色谱  2015, Vol. 33 Issue (1): 58-64   PDF (843 KB)    
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李晓萍
王强
李玲
丁艳萍
反气相色谱法表征离子液体1-己基-3-甲基咪唑四氟硼酸盐的热力学参数
李晓萍1,2, 王强1 , 李玲2, 丁艳萍1,2    
1. 新疆大学理化测试中心, 新疆 乌鲁木齐 830046;
2. 新疆大学化学化工学院, 新疆 乌鲁木齐 830046
收稿日期：2014-09-10
基金项目：National Natural Science Foundation of China (Grant No. 21366029)
通讯作者：王强,E-mail:xjuwq@sina.com
摘要：采用反气相色谱法(IGC)表征了离子液体(IL)1-己基-3-甲基咪唑四氟硼酸盐([HMIM]BF4)在343.15~373.15 K温度范围内的热力学参数.使用了一系列不同化学结构的探针分子测定[HMIM]BF4与溶剂之间的相互作用力.根据探针分子的保留时间计算得到探针分子与[HMIM]BF4之间的Flory-Huggins相互作用参数、摩尔吸附焓、无限稀释摩尔混合焓、摩尔蒸发焓、无限稀释活度系数以及[HMIM]BF4的溶解度参数.结果表明,n-C6、n-C7、n-C8、n-C9、乙醚、四氢呋喃、苯、环己烷为[HMIM]BF4的不良溶剂;甲苯、间二甲苯、甲醇、乙醇、二氯甲烷、四氯化碳、氯仿、丙酮、乙酸乙酯、乙酸甲酯为[HMIM]BF4的良溶剂.运用外推法得到了[HMIM]BF4在室温(298.15K)时的溶解度参数为23.70 (J·-3)0.5.实验结果证明反气相色谱法是一种简便准确的获得离子液体热力学参数的方法.获得的热力学参数体现了这种离子液体与探针分子之间的相互作用力.本研究为离子液体的进一步应用提供了参考.
关键词： 反气相色谱法     离子液体     Flory-Huggins相互作用参数     无限稀释活度系数     溶解度参数    
Determination of the thermodynamic parameters of ionic liquid 1-hexyl-3-methylimidazolium tetrafluoroborate by inverse gas chromatography
LI Xiaoping1,2 , WANG Qiang1, LI Ling2, DING Yanping1,2    
1. Center for Physical and Chemical Analysis, Xinjiang University, Urumqi 830046, China;
2. College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, China
Received: 2014-09-10
Abstract: Inverse gas chromatography (IGC) was used to characterize the thermodynamic properties of ionic liquid (IL) 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4) in the temperature range from 343.15 K to 373.15 K. A series of solvents with different chemical natures were used to determine the [HMIM]BF4-solvent interactions. The thermodynamic parameters including Flory-Huggins interaction parameter, partial molar heats of sorption, mixing and evaporation as well as the activity coefficient at infinite dilution were obtained to judge the interactions between [HMIM]BF4 and the selected solvents. In addition, the solubility parameters of [HMIM]BF4 at different temperatures were determined. The results showed that among the selected solvents, n-C6, n-C7, n-C8, n-C9, diethyl ether, tetrahydrofuran, benzene and cyclohexane were poor solvents for [HMIM]BF4, while toluene, m-xylene, methanol, ethanol, dichloromethane, tetrachloromethane, chloroform, acetone, ethyl acetate and methyl acetate were the favorite ones. The solubility parameter of [HMIM]BF4 at room temperature (298.15 K) was 23.70 (J·-3)0.5, which was obtained by the linear extrapolation method. The experiment proved that IGC is a simple and accurate method to obtain the thermodynamic properties of ionic liquids. The obtained thermodynamic parameters revealed the strength of the interactions between the selected solvents and the ionic liquid, which could be used as a reference to the further applications of the ionic liquid.
Key words: inverse gas chromatography (IGC)     ionic liquid (IL)     Flory-Huggins interaction parameter     activity coefficient at infinite dilution     solubility parameter    
In recent years,ionic liquids (ILs) have emerged as environmentally benign green replacement for traditionally used volatile organic solvents in many scientific fields. ILs are molten salts at room temperature or near room temperature and entirely composed of cations and anions [1]. Because of the fascinating physicochemical properties,ILs have attracted much interest to chemists and bioengineers [2],like negligible vapor pressure,wide liquid range,high thermal solubility,good dissolution properties,etc [3]. To better understand how ILs work and be applied,several studies have been made to investigate the various physicochemical parameters of ILs,such as Flory-Huggins interaction parameter [4],activity coefficient at infinite dilution [5, 6],the solubility parameter [7]. To get these parameters,some techniques could be used,such as viscosimetry [8],differential scanning calorimetry,mechanical measurement [9]. However,these techniques are often time-consuming and laborious [10]. Since Smidsrod and Guillet developed the inverse gas chromatography (IGC) technology in 1969 [11]. This technology has been shown to be a simple,accurate and versatile tool to obtain the thermodynamic properties of various materials over a wide temperature range [12, 13]. IGC means that the stationary phase of the chromatographic column is the object of investigation,and the retention times of the probes are measured [14]. IGC has also been used to understand the miscibility of different materials [15, 16, 17, 18, 19]. The 1-alkyl-3-methyl-imidazolium family ((CnMIM)+) cations appear to be very useful for fine-tuning the physical properties of ILs,through variations in the alkyl chain length or the appropriate choice of the anions [7, 12, 13]. 1-Hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4) is one of the commonly investigated ILs. Taguchi et al. [20] determined the high-pressure densities of [HMIM]BF4. Costantini et al. [21] studied the phase behavior of a binary mixture consisting of carbon dioxide (CO2) and [HMIM]BF4. Although [HMIM]BF4 is important and interesting,its extensive utilization has been restricted because of the lack of the thermodynamic data. In this study,we investigated the thermodynamic parameters of [HMIM]BF4 by the IGC technique. The thermodynamic parameters,including Flory-Huggins interaction parameter partial molar heats of sorption,mixing and evaporation,activity coefficient at infinite dilution were obtained to characterize the interactions between [HMIM]BF4 and solvents. Furthermore,the solubility parameters of [HMIM]BF4 at different temperatures were calculated.
1 Theoretical background
1.1 Thermodynamic parameters
The specific retention volume,Vg0,which enables the determination of the thermodynamic parameters of the system under study,can be calculated according to the following equation [22]:

