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Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromatography
Journal of Molecular Liquids xxx (xxxx) xxx
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Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromatography Qiang Wang a,b, Wen-Na Wang a,b, Bin Wang a,b, Jun Tang a,b, Qiang Wang a,b,⁎ a
Center for Physical and Chemical Analysis, Xinjiang University, Urumqi 830046, PR China Key Laboratory of Coal Cleaning Conversion and Chemical Engineering Process, Xinjiang Uyghur Autonomous Region, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, Xinjiang 830046, PR China
b
a r t i c l e
i n f o
Article history: Received 29 June 2019 Received in revised form 20 November 2019 Accepted 22 November 2019 Available online xxxx Keywords: Ionic liquids Inverse gas chromatography Surface properties Dispersive free energy Lewis acid-base parameters
a b s t r a c t The surface properties of 1-alkyl-3-vinylimidazolium hexafluorophosphate ([CnVIM][PF6], n = 2,4,6,8) ionic liquids and 1-allyl-3-vinylimidazolium ([AVIM]+), ionic-based liquids with different anions (Cl−, Br−, [BF4]−, [Tf2N]−) were investigated at temperatures between 303.15 and 343.15 K on the basis of inverse gas chromatography (IGC). The Schultz method and Dorris-Gray methods were used to estimate the dispersive free energy based on the net retention volumes of homogeneous n-alkanes, and it was found that the discrepancy between the two methods increases with increasing temperature, with the Dorris-Gray method providing slightly higher values than the Schultz method at the same test temperature. The Lewis acid-base parameters, KA and KB, were likewise assessed by the method proposed by Schultz, Flour and Papirer, Brookman and Sawyer, respectively. Although the difference between the values of acid-base parameters were observed, the ratios of KB/KA obtained by three methods presented the same tendency and implied that the surface of all the tested ionic liquids exhibited the Lewis amphoteric and stronger basic character. In addition, the values of the enthalpies of adsorption between the probes and ionic liquids were also determined. © 2018 Published by Elsevier B.V.
1. Introduction Generally speaking, ionic liquids (ILs) are a class of salts, and it is difficult to form an ordered crystal structure between anions and cations, which results in the melting point of ILs measuring somewhere below 100 °C [1–4]. ILs are considered to be “green solvents” and are proposed as a substitute for traditional industrial solvents, with the advantage of having a wide liquid range, recyclability, good stability and solubility, thermal nonflammability and so on [5–8]. When ILs are used in reactions and applications that occurred at the interface of a two-phase system, such as adsorption, catalysis, and extraction, the interactions and chemical to exchange strongly depend on the surface properties of those ILs [9–12]. The experimental data regarding the surface energy, and especially the acid-base properties, of ILs are still limited or lacking. At present, the techniques that can be used for the surface characterization of materials are mainly the sessile drop method, the Wilhelmy plate method, the NMR imaging method and contact-angle measurement [13–17]. These static adsorption techniques are susceptible to ⁎ Corresponding author at: Center for Physical and Chemical Analysis, Xinjiang University, Urumqi 830046, PR China. E-mail address: [email protected] (Q. Wang).
external factors, such as temperature and the surface roughness of materials [18]. As a kinetic adsorption characterization technique [19], inverse gas chromatography (IGC) allows access to more thermodynamic parameters [20–22] and physicochemical properties [23–27] of pure substances and binary mixtures. As such, it has been widely used in biological materials, polymers, the pharmaceutical industry, etc. This paper aims to develop at a better understanding on the surface properties of a series of 1-alkyl-3-vinylimidazolium hexafluorophosphate ([CnVIM][PF6]) ILs and 1-allyl-3vinylimidazolium ([AVIM]) based ILs. Some surface parameters were obtained from the interactions of ILs with polar and non-polar probes by IGC. Different evaluation methods were used to calculate the value of the dispersive surface energy and acid-base constants. 2. IGC calculation methods 2.1. Enthalpies of adsorption By measuring the retention time of probe solvents through the column packed with ILs, the net retention volumes, Vn and the specific retention volumes, V0g can be determined using Eqs. (1) and (2) [28].
https://doi.org/10.1016/j.molliq.2019.112202 0167-7322/© 2018 Published by Elsevier B.V.
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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When plotting the retention diagrams (InV0gversus 1/T) using Eq. (3), ΔHsa can be calculated from the slope (‐ΔHsa/R) [29,30]. ðP o ‐P w Þ 3ðP i ‐P 0 Þ2 ‐1 ðt r ‐t 0 Þ Vn ¼ F Po 2ðP i ‐P 0 Þ3 ‐1 V og ¼
273 Vn mT a
ΔH sa ¼ ‐R
∂ InV g o ∂ð1=T Þ
ð1Þ
ð2Þ ð3Þ
where F is the carrier gas flow rate measured at room temperature, tr is the retention time of the probe through packed column, t0 is the retention time of a non-retained marker, T and Ta are the column and room temperatures, respectively, Pi and Po are the packed column inlet and outlet pressures, respectively, Pw is the saturated vapor pressure of water at room temperature, m is the mass of coated ILs, and R is the ideal gas constant. 2.2. Dispersive component of the surface energy In IGC theory, there are two basic calculation methods for the determination of dispersive surface energy, γds of the stationary phase. The first method, proposed by Schultz, can be expressed in the following form [31]: 1=2 þ K0 RT ln V n ¼ 2Na γ ds γdL
ð4Þ
where γdL is the dispersive component of the surface energy of the liquid probe molecule, a is the molecular surface area of the probe, and N is Avogadro's number. When a series of n-alkanes of known physical and chemical properties are used as probes, by plotting the RT ln Vn values of n-alkanes versus the term of 2Na(γdL )1/2, the value of γds can be derived from the slope. An alternative method used for the calculation of γds Dorris and Gray'method [32] is described by the following relationships: ΔGCH2 ¼ ‐RTIn V n;nþ1 =V n;n
γ ds ¼
ΔG2CH2 4N2 a2CH2 γ CH2
ð5Þ
ð6Þ
where Vn, n+1 and Vn, n are the net retention volumes of n-alkane with n + 1 and n carbon atoms, respectively, aCH 2 and γCH 2 are the crosssectional area and the surface free energy of a methylene group, respectively, and γCH 2 has a relationship of temperature:γCH 2 = 35.6 ‐ 0.058t. The free energy of adsorption of a methylene group (ΔGCH 2) can be calculated from the slope by plotting the RT ln Vn values of n-alkanes versus the number of their carbon atoms (n). Then, γds can be derived from Eq. (6).
