initially constructing the 3D molecular structure. Afterwards, the molecular orbital was generated at basic set def-TZVP (6311G***) and the calculation was run at 1000 to 5000 self-consistent field (SCF) with 10,000 to 30,000 cycles. The optimisation of the structures was performed for both isolated ions and ion-pair of PILs, respectively. 3. Results and discussion 3.1. 1H NMR spectroscopy Six phenolic based PILs were successfully synthesised through neutralisation reaction. The structure and purity of the synthesised PILs were determined using 1H NMR. The spectra in Figs. S1–S5 show that the approximate ratios of the integration lines in the spectra are consistent with the ratios of the number of protons on the predicted structures. 2-Hydroxy-N-methylethanaminium salicylate (2HMES). Colourless liquid (Yield: 97.56%) 1H NMR (CDCl3) δ(ppm): 2.66 (s, 3H), 3.04 (t, 2H), 3.89 (t, 2H), 6.83 (m, 1H), 6.92 (m, 1H), 7.35 (m, 1H), 7.84 (m, 1H). 2-Hydroxy-N-propylethanaminium salicylate (2HPES). Colourless liquid (Yield: 90.01%) 1H NMR (CDCl3) δ(ppm): 0.94 (t, 3H), 1.75 (m, 2H), 2.89 (t, 2H), 3.12 (t, 2H), 3.94 (t, 2H), 6.82 (m, 1H), 6.89 (m, 1H), 7.33 (m, 1H), 7.84 (m, 1H).

N.A. Ahmad et al. / Journal of Molecular Liquids 308 (2020) 113062


Table 2 Name, abbreviation and chemical structure of synthesised PILs.

2-Hydroxy-N-butylethanaminium salicylate (2HBES). Pale yellow liquid (Yield: 97.59%) 1H NMR (CDCl3) δ(ppm): 0.99 (t, 3H), 1.44 (m, 2H), 1.69 (m, 2H), 3.02 (t, 2H), 3.13 (t, 2H), 3.81 (t, 2H), 6.79 (m, 2H), 7.29 (m, 1H), 7.84 (m, 1H). 2-Hydroxy-N-methylethanaminium vanillate (2HMEV). Brownish solid (Yield: 98.16%) 1H NMR (CD3OD) δ(ppm): 2.71 (t, 3H), 3.09 (m, 2H), 3.79 (m, 2H), 3.90 (s, 3H), 6.78 (m, 1H), 7.50 (m, 1H), 7.59 (m, 1H). 2-Hydroxy-N-propylethanaminium vanillate (2HPEV). Brownish viscous liquid (Yield: 91.12%) 1H NMR (CD3OD) δ(ppm): 1.02 (t, 3H), 1.71 (m, 2H), 2.95 (m, 2H), 3.09 (t, 2H), 3.80 (t, 2H), 3.90 (s, 1H), 6.78 (m, 1H), 7.48 (m, 1H), 7.59 (m, 1H).

3.3. Thermal stability Fig. 2 shows the thermogravimetric curves of the synthesised PILs. It can be seen that the salicylate-based PILs are more stable compared to the vanillate-based PILs due to their higher thermal decomposition temperatures (Td). As shown in Table 3, both vanillate-based PILs (2HMEV and 2HPEV) have low Td values (423.64 and 429.49 K) than 2HMES, 2HPES and 2HBES (473.15, 475.31, and 476.35 K, respectively). The presence of the methoxy substituent (–OCH3) on the vanillic structure has a strong influence on the thermal stability of the vanillate-based PILs [16]. This is because –OCH3 reduces the electron density of the benzene ring which causes the PILs to decompose easily, hence decreases the thermal stability of the synthesised compounds.