where m is the mass of IL; Ta is column temperature; tr is the retention time of the probe and t0 is the retention time of the non-interacting probe (such as methane); F is the flow rate of the carrier gas measured at room temperature; Pw is the saturated vapor pressure of water at ambient temperature,and Pi and Po are the inlet and outlet pressures,respectively. Vg0 can also be used to calculate other important thermodynamic qualities,such as the weight fraction activity coefficient,Ω 1∞,the molar heat (enthalpy),Δ H1S,of the probe’s absorption in the IL,the partial molar enthalpy of mixing at infinite dilution,Δ Hl∞,and the values of heat of vaporization,Δ Hv,which can be calculated as follows [23, 24]:

where T is the column temperature; R is the gas constant; M1 is the molecular mass of the probe; P10 is the saturated vapor pressure of the probe at temperature T; and B11 is the second virial coefficient of the probe in the gaseous state at temperature T (B11/Vc=0.430-0.886(Tc/T)-0.694(Tc/T)2-0.037 5(n-1)(Tc/T)4.5,where n is the amount of carbon atoms in the probe; Vc and Tc are the critical molar volume and the critical temperature of the probe,respectively); V1 represents the molar volume of the probe. Moreover,from the retention data determined with IGC experiments,the activity coefficients at infinite dilution ( γ 12∞) for the solute in the IL could be calculated using the expression [25]:

where n2 is the number of moles of stationary phase component within the column,B13 is the mutual virial coefficient between probe 1 and the carrier gas (dried N2,denoted by “3”). V10 is the molar volume of the probes,and the partial molar volume of the probe at infinite dilution V1∞ was assumed to be equal to V10. The factor J that corrects the influence of the pressure drop along the column is given by the following equation:

1.2 Flory-Huggins interaction parameter
The Flory-Huggins parameter,χ 12∞,which indicates the strength of the interactions between the materials and probes,can be calculated from the following equation [26, 27]:

where V2 is the specific volume of the IL.
1.3 Solubility parameter
In the early 1970s,Guillet et al. [11] calculated the solubility of materials by IGC. The solubility parameter of the IL,δ 2,can be calculated by the following equation [28, 29]:

By plotting the left-hand side of equation (9) as a function of δ 1,the solubility parameter of the IL,δ 2,can be obtained from the slope of the straight line.
2 Experimental
2.1 Apparatus and chemicals
The IL [HMIM]BF4 used in the experiment was obtained from Chengjie Chemical Co. Ltd.,China with a mass fraction purity greater than 0.99. [HMIM]BF4 was further purified by vacuum evaporation to remove the trace of volatile impurities prior to use. A homologous series of n-alkanes from n-hexane to n-nonane (analytical grade,Bodi Chemical Holding Co. Ltd,China) were used as the non-polar solvents. Chloroform,dichloromethane,carbon tetrachloride,ether,acetone,ethanol,methanol,ethyl acetate,methyl acetate,cyclohexane,benzene,toluene,m-xylene,tetrahydrofuran were purchased from Baishi Chemical Industry Co. Ltd.,China with a high purity (＞ 99.5% ) in quantities. All the studied solvents were used without further purification because the chromatography technique efficiently separates any impurities on the column.
2.2 Chromatographic procedure
The IGC experiments were carried out using a commercial Hewlett-Packard 6890 gas chromatograph equipped with a flame ionization detector (FID). Chemstation software (version A.06.01) was used to directly record the detector signals. The injector and detector temperatures were kept at 523.15 K during all experiments. Dried nitrogen was used as the carrier gas with a flow-rate of about 10 mL/min measured at the end of the column using a soap bubble flow meter. Methane was used to determine the column hold-up time for calculating the retention time of other probes. The temperature of the oven ranged from 343.15 K to 373.15 K with 10 K increments. Each experiment was repeated at least thrice to ensure the reproducibility. The stationary phase used in the experiment was prepared by dissolving a weighed sample of the [HMIM]BF4 in dichloromethane and followed by depositing the resulting solution onto a weighed amount of silicon alkylation 102 monomer support (60-80 mesh). The mixture was slowly dried under a rotary evaporator by slow evaporation,and stirred to ensure a homogeneous mixture. The stationary phase consisted of 10% (mass percentage) of the [HMIM]BF4. The coated support was packed into a stainless steel column with 0.2 cm of inner diameter and 60 cm long and then equilibrated under nitrogen for 8 h at 453.15 K prior to use.
3 Results and discussion
The specific retention volume,Vg0,is an essential term in the IGC experiment to determine the thermodynamic parameters of a system under study. The Vg0 of the 18 solvents on the [HMIM]BF4 were obtained according to equation (1) from 343.15 K to 373.15 K. Fig. 1 and Fig. 2 are the retention diagrams for the solvents-[HMIM]BF4 systems. The results show that the values of Vg0 of the probes on [HMIM]BF4 varied for each probe and were linearly decreased with the increase of the temperature. As for n-alkanes,the Vg0 increased with the increase of CH2 groups added to the probes.
Fig.1 Plot of lnVg0 versus 1/T for the probes

Fig.2 Plot of lnVg0 versus 1/T for the probes 1. dichloromethane; 2. acetone; 3. chloroform; 4. ethyl acetate; 5. tetrahydrofuran; 6. diethyl ether; 7. tetrachloromethane; 8. methyl acetate; 9. ethanol; 10. methanol.

According to equations (2,3,4),the molar heat of sorption of the probes into the [HMIM]BF4,Δ H1S,the molar enthalpy of mixing at infinite dilution,Δ Hl∞,and the heat of vaporization,Δ Hv,in the temperature range from 343.15 to 373.15 K can be obtained,and the values are shown in Table 1. The sorption process was dependent on the chemical nature of IL and the interactions between the [HMIM]BF4 and probes. Probes with different chemical natures have different interactions with [HMIM]BF4. Alkanes would reveal their dispersive interactions with [HMIM]BF4,while acetates,halogenated and oxy probes revealed the dipole-dipole and H-bonding interactions,aromatic and cyclic probes revealed the van der Waal’s interactions. The data in Table 1 can reflect which solvent is closer to dissolve [HMIM]BF4 and which solvent shows a stronger interaction with [HMIM]BF4. It can be seen from Table 1 that the partial molar heat of mixing,Δ Hl∞,for n-alkanes,alcohols and halogenated probes have negative values,which reflect exothermic mixing. However,the values are positive for tetrahydrofuran,acetone,acetates,ether,aromatic and cyclic probes that reflect endothermic mixing. According to equation (2),the molar heats of sorption,Δ H1S,of the probes into the [HMIM]BF4 were obtained. As for the n-alkanes,the number of CH2 groups in the n-alkanes series would affect the values of the exothermic molar heat of sorption. When more CH2 groups were added to the probes,the heat of sorption became more exothermic,which led to the stronger interaction between the probes and [HMIM]BF4. In addition,the values of Δ H1S for dichloromethane,chloroform,and carbon tetrachloride increased with the number of Cl atoms in the probe molecules increasing. Specifically,the CH2 groups exhibited dispersive forces during interaction of the CH2 groups of [HMIM]BF4,whereas the polar groups had the dipole-dipole forces during interaction with the polar groups of [HMIM]BF4.