be obtained according to the following relation: ‐ΔGsp ¼ ð−ΔGÞ− −ΔGref ¼ RT ln V n =V n;ref
ð7Þ
where Vn and Vn, ref are the net retention volume of the polar probe and a hypothetical alkane with the same physicochemical properties. The specific enthalpy of adsorption, ΔHsp, can be calculated from the slope by making a plot of ΔGsp/Tas a function of 1/T, as described by Eq. (8). ΔGsp ΔHsp ¼ −ΔSsp T T
ð8Þ
The Lewis acid-base parameters, KA and KB, can be determined by either using ΔGsp or ΔHsp values of selected ploar probes and are described by the following relationships: ΔGsp DN ≈ KA þ KB AN AN
ð9Þ
ΔH sp DN ¼ KA þ KB AN AN
ð10Þ
where DN and AN ∗ are the electron donor numbers and the corrected electron acceptor numbers of selected polar probes, respectively. Using ΔGsp leads to temperature-dependent KA and KB values, which also contain the specific entropic factor, whereas the utilization of ΔHsp gives constant values for each parameter corresponding to the temperature range of the study. In our case, we used the latter method to determine KA and KB values. The final acidity-basicity of the stationary phase surface is usually assessed by the ratio of KB/KA [25]. The value KB/KA N 1 represents a surface that is considered to be basic, while the value of KB/KA b 1 is considered to be acidic. 3. Experimental section 3.1. Materials and chemicals The mass fraction of ILs used in this work was N0.99 and were acquired from Chengjie Chemical Co. Ltd. (Shanghai, China), including 1ethyl-3-vinylimidazolium hexafluorophosphate ([EVIM][PF6]), 1butyl-3-vinylimidazolium hexafluorophosphate ([BVIM][PF6]), 1hexyl-3-vinylimidazolium hexafluorophosphate ([HVIM][PF6]), 1octyl-3-vinylimidazolium hexafluorophosphate ([OVIM][PF6]) 1-allyl3-vinylimidazolium bis(trifluorompropylsulfonyl) imide ([AVIM] [Tf2N]), 1-allyl-3-vinylimidazolium tetrafluoroborate ([AVIM][BF4]), 1allyl-3-vinylimidazolium chloride ([AVIM]Cl), 1-allyl-3vinylimidazolium bromide ([AMIM]Br). The probes used in experiment and their properties are listed in Table 1 [19,26,27,29]. In order to minimize test error, the purity of all probe solvents was of the highest available quality and was used as received without purification from J&K Scientific Ltd. 3.2. Column preparation
2.3. Lewis acid-base parameters The free energy of the adsorption of polar probes in the stationary phase is divided into the dispersive section (ΔGd) and the specific section (ΔGsp). In the literature, one can find three methods for the determination of ΔGspproposed by Schultz [31], Flour and Papirer [33] and Brookman and Sawyer, respectively [34]. In three methods, the estimation of ΔGsp is based on the same assumption that the value of ΔGd between a polar probe and the stationary phase is equal to the free energy of the adsorption of a hypothetical n-alkanes (ΔGref) with the same physicochemical properties. ΔGsp can
Stainless steel tubing (1200 mm × 2 mm I.D.) was selected to serve as the packed column. In order to reduce the water content and volatile compounds to negligible values, all ILs were subjected to a vacuum, at medium temperature, for at least 48 h prior to measurement. The water content in all liquid state ILs was b500 ppm, as checked by KarlFisher method. The stationary phase was used with the packed columns by dissolving a certain amount of ILs with methanol, and silicon alkylation 102 monomer support was used to load the solution. After the evaporation of the excess methanol under a rotary evaporator, the stationary phase was dried at 363.15 K until the mass was constant. The
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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Table 1 Physical and chemical properties of selected probes. Probe
n-Hexane n-Heptane n-Octane n-Decane Dichloromethane Chloroform Acetone Ethyl acetate Tetrahydrofuran
Tb
po
a × 1020
γdL
AN*
DN
(°C)
(kPa)
(m2)
(mJ/m)
(kJ/mol)
(kJ/mol)
68.74 98.5 125.6 150.8 39.75 61.3 56.53 77.2 66
20.14 6.093 1.871 0.580 57.93 26.16 30.56 1.781 3.671
0.515 0.570 0.630 0.690 0.315 0.440 0.425 0.480 0.450
18.4 20.3 21.3 22.7 27.6 25.9 16.5 19.6 22.5
– – – – 16.3 22.7 10.5 6.30 2.10
– – – – 0 0 71.4 71.1 84.4
Character
Neutral Neutral Neutral Neutral Acidic Acidic Amphoteric Amphoteric Basic
stationary phase was filled in the column with a vacuum pump and the mass of the packing ILs was calculated from the mass of the packed versus the empty column. In this experiment, the stationary phases consisted of 25–30% (w/w) of ILs. The amounts of analysed quality in the IGC experiments for eight ILs ranged from 0.65 to 0.71 g. Before the test, the packed column needed to be aged at 363.15 K and the nitrogen flow rate was 30 ml/min for 24 h.