3.2. FTIR spectroscopy The formation of PILs was further validated by observing the pres− ence of protonated amine (NH+ 2 ) and carboxylate (COO ) functional groups on the IR spectrum. Fig. 1 shows the IR spectra of 2HBES, neat amine, and acid. The neat acid (SA) spectrum displays a sharp peak at 1650.77 cm−1 which represents the vibrational frequency of the carbonyl group (C=O) of carboxylic acid [24]. The peak is slightly shifted to a lower frequency due to the conjugated system of the carbonyl group [25]. A strong band is also observed at 1455.993 cm−1 in 2BAE spectrum which corresponds to the vibration of the N\\H bending for secondary amine. However, a new peak was detected in the 2HBES spectrum at 1594.84 cm−1 which belonged to the deformation vibration of protonated secondary amine –NH+ 2 [26,27]. Moreover, another strong peak was discovered at 1347.03 cm−1 represented the carboxylate anion (COO−) of SA. The presence of these peaks is believed to occur due to the transfer of hydrogen atom from the hydroxyl group of SA towards the nitrogen atom of secondary alkanolamine as well as due to the resonance effect of the carboxylate anion. This is in agreement with other similar findings reported by other researchers [28,29].

Fig. 1. FTIR spectrum of 2-hydroxy-N-butylethanaminium salicylate (2HBES).


N.A. Ahmad et al. / Journal of Molecular Liquids 308 (2020) 113062 Table 4 Percentage of water in synthesised PILs.

Weight percent (%)






Percentage of water (%)


1.39 1.20 1.15 1.48 1.37



0 400




Temperature (K) Fig. 2. Thermogravimetric curves of synthesised PILs at the temperature range of 323.15 to 723.15 K.

molecules and consequently reduces the absorption of water [38]. A similar trend has been reported by Freire et al. [39], where they found that the water content in the synthesised imidazolium-based ILs decreased in the order of [C4mim][PF6] N [C6mim][PF6] N [C8mim][PF6]. Furthermore, the different anions used in the present work do not contribute a major effect towards the water content as both salicylate and vanillate anions possess hydrophobic character [40–42]. Moreover, the precursors for these two anions were derived from the same phenolic group namely hydroxybenzoic acid which have similar characteristic [43,44]. 3.5. Density

Furthermore, the cation structure does not show any significant effects on the thermal stability of the synthesised PILs. However, there is a slight change in Td as the alkyl chain length of the cation elongated. From the results obtained, the Td of the synthesised PILs increases in the order of 2HMEV N 2HPEV N 2HMES N 2HPES N 2HBES. The longer alkyl chain length results in higher thermal stability due to the stronger van der Waals force of interaction [30]. Similar findings have been observed in other studies as well [31]. 3.4. Water content As mentioned earlier, the synthesised PILs have a slightly hygroscopic characteristic, therefore, it is important to report their water content. According to Kurnia et al. [32], it is impossible to completely remove the water in the ILs since they have a high tendency to form a hydrogen bond with the water molecules. This phenomenon can be seen based on our previous study where water molecules were found in the synthesised crystallographic structure of tetraethylammonium L-tartarate and tetraethylammonium L-malate ILs after the crystallisation process [33,34]. Since high polarity of cation and anion are used to synthesis these ILs, strong hydrogen bonds with water molecules are formed, leading to the formation of hygroscopic compounds [35]. Table 3 shows the percentage of water content in the synthesised PILs. Based on the data obtained, all the synthesised PILs show water content of b2% which lies in the acceptable range of water content of ILs (5 to 10%) [36]. Even though the presence of a small amount of water slightly affects the physical properties of ILs, however their chemical properties remain the same [37] (Table 4). The percentage of water in PILs reduces when the alkyl chain length of cation increases from 2HMES to 2HBES (1.39, 1.20 and 1.15, respectively) and 2HMEV (1.48) and 2HPEV (1.37). This is due to the strong hydrophobic character of cation as the alkyl chain length increases. The hydrophobicity of ILs increases as the alkyl chain lengthens, thus leading to a weak intermolecular hydrogen bond with the water