Table 1 Molar enthalpy of probe’s absorption,Δ H1S, partial molar enthalpy of mixing at infinite dilution,Δ Hl∞,and molar enthalpy of vaporization,Δ Hv,between probes and [HMIM]BF4

The Flory-Huggins interaction parameter between IL and solvents ( χ 12∞) played a significant role in predicting the thermodynamic state of IL mixed with the solvents,swelled with solid,blended with another liquid,and in many other applications. It can be calculated based on equation (8),and the results are given in Table 2. χ 12∞ has been proposed to indicate the interactions between IL-solute systems. The following rule has been formulated among researchers in thermodynamics: χ 12∞＞1,bad solvents; χ 12∞＜0.5,good solvents [30]. For all examined solvents,high Flory-Huggins interaction parameter values reflected the poor compatibility/miscibility and were observed for n-C6,n-C7,n-C8,n-C9,diethyl ether,tetrahydrofuran,benzene and cyclohexane,which indicated they are bad solvents for [HMIM]BF4. While low Flory-Huggins interaction parameters reflected good solubility and were observed for toluene,m-xylene,ethanol,methanol,dichloromethane,tetrachloromethane,chloroform,acetone,ethyl acetate and methyl acetate,which indicated they are good solvents.

Table 2 Flory-Huggins interaction parameters,χ 12∞, between probes and [HMIM]BF4 at various temperatures

The activity coefficients at infinite dilution provide great information about the strength of the ionic liquid with organic solvents. The values were calculated based on equation (6),as listed in Table 3. The increase of the temperature resulted in the decrease of γ 12∞ for n-alkanes,ether,alcohols,tetrahydrofuran,benzene,toluene,m-xylene and cyclohexane. However,the opposite trend was observed for some probes,especially for dichloromethane,tetrachloromethane,chloroform,acetone,ethyl acetate and methyl acetate,as shown clearly in Fig. 3 and Fig. 4. Higher values of γ 12∞ were observed for n-C6,n-C7,n-C8,n-C9 benzene,tetrahydrofuran,diethyl ether,and cyclohexane,which indicated weak interactions between the probes and [HMIM]BF4. Smaller values of γ 12∞ were observed for alcohols,acetone,dichloromethane,tetrachloromethane,chloroform,ethyl acetate and methyl acetate,toluene,and m-xylene,which indicated the stronger interaction with [HMIM]BF4. All the results were consistent with the χ 12∞ values.

Table 3 Activity coefficients at infinite dilution,γ 12∞, of probe-[HMIM]BF4 systems at various temperatures

Fig.3 Plot of ln γ 12∞ versus 1/T for the probe-[HMIM]BF4 systems 1. n-C6; 2. n-C7; 3. n-C8; 4. n-C9; 5. cyclohexane; 6. benzene; 7. toluene; 8. m-xylene.

Fig.4 Plot of ln γ 12∞ versus 1/T for the probe-[HMIM]BF4 systems 1. dichloromethane; 2. acetone; 3. chloroform; 4. ethyl acetate; 5. tetrahydrofuran; 6. diethyl ether; 7. tetrachloromethane; 8. methyl acetate; 9. ethanol; 10. methanol.

Knowledge of the solubility parameters of IL is of vital importance for the selection of solvents for ILs. The solubility parameter,δ 2,for [HMIM]BF4 was obtained from the slope of the plot of δ 12/(RT)- χ 12∞/V1 against δ 1 by equation (9) (Fig. 5). The δ 2 for [HMIM]BF4 were measured at four temperatures to obtain the temperature dependence of the solubility parameters and the results are listed in Table 4. The δ 2 linearly increased with the increase of temperature (