3.3. Instruments and methods All measurements were carried out using an Agilent 6890 GC, which was equipped with a high sensitivity flame ionization detector. High purity nitrogen (N99.99%) was used as a carrier gas and the flow rate was determined experimentally at 30 mL/min. During all experiments, the injector and detector temperatures were kept at 523.15 K and 553.15 K, respectively. The oven temperature was maintained at a temperature range of (303.15–343.15 K) with an interval of 10 K under atmospheric pressure. The ionic liquids analysed under the experimental temperature range were either solid ([EVIM][PF6], [BVIM][PF6], [OVIM] [PF6], [AVIM][BF4]) or liquid ([HVIM][PF6], [AVIM][Tf2N], [AVIM]Cl, [AMIM]Br), which was determined by drying under a vacuum oven. In order to prevent that the bulk adsorption of the probe inside the liquid phase, which would affect the retention time of the probe, the retention times were determined using the maximum of the peaks and the interaction between the probes and the ILs, which can be considered only as a surface interaction [24]. We repeated the test three times for each probe and took an average of the retention times for subsequent calculations. The methane was used as calibration for dead time compared to other probe solvents.
4. Results and discussion 4.1. Enthalpies of adsorption After the retention time was determined, the specific retention volumes of the probes were calculated using Eq. (2). A linear relationship of InV0g and 1/T was acquired within the experimental temperature range for probes operating on the examined ILs, suggesting that the adsorption equilibrium had been achieved. This also indicated that the experimentally measured V0g values were suitable for thermodynamic analysis. The enthalpies of the adsorption of the probes on the examined ILs were calculated from the slope using Eq. (3) and the presented values were provided for uncertain surface coverage. For all examined ILs, ΔHsa values of n-alkanes increased with the increase of carbon atoms (Fig. 1). This suggests that the dispersive force between the examined ILs and n-alkanes became stronger as the CH2 chain in the probe molecule increased. Due to specific interactions, such as dipole-dipole interactions and hydrogen bonding between the polar probes and tested ILs, the ΔHsa values obtained were varied compared to the n-alkanes [25].
Fig. 1. Enthalpies of adsorption, ΔHsa, of probes on examined ILs; (a) ▼ [OVIM][PF6], ▲ [HVIM][PF6], ● [BVIM][PF6], ■ [EVIM][PF6], ◇ [BMIM][PF6] [35]; (b) ● [AVIM][BF4], ■ [AVIM][Tf2N], ☆ [AVIM]Cl, ▼ [AMIM]Cl [27], ◇ [AVIM]Br.
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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Q. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx Table 2 Dispersive component of the surface free energy, γds , of the tested ILs at various temperatures. ILs
γds /(mJ·m−2) 298.15 Ka
303.15 K
313.15 K
323.15 K
333.15 K
343.15 K
Schultz method [EVIM][PF6] 56.7 [BVIM][PF6] 47.6 [HVIM][PF6] 41.7 [OVIM][PF6] 37.3 [AVIM]Cl 56.4 [AVIM]Br 54.4 [AVIM][BF4] 52.8 [AVIM][Tf2N] 34.3
55.5 46.8 41.4 36.7 55.9 54.0 52.4 33.9
53.9 45.0 40.8 35.7 55.2 53.2 51.6 33.1
51.6 43.2 34.0 35.0 54.1 52.6 50.8 32.2
49.8 42.2 39.3 33.9 53.1 51.6 49.9 31.5
47.5 39.8 38.7 32.7 52.6 51.2 49.2 30.7
Dorris-Gray method [EVIM][PF6] 58.4 [BVIM][PF6] 49.1 [HVIM][PF6] 43.6 [OVIM][PF6] 38.6 [AVIM]Cl 57.8 [AVIM]Br 55.9 [AVIM][BF4] 54.0 [AVIM][Tf2N] 35.7
58.0 48.7 43.3 38.4 57.6 55.8 53.7 35.6
57.5 47.9 42.9 38.0 57.3 55.4 53.4 35.3
56.8 46.9 42.3 37.6 56.6 55.2 52.9 35.0
56.0 46.2 41.8 37.2 56.3 54.6 52.5 34.9
55.5 45.4 41.3 36.8 55.9 54.5 52.0 34.6
Standard uncertainties are as follows: u(T) = ±0.5 K, u(γds ) = ±0.2 mJ·m−2. a Obtained by extrapolation.
When the anion remained the same [PF6] -, the values of γds obtained by two methods presented the same tendency and decreased as the alkyl side chains on the imidazole cation become longer. When the cation was maintained as [AVIM]+, the values of γds increased along the sequence:[Tf2N]− b [BF4]− b Br− b Cl−. This phenomenon can be attributed to the complex interactions between the cation and anion groups. With the extension of the alkyl side chain, the spread of the ionic charge enhances, thereby reducing the strength of the Coulomb interaction. The increase of the anion group size thus enhances the negative charge diffusion on ILs and weakens the strength of the cationanion interactions, thereby inducing a decrease in the surface energy [37]. At the same time, the values of γds of all the tested ILs were repetitive and decreased with increasing experimental temperatures. To the best of our knowledge, the surface energy of the ILs we examined has not been reported in other literature. A rough comparison was made with the ILs having a similar structure (Table 3), and the calculated results in our work showed the same trend and as would have been expected. 4.3. Lewis acid-base constants
Fig. 2. Determination of the dispersive component of the Surface energy, γds , ■ 303.15 K, ● 313.15 K, ▲ 323.15 K,▼ 333.15 K, ◆ 343.15 K. (a) Schultz method, (b) Dorris-Gray method.
4.2. Dispersive component of the surface free energy At the microscopic level, the surface free energy is a powerful means of exploring the interaction mechanisms between ILs and other molecules [36]. Take [OVIM][PF6] as an example: Fig. 2 demonstrated the dispersive component of the surface energy using Schultz and Dorris-Gray methods based on Eqs. (4) and (5). The final, calculated γds results of all the tested ILs by the two methods are shown in Table 2. For all the examined temperatures, the higher γds values for tested ILs were obtained with the aid of the Dorris-Gray method, more so than Schultz method. Moreover, the discrepancy between the two methods depended on the temperature, which increased with increasing temperatures. This phenomenon may be attributed to the fact that the Schultz method is independent of temperature and has a large calculation error at temperatures that deviate from ambient conditions.