The density of the liquid PILs at various temperatures was fitted using linear equation (Eq. (1)) as follows: ρ ¼ A þ BT


where ρ is the density of PILs, A and B are fitting parameters (Table 6). However, the density measurement was only attainable for salicylatebased PILs since the vanillate-based PILs were highly viscous and formed amorphous solid. From the results obtained in Fig. 3 and Table 5, the density of PILs increases gradually as the temperature decreases. This is corresponding to the enlargement of interionic separation at elevated temperature, which in turn leads to poor packing efficiency. The poor packing efficiency of compound consequently reduces its mass per volume and density [45]. Furthermore, from the results, it can be seen that the density of PILs declines as alkyl chain length elongates from 2HMES, 2HPES to 2HBES. The dispersive interactions are believed to influence this condition since the increase of alkyl chain length will lead to the increase of the dispersive interaction between the carbon chain, thus resulting in a nanostructural organisation in polar and nonpolar region (domain) [46]. In this study, the alkyl group of cations is considered as the nonpolar region whereas the cationic head groups and the anions of PILs are the polar region. As the

Table 3 Thermal decomposition temperature (Td) of PILs. PILs



473.15 475.31 476.35 423.64 429.49

Fig. 3. Temperature dependence of density for 2HMES, 2HPES and 2HBES at 0.1 MPa and temperature range of 293 to 363 K.

N.A. Ahmad et al. / Journal of Molecular Liquids 308 (2020) 113062


Table 5 Density of PILs at different temperature range from T = (293–363 K) and p = (0.1 MPa). ρ/kg.m−3

PILs/T (K)











1158.40 1133.31 1110.60

1155.12 1129.92 1107.10

1151.71 1126.63 1103.70

1145.33 1119.80 1097.00

1138.91 1113.30 1090.40

1132.60 1106.90 1083.70

1126.21 1100.30 1077.10

1119.82 1093.20 1070.01

1113.41 1087.60 1062.60

Standard uncertainties are: u(ρ) = ± 2.1 kg m−3, u(T) = ±0.01 K, u(P) = ±1 kPa.

alkyl chain length increases, the nonpolar region of PILs enlarges and occupies more space, which contributes to a lower density [47]. In addition, the packing of chains within the nonpolar domain of PILs is limited by the existence of the polar region due to the charge of head groups covalently attached to the chain. This trend is also observed in other studies [11,48]. 3.6. Viscosity Viscosity is one of the important properties that needs to be considered in the synthesis of PILs since its value is beneficial in various applications [49–51]. The temperature dependence of viscosity was fitted in Vogel-Tammann-Fulcher equation (Eq. (2)):   B η ¼ ηo exp T−T o


where η is the viscosity, T is temperature, ηo, B and To are fitting parameters (Table 6). Fig. 4 and Table 7 show the viscosity of the three salicylatebased PILs. Basically, the viscosity of PILs decreases progressively as the temperature rises steadily and the alkyl chain length of the cation shortens [52]. As the temperature elevates, the interionic distance of the molecules lengthens due to the weakened intermolecular forces and thus, reduces the viscosity of PILs [53]. On the other hand, the increase in alkyl chain length leads to the increment of the viscosity value of PILs. Van der Waal force of attraction and the formation of hydrogen bonding between the aliphatic alkyl chain rationalise this behaviour [54]. 3.7. Computational modelling Fig. 5 displays the most stable conformation of the PILs where the – COO group of the anion prefers to have close contact with the –NH group of the cation. The strong positive charge at the nitrogen atom and negative charge at the oxygen atom lead to the formation of the directional intermolecular hydrogen bonding between the cation and anion (N–O–H). Furthermore, the 2HPES and 2HBES were found to have an extra hydrogen bond between the labile hydrogen (–OH group) on the cation with the oxygen atom of the –COO group of the anion. This behaviour occurred due to the bending and flexibility of the cation as the alkyl chain length increased. Thus, it can be concluded that the experimental data is supported by the computational method due to the possible interaction between the –NH and –COO groups. This is due to the fact that the formation of PILs is established via the transferral of the hydrogen atom from the acid (anion) to the base (cation). In addition, the interaction energy (Eint) was also computed in which the more negative interaction energy of the ion pairs signified the most

stable structure of the PILs. The more negative value indicates strong cation-anion interaction, thus implying the high stability of the compounds [55]. Therefore, the Eint is a crucial parameter needs to be considered in determining the most stable conformation of the ionic liquids (ILs). According to the theory, the compound is considered stable when it possesses high Eint due to the fact that, greater energy is required to break the bond of the molecules [55]. The Eint was calculated using Eq. (3): Eint ¼ Eion−pair −ðEcation þ Eanion Þ