The specific enthalpy of adsorption, ΔGsp, was obtained from the vertical distance between the RT ln Vn values of the polar probes and
Table 3 Dispersive component of the surface energy, γds , and surface tension, γ, of various ILs taken from the literature. ILs
T/K
γds (mJ·m−2)
ref
ILs
T/K
γ (mJ·m−2)
ref
[AMIM]Cl
343.15 353.15 363.15 373.15 343.15 353.15 363.15 373.15 343.15 353.15 363.15 373.15
52.26 50.82 46.08 42.05 46.55 45.82 45.04 44.70 46.37 43.48 40.19 39.29
[38]a
[HVIM]Br
[OVIM]Br
56.81 51.15 45.67 44.40 54.04 48.55 43.18 38.24
[41]b
[39]a
303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15
[HMIM]Cl
[HMIM][PF6]
a b
[40]a
Measured by the Schultz method. Measured by the density functional method.
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the reference line, as shown in Fig. 3. In Fig. 3, the straight reference line of n-alkanes obtained under different abscissa conditions defined the measurement of the dispersive interactions for the polar probes. The ΔGsp values of all polar probes obtained by the three methods were relatively high and gradually decreased with increasing experimental temperature. This showed that the specific surface interactions weakened with higher temperatures. The specific enthalpy of adsorption was calculated via a plot of ΔGsp/T versus the reciprocal temperature, using Eq. (8). The values of ΔHsp of each polar probe estimated with the three methods were slightly different and were all negative in every case. The specific intermolecular interactions are mainly derived from the interaction between the polar probes and the Lewis acidic-basic sites on the surface of the material [42]. The determination of the acid-base constants is critical for the modification based on the ILs. By plotting −ΔHsp/AN ∗ as a function of DN/AN ∗ using Eq. (10), the acid-base constants were obtained and are listed in Table 4. From Table 4, the difference between the three methods is mainly concentrated on the values of KB. That is, higher KB values were obtained by the Schultz method than those obtained from other methods. That the KB values were all greater than KA through the three methods suggests that the surface of the examined ILs has more basic sites than acidic sites. The Lewis basic sites are identified as nitrogen, oxygen and with halogen atoms, while the Lewis acidic sites are identified as methylene and methyl groups [27]. For the studied ILs, the KB/KA ratios obtained are plotted in Fig. 4. The KB/KA ratio obtained by different methods followed the subsequent order: Schultz N Brookman-Sawyer N Flour-Papirer, and were all N1, which confirms that the surface of all the tested ILs is placed in the same relative position on the acidity-basicity scale. Using the same method, the KB/KA ratios obtained decreased with the increase of the alkyl side chain on the imidazolium cation and with the increase in anion size, respectively. For the [CnVIM][PF6], the acidic sites increased as the alkyl side chains of the cation become longer and the tendency of the alkyl side chain to rise to the surface increased due to the influence of Coulomb forces in the imidazolium cation, thereby lowering the basicity. For [AVIM] based ILs, the basicity become low, which may be associated with the electronegativity of the anion group.
5. Conclusions In this paper, IGC was successfully used to characterize the surface properties of [CnVIM][PF6] (n = 2,4,6,8) ILs and [AVIM]-based ILs with different anions (Cl−, Br−, [BF4]−, [Tf2N]]−). The dispersive free energy analysis has shown that the value of γds is extremely sensitive to the calculation method, especially at higher experimental temperatures. The difference of calculated Lewis acid-base constants by means of the Vn of selected polar probes among the three methods was observed, and the three methods lead to the same conclusions through KB/KA. For the tested eight imidazolium-based ILs, the adsorption of the selected probes on the surface was found to be exothermic, and the values of ΔHsa varied from 25 to 45 kJ·mol−1. The obtained γds values via DorrisGray and Schultz methods all decreased as the temperature increased and the surface was found to be Lewis amphoteric with a stronger basic character. As the alkyl side chains of the cation become longer and the size of the anion group increased, lower γds and basicity were observed. In addition, in our case, the basicity obtained by the different methods followed the subsequent order: Schultz N Brookman and Sawyer N Flour and Papirer.
Fig. 3. Estimation of the specific energy of adsorption, ΔGsp, of the polar probes on the surface of [OVIM][PF6] at 303.15 K, (a) Schultz method, (b) Flour-Papirer method, (c) Brookman-Sawyer method.
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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Table 4 Surface Lewis acid-base parameters, KA and KB, of investigated ILs. ILs
[EVIM][PF6] [BVIM][PF6] [HVIM][PF6] [OVIM][PF6] [AVIM]Cl [AVIM]Br [AVIM][BF4] [AVIM][Tf2N]
Schultz
Flour and Papirer
Brookman and Sawyer
KA
KB
KA
KB
KA
KB
0.078 0.108 0.117 0.108 0.072 0.078 0.089 0.084
0.674 0.642 0.577 0.489 0.674 0.631 0.500 0.466
0.088 0.116 0.126 0.114 0.077 0.104 0.116 0.106
0.543 0.505 0.428 0.374 0.556 0.519 0.457 0.404
0.078 0.104 0.116 0.106 0.078 0.107 0.117 0.108
0.556 0.519 0.457 0.404 0.674 0.641 0.576 0.489
Standard uncertainties are as follows: u(Ka) = ±0.001, u(Kb) = ±0.001.
Fig. 4. Comparison of the Lewis acidity-basicity of examined ILs with different methods, ■ Schultz, ▲ Flour and Papirer, ● Brookman and Sawyer.
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Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromatography Qiang Wang a,b, Wen-Na Wang a,b, Bin Wang a,b, Jun Tang a,b, Qiang Wang a,b,⁎ a
Center for Physical and Chemical Analysis, Xinjiang University, Urumqi 830046, PR China Key Laboratory of Coal Cleaning Conversion and Chemical Engineering Process, Xinjiang Uyghur Autonomous Region, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, Xinjiang 830046, PR China
b
a r t i c l e
i n f o
Article history: Received 29 June 2019 Received in revised form 20 November 2019 Accepted 22 November 2019 Available online xxxx Keywords: Ionic liquids Inverse gas chromatography Surface properties Dispersive free energy Lewis acid-base parameters
a b s t r a c t The surface properties of 1-alkyl-3-vinylimidazolium hexafluorophosphate ([CnVIM][PF6], n = 2,4,6,8) ionic liquids and 1-allyl-3-vinylimidazolium ([AVIM]+), ionic-based liquids with different anions (Cl−, Br−, [BF4]−, [Tf2N]−) were investigated at temperatures between 303.15 and 343.15 K on the basis of inverse gas chromatography (IGC). The Schultz method and Dorris-Gray methods were used to estimate the dispersive free energy based on the net retention volumes of homogeneous n-alkanes, and it was found that the discrepancy between the two methods increases with increasing temperature, with the Dorris-Gray method providing slightly higher values than the Schultz method at the same test temperature. The Lewis acid-base parameters, KA and KB, were likewise assessed by the method proposed by Schultz, Flour and Papirer, Brookman and Sawyer, respectively. Although the difference between the values of acid-base parameters were observed, the ratios of KB/KA obtained by three methods presented the same tendency and implied that the surface of all the tested ionic liquids exhibited the Lewis amphoteric and stronger basic character. In addition, the values of the enthalpies of adsorption between the probes and ionic liquids were also determined. © 2018 Published by Elsevier B.V.
1. Introduction Generally speaking, ionic liquids (ILs) are a class of salts, and it is difficult to form an ordered crystal structure between anions and cations, which results in the melting point of ILs measuring somewhere below 100 °C [1–4]. ILs are considered to be “green solvents” and are proposed as a substitute for traditional industrial solvents, with the advantage of having a wide liquid range, recyclability, good stability and solubility, thermal nonflammability and so on [5–8]. When ILs are used in reactions and applications that occurred at the interface of a two-phase system, such as adsorption, catalysis, and extraction, the interactions and chemical to exchange strongly depend on the surface properties of those ILs [9–12]. The experimental data regarding the surface energy, and especially the acid-base properties, of ILs are still limited or lacking. At present, the techniques that can be used for the surface characterization of materials are mainly the sessile drop method, the Wilhelmy plate method, the NMR imaging method and contact-angle measurement [13–17]. These static adsorption techniques are susceptible to ⁎ Corresponding author at: Center for Physical and Chemical Analysis, Xinjiang University, Urumqi 830046, PR China. E-mail address: [email protected] (Q. Wang).
external factors, such as temperature and the surface roughness of materials [18]. As a kinetic adsorption characterization technique [19], inverse gas chromatography (IGC) allows access to more thermodynamic parameters [20–22] and physicochemical properties [23–27] of pure substances and binary mixtures. As such, it has been widely used in biological materials, polymers, the pharmaceutical industry, etc. This paper aims to develop at a better understanding on the surface properties of a series of 1-alkyl-3-vinylimidazolium hexafluorophosphate ([CnVIM][PF6]) ILs and 1-allyl-3vinylimidazolium ([AVIM]) based ILs. Some surface parameters were obtained from the interactions of ILs with polar and non-polar probes by IGC. Different evaluation methods were used to calculate the value of the dispersive surface energy and acid-base constants. 2. IGC calculation methods 2.1. Enthalpies of adsorption By measuring the retention time of probe solvents through the column packed with ILs, the net retention volumes, Vn and the specific retention volumes, V0g can be determined using Eqs. (1) and (2) [28].
https://doi.org/10.1016/j.molliq.2019.112202 0167-7322/© 2018 Published by Elsevier B.V.
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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Q. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
When plotting the retention diagrams (InV0gversus 1/T) using Eq. (3), ΔHsa can be calculated from the slope (‐ΔHsa/R) [29,30]. ðP o ‐P w Þ 3ðP i ‐P 0 Þ2 ‐1 ðt r ‐t 0 Þ Vn ¼ F Po 2ðP i ‐P 0 Þ3 ‐1 V og ¼
273 Vn mT a
ΔH sa ¼ ‐R
∂ InV g o ∂ð1=T Þ
ð1Þ
ð2Þ ð3Þ
where F is the carrier gas flow rate measured at room temperature, tr is the retention time of the probe through packed column, t0 is the retention time of a non-retained marker, T and Ta are the column and room temperatures, respectively, Pi and Po are the packed column inlet and outlet pressures, respectively, Pw is the saturated vapor pressure of water at room temperature, m is the mass of coated ILs, and R is the ideal gas constant. 2.2. Dispersive component of the surface energy In IGC theory, there are two basic calculation methods for the determination of dispersive surface energy, γds of the stationary phase. The first method, proposed by Schultz, can be expressed in the following form [31]: 1=2 þ K0 RT ln V n ¼ 2Na γ ds γdL
ð4Þ
where γdL is the dispersive component of the surface energy of the liquid probe molecule, a is the molecular surface area of the probe, and N is Avogadro's number. When a series of n-alkanes of known physical and chemical properties are used as probes, by plotting the RT ln Vn values of n-alkanes versus the term of 2Na(γdL )1/2, the value of γds can be derived from the slope. An alternative method used for the calculation of γds Dorris and Gray'method [32] is described by the following relationships: ΔGCH2 ¼ ‐RTIn V n;nþ1 =V n;n
γ ds ¼
ΔG2CH2 4N2 a2CH2 γ CH2
ð5Þ
ð6Þ
where Vn, n+1 and Vn, n are the net retention volumes of n-alkane with n + 1 and n carbon atoms, respectively, aCH 2 and γCH 2 are the crosssectional area and the surface free energy of a methylene group, respectively, and γCH 2 has a relationship of temperature:γCH 2 = 35.6 ‐ 0.058t. The free energy of adsorption of a methylene group (ΔGCH 2) can be calculated from the slope by plotting the RT ln Vn values of n-alkanes versus the number of their carbon atoms (n). Then, γds can be derived from Eq. (6).