Liu et al. [56] reported that the Eint of the ILs was highly influenced by the hydrogen bonding interaction. This is because the formation of PILs is prominently dependent on the strength of hydrogen bond [57]. The calculated Eint is summarised in Table 8. The salicylate-based PILs show slightly higher Eint than vanillate-based PILs. The presence of the methoxy group on the vanillate anion could affect the strength of hydrogen bonding with the cation which leads to low Eint. [58]. The [2MAE]-based PILs was found to exhibit the strongest Eint than [2PAE]- and [2BAE]-based PILs. This is because the small structure of [2MAE]+ causes favourable interaction with the anion, and hence increases the interaction energy. 4. Conclusion In this work, it was discovered that the alkyl chain length of the cation and temperature contributed significant effect towards the properties of the PILs. From the results obtained, the viscosity of the synthesised PILs increased as the alkyl chain lengthened. For instance, the viscosity of 2HMES, 2HPES, and 2HBES at temperature 293 K increases (541.69, 549.89 and 1138.00 kg.m−3, respectively) due to the intermolecular interaction of the ionic liquid molecules. However, longer alkyl chain length causes the increase in dispersive interaction and consequently reduces the density of PILs. Furthermore, the density of PILs also decreases at increasing temperature due to the increment in

Table 6 Fitting parameters of linear equation (Eq. (1)) and Vogel-Tammann-Fulcher (Eq. (2)). PILs








1346.08 1325.71 1309.48

0.6411 0.6573 0.6787

1.2406 1.5856 1.7226

425.05 544.22 573.40

186 178 165

0.999 0.999 0.999

Standard uncertainties are u(P) = ±1 kPa and u(T) = ±0.01 K.

Fig. 4. Temperature dependence of viscosity for 2HMES, 2HPES and 2HBES at 0.1 MPa and temperature range of 293 to 363 K.


N.A. Ahmad et al. / Journal of Molecular Liquids 308 (2020) 113062

Table 7 Viscosity of PILs at different temperatures and p = (0.1 MPa). PILs/T(K)


ƞ/mPa.s 293









541.69 549.89 1383.00

357.82 376.08 876.02

249.12 261.80 582.79

128.20 136.79 275.53

72.71 78.08 145.03

44.62 48.03 83.13

29.18 31.84 51.32

20.13 21.27 33.71

14.51 15.14 22.94

Standard uncertainties are u(η) = ±1.2% and u(T) = ±0.01 K, u(P) = ±1 kPa.

the interionic separation thus, leading to the poor packing efficiency. Similarly, as the temperature increases, the intermolecular forces in the molecules weakens accordingly which in turn promotes molecular changes and reduces the viscosity. In addition, the optimised geometry

of the PILs was validated by the computational study in which the possible interaction of PILs occurred at the –NH and –COO groups of the cation and anion, respectively which justified the data obtained from FTIR spectral analyses.

Fig. 5. Optimised structure of the PILs at B3-LYP/6-311G*** level using DFT method.

N.A. Ahmad et al. / Journal of Molecular Liquids 308 (2020) 113062 Table 8 Calculated interaction energy of PILs. PILs

Eint (kcal/mol)


−555.58 −524.86 −549.04 −514.99 −474.22

CRediT authorship contribution statement Nur Afiqah Ahmad: Formal analysis, Investigation, Writing - original draft. Khairulazhar Jumbri: Conceptualization, Methodology, Software, Funding acquisition, Supervision. Anita Ramli: Resources, Supervision, Writing - review & editing. Haslina Ahmad: Visualization, Investigation, Writing - review & editing. Mohd Basyaruddin Abdul Rahman: Validation, Writing - review & editing. Roswanira Abdul Wahab: Software, Writing - review & editing.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was funded by Yayasan Universiti Teknologi Petronas (YUTP 015LC0-070). Nur Afiqah Ahmad acknowledges the UTP Graduate Assistantship (GA) scheme, Universiti Teknologi PETRONAS. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2020.113062.

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