be obtained according to the following relation: ‐ΔGsp ¼ ð−ΔGÞ− −ΔGref ¼ RT ln V n =V n;ref
ð7Þ
where Vn and Vn, ref are the net retention volume of the polar probe and a hypothetical alkane with the same physicochemical properties. The specific enthalpy of adsorption, ΔHsp, can be calculated from the slope by making a plot of ΔGsp/Tas a function of 1/T, as described by Eq. (8). ΔGsp ΔHsp ¼ −ΔSsp T T
ð8Þ
The Lewis acid-base parameters, KA and KB, can be determined by either using ΔGsp or ΔHsp values of selected ploar probes and are described by the following relationships: ΔGsp DN ≈ KA þ KB AN AN
ð9Þ
ΔH sp DN ¼ KA þ KB AN AN
ð10Þ
where DN and AN ∗ are the electron donor numbers and the corrected electron acceptor numbers of selected polar probes, respectively. Using ΔGsp leads to temperature-dependent KA and KB values, which also contain the specific entropic factor, whereas the utilization of ΔHsp gives constant values for each parameter corresponding to the temperature range of the study. In our case, we used the latter method to determine KA and KB values. The final acidity-basicity of the stationary phase surface is usually assessed by the ratio of KB/KA [25]. The value KB/KA N 1 represents a surface that is considered to be basic, while the value of KB/KA b 1 is considered to be acidic. 3. Experimental section 3.1. Materials and chemicals The mass fraction of ILs used in this work was N0.99 and were acquired from Chengjie Chemical Co. Ltd. (Shanghai, China), including 1ethyl-3-vinylimidazolium hexafluorophosphate ([EVIM][PF6]), 1butyl-3-vinylimidazolium hexafluorophosphate ([BVIM][PF6]), 1hexyl-3-vinylimidazolium hexafluorophosphate ([HVIM][PF6]), 1octyl-3-vinylimidazolium hexafluorophosphate ([OVIM][PF6]) 1-allyl3-vinylimidazolium bis(trifluorompropylsulfonyl) imide ([AVIM] [Tf2N]), 1-allyl-3-vinylimidazolium tetrafluoroborate ([AVIM][BF4]), 1allyl-3-vinylimidazolium chloride ([AVIM]Cl), 1-allyl-3vinylimidazolium bromide ([AMIM]Br). The probes used in experiment and their properties are listed in Table 1 [19,26,27,29]. In order to minimize test error, the purity of all probe solvents was of the highest available quality and was used as received without purification from J&K Scientific Ltd. 3.2. Column preparation
2.3. Lewis acid-base parameters The free energy of the adsorption of polar probes in the stationary phase is divided into the dispersive section (ΔGd) and the specific section (ΔGsp). In the literature, one can find three methods for the determination of ΔGspproposed by Schultz [31], Flour and Papirer [33] and Brookman and Sawyer, respectively [34]. In three methods, the estimation of ΔGsp is based on the same assumption that the value of ΔGd between a polar probe and the stationary phase is equal to the free energy of the adsorption of a hypothetical n-alkanes (ΔGref) with the same physicochemical properties. ΔGsp can
Stainless steel tubing (1200 mm × 2 mm I.D.) was selected to serve as the packed column. In order to reduce the water content and volatile compounds to negligible values, all ILs were subjected to a vacuum, at medium temperature, for at least 48 h prior to measurement. The water content in all liquid state ILs was b500 ppm, as checked by KarlFisher method. The stationary phase was used with the packed columns by dissolving a certain amount of ILs with methanol, and silicon alkylation 102 monomer support was used to load the solution. After the evaporation of the excess methanol under a rotary evaporator, the stationary phase was dried at 363.15 K until the mass was constant. The
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
Q. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
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Table 1 Physical and chemical properties of selected probes. Probe
n-Hexane n-Heptane n-Octane n-Decane Dichloromethane Chloroform Acetone Ethyl acetate Tetrahydrofuran
Tb
po
a × 1020
γdL
AN*
DN
(°C)
(kPa)
(m2)
(mJ/m)
(kJ/mol)
(kJ/mol)
68.74 98.5 125.6 150.8 39.75 61.3 56.53 77.2 66
20.14 6.093 1.871 0.580 57.93 26.16 30.56 1.781 3.671
0.515 0.570 0.630 0.690 0.315 0.440 0.425 0.480 0.450
18.4 20.3 21.3 22.7 27.6 25.9 16.5 19.6 22.5
– – – – 16.3 22.7 10.5 6.30 2.10
– – – – 0 0 71.4 71.1 84.4
Character
Neutral Neutral Neutral Neutral Acidic Acidic Amphoteric Amphoteric Basic
stationary phase was filled in the column with a vacuum pump and the mass of the packing ILs was calculated from the mass of the packed versus the empty column. In this experiment, the stationary phases consisted of 25–30% (w/w) of ILs. The amounts of analysed quality in the IGC experiments for eight ILs ranged from 0.65 to 0.71 g. Before the test, the packed column needed to be aged at 363.15 K and the nitrogen flow rate was 30 ml/min for 24 h.
3.3. Instruments and methods All measurements were carried out using an Agilent 6890 GC, which was equipped with a high sensitivity flame ionization detector. High purity nitrogen (N99.99%) was used as a carrier gas and the flow rate was determined experimentally at 30 mL/min. During all experiments, the injector and detector temperatures were kept at 523.15 K and 553.15 K, respectively. The oven temperature was maintained at a temperature range of (303.15–343.15 K) with an interval of 10 K under atmospheric pressure. The ionic liquids analysed under the experimental temperature range were either solid ([EVIM][PF6], [BVIM][PF6], [OVIM] [PF6], [AVIM][BF4]) or liquid ([HVIM][PF6], [AVIM][Tf2N], [AVIM]Cl, [AMIM]Br), which was determined by drying under a vacuum oven. In order to prevent that the bulk adsorption of the probe inside the liquid phase, which would affect the retention time of the probe, the retention times were determined using the maximum of the peaks and the interaction between the probes and the ILs, which can be considered only as a surface interaction [24]. We repeated the test three times for each probe and took an average of the retention times for subsequent calculations. The methane was used as calibration for dead time compared to other probe solvents.
4. Results and discussion 4.1. Enthalpies of adsorption After the retention time was determined, the specific retention volumes of the probes were calculated using Eq. (2). A linear relationship of InV0g and 1/T was acquired within the experimental temperature range for probes operating on the examined ILs, suggesting that the adsorption equilibrium had been achieved. This also indicated that the experimentally measured V0g values were suitable for thermodynamic analysis. The enthalpies of the adsorption of the probes on the examined ILs were calculated from the slope using Eq. (3) and the presented values were provided for uncertain surface coverage. For all examined ILs, ΔHsa values of n-alkanes increased with the increase of carbon atoms (Fig. 1). This suggests that the dispersive force between the examined ILs and n-alkanes became stronger as the CH2 chain in the probe molecule increased. Due to specific interactions, such as dipole-dipole interactions and hydrogen bonding between the polar probes and tested ILs, the ΔHsa values obtained were varied compared to the n-alkanes [25].
Fig. 1. Enthalpies of adsorption, ΔHsa, of probes on examined ILs; (a) ▼ [OVIM][PF6], ▲ [HVIM][PF6], ● [BVIM][PF6], ■ [EVIM][PF6], ◇ [BMIM][PF6] [35]; (b) ● [AVIM][BF4], ■ [AVIM][Tf2N], ☆ [AVIM]Cl, ▼ [AMIM]Cl [27], ◇ [AVIM]Br.
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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Q. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx Table 2 Dispersive component of the surface free energy, γds , of the tested ILs at various temperatures. ILs
γds /(mJ·m−2) 298.15 Ka
303.15 K
313.15 K
323.15 K
333.15 K
343.15 K
Schultz method [EVIM][PF6] 56.7 [BVIM][PF6] 47.6 [HVIM][PF6] 41.7 [OVIM][PF6] 37.3 [AVIM]Cl 56.4 [AVIM]Br 54.4 [AVIM][BF4] 52.8 [AVIM][Tf2N] 34.3
55.5 46.8 41.4 36.7 55.9 54.0 52.4 33.9
53.9 45.0 40.8 35.7 55.2 53.2 51.6 33.1
51.6 43.2 34.0 35.0 54.1 52.6 50.8 32.2
49.8 42.2 39.3 33.9 53.1 51.6 49.9 31.5
47.5 39.8 38.7 32.7 52.6 51.2 49.2 30.7
Dorris-Gray method [EVIM][PF6] 58.4 [BVIM][PF6] 49.1 [HVIM][PF6] 43.6 [OVIM][PF6] 38.6 [AVIM]Cl 57.8 [AVIM]Br 55.9 [AVIM][BF4] 54.0 [AVIM][Tf2N] 35.7
58.0 48.7 43.3 38.4 57.6 55.8 53.7 35.6
57.5 47.9 42.9 38.0 57.3 55.4 53.4 35.3
56.8 46.9 42.3 37.6 56.6 55.2 52.9 35.0
56.0 46.2 41.8 37.2 56.3 54.6 52.5 34.9
55.5 45.4 41.3 36.8 55.9 54.5 52.0 34.6
Standard uncertainties are as follows: u(T) = ±0.5 K, u(γds ) = ±0.2 mJ·m−2. a Obtained by extrapolation.
When the anion remained the same [PF6] -, the values of γds obtained by two methods presented the same tendency and decreased as the alkyl side chains on the imidazole cation become longer. When the cation was maintained as [AVIM]+, the values of γds increased along the sequence:[Tf2N]− b [BF4]− b Br− b Cl−. This phenomenon can be attributed to the complex interactions between the cation and anion groups. With the extension of the alkyl side chain, the spread of the ionic charge enhances, thereby reducing the strength of the Coulomb interaction. The increase of the anion group size thus enhances the negative charge diffusion on ILs and weakens the strength of the cationanion interactions, thereby inducing a decrease in the surface energy [37]. At the same time, the values of γds of all the tested ILs were repetitive and decreased with increasing experimental temperatures. To the best of our knowledge, the surface energy of the ILs we examined has not been reported in other literature. A rough comparison was made with the ILs having a similar structure (Table 3), and the calculated results in our work showed the same trend and as would have been expected. 4.3. Lewis acid-base constants
Fig. 2. Determination of the dispersive component of the Surface energy, γds , ■ 303.15 K, ● 313.15 K, ▲ 323.15 K,▼ 333.15 K, ◆ 343.15 K. (a) Schultz method, (b) Dorris-Gray method.
4.2. Dispersive component of the surface free energy At the microscopic level, the surface free energy is a powerful means of exploring the interaction mechanisms between ILs and other molecules [36]. Take [OVIM][PF6] as an example: Fig. 2 demonstrated the dispersive component of the surface energy using Schultz and Dorris-Gray methods based on Eqs. (4) and (5). The final, calculated γds results of all the tested ILs by the two methods are shown in Table 2. For all the examined temperatures, the higher γds values for tested ILs were obtained with the aid of the Dorris-Gray method, more so than Schultz method. Moreover, the discrepancy between the two methods depended on the temperature, which increased with increasing temperatures. This phenomenon may be attributed to the fact that the Schultz method is independent of temperature and has a large calculation error at temperatures that deviate from ambient conditions.
The specific enthalpy of adsorption, ΔGsp, was obtained from the vertical distance between the RT ln Vn values of the polar probes and
Table 3 Dispersive component of the surface energy, γds , and surface tension, γ, of various ILs taken from the literature. ILs
T/K
γds (mJ·m−2)
ref
ILs
T/K
γ (mJ·m−2)
ref
[AMIM]Cl
343.15 353.15 363.15 373.15 343.15 353.15 363.15 373.15 343.15 353.15 363.15 373.15
52.26 50.82 46.08 42.05 46.55 45.82 45.04 44.70 46.37 43.48 40.19 39.29
[38]a
[HVIM]Br
[OVIM]Br
56.81 51.15 45.67 44.40 54.04 48.55 43.18 38.24
[41]b
[39]a
303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15
[HMIM]Cl
[HMIM][PF6]
a b
[40]a
Measured by the Schultz method. Measured by the density functional method.
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the reference line, as shown in Fig. 3. In Fig. 3, the straight reference line of n-alkanes obtained under different abscissa conditions defined the measurement of the dispersive interactions for the polar probes. The ΔGsp values of all polar probes obtained by the three methods were relatively high and gradually decreased with increasing experimental temperature. This showed that the specific surface interactions weakened with higher temperatures. The specific enthalpy of adsorption was calculated via a plot of ΔGsp/T versus the reciprocal temperature, using Eq. (8). The values of ΔHsp of each polar probe estimated with the three methods were slightly different and were all negative in every case. The specific intermolecular interactions are mainly derived from the interaction between the polar probes and the Lewis acidic-basic sites on the surface of the material [42]. The determination of the acid-base constants is critical for the modification based on the ILs. By plotting −ΔHsp/AN ∗ as a function of DN/AN ∗ using Eq. (10), the acid-base constants were obtained and are listed in Table 4. From Table 4, the difference between the three methods is mainly concentrated on the values of KB. That is, higher KB values were obtained by the Schultz method than those obtained from other methods. That the KB values were all greater than KA through the three methods suggests that the surface of the examined ILs has more basic sites than acidic sites. The Lewis basic sites are identified as nitrogen, oxygen and with halogen atoms, while the Lewis acidic sites are identified as methylene and methyl groups [27]. For the studied ILs, the KB/KA ratios obtained are plotted in Fig. 4. The KB/KA ratio obtained by different methods followed the subsequent order: Schultz N Brookman-Sawyer N Flour-Papirer, and were all N1, which confirms that the surface of all the tested ILs is placed in the same relative position on the acidity-basicity scale. Using the same method, the KB/KA ratios obtained decreased with the increase of the alkyl side chain on the imidazolium cation and with the increase in anion size, respectively. For the [CnVIM][PF6], the acidic sites increased as the alkyl side chains of the cation become longer and the tendency of the alkyl side chain to rise to the surface increased due to the influence of Coulomb forces in the imidazolium cation, thereby lowering the basicity. For [AVIM] based ILs, the basicity become low, which may be associated with the electronegativity of the anion group.
5. Conclusions In this paper, IGC was successfully used to characterize the surface properties of [CnVIM][PF6] (n = 2,4,6,8) ILs and [AVIM]-based ILs with different anions (Cl−, Br−, [BF4]−, [Tf2N]]−). The dispersive free energy analysis has shown that the value of γds is extremely sensitive to the calculation method, especially at higher experimental temperatures. The difference of calculated Lewis acid-base constants by means of the Vn of selected polar probes among the three methods was observed, and the three methods lead to the same conclusions through KB/KA. For the tested eight imidazolium-based ILs, the adsorption of the selected probes on the surface was found to be exothermic, and the values of ΔHsa varied from 25 to 45 kJ·mol−1. The obtained γds values via DorrisGray and Schultz methods all decreased as the temperature increased and the surface was found to be Lewis amphoteric with a stronger basic character. As the alkyl side chains of the cation become longer and the size of the anion group increased, lower γds and basicity were observed. In addition, in our case, the basicity obtained by the different methods followed the subsequent order: Schultz N Brookman and Sawyer N Flour and Papirer.
Fig. 3. Estimation of the specific energy of adsorption, ΔGsp, of the polar probes on the surface of [OVIM][PF6] at 303.15 K, (a) Schultz method, (b) Flour-Papirer method, (c) Brookman-Sawyer method.
Please cite this article as: Q. Wang, W.-N. Wang, B. Wang, et al., Surface properties of ionic liquids: A study of different calculation methods in inverse gas chromat..., Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.112202
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Q. Wang et al. / Journal of Molecular Liquids xxx (xxxx) xxx
Table 4 Surface Lewis acid-base parameters, KA and KB, of investigated ILs. ILs
[EVIM][PF6] [BVIM][PF6] [HVIM][PF6] [OVIM][PF6] [AVIM]Cl [AVIM]Br [AVIM][BF4] [AVIM][Tf2N]
Schultz
Flour and Papirer
Brookman and Sawyer
KA
KB
KA
KB
KA
KB
0.078 0.108 0.117 0.108 0.072 0.078 0.089 0.084
0.674 0.642 0.577 0.489 0.674 0.631 0.500 0.466
0.088 0.116 0.126 0.114 0.077 0.104 0.116 0.106
0.543 0.505 0.428 0.374 0.556 0.519 0.457 0.404
0.078 0.104 0.116 0.106 0.078 0.107 0.117 0.108
0.556 0.519 0.457 0.404 0.674 0.641 0.576 0.489
Standard uncertainties are as follows: u(Ka) = ±0.001, u(Kb) = ±0.001.
Fig. 4. Comparison of the Lewis acidity-basicity of examined ILs with different methods, ■ Schultz, ▲ Flour and Papirer, ● Brookman and Sawyer.
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. Acknowledgments Funding for this work was provided by the National Natural Science Foundation of China (Grant No. 21566036, 21762041 and 21868037), and 111 Project (Grant No. D18022). References [1] T. Cremer, M. Killian, J.M. Gottfried, N. Paape, P. Wasserscheid, F. Maier, H.P. Steinrück, Physical vapor deposition of [EMIM][Tf2N]: a new approach to the modification of surface properties with ultrathin ionic liquid films, Chemphyschem 9 (2010) 2185–2190. [2] S.L.I. Toh, J. Mcfarlane, C. Tsouris, D.W. Depaoli, H. Luo, S. Dai, Room-temperature ionic liquids in liquid–liquid extraction: effects of solubility in aqueous solutions on surface properties, Solvent Extraction & Ion Exchange 24 (2006) 33–56. [3] M.V. Fedorov, A.A. Kornyshev, Ionic liquids at electrified interfaces, Chem. Rev. 114 (2014) 2978–3036. [4] S.P. Ventura, E.S. Fa, M.V. Quental, D. Mondal, M.G. Freire, J.A. Coutinho, Ionic-liquidmediated extraction and separation processes for bioactive compounds: past, present, and future trends, Chem. Rev. 117 (2017) 6984–7052.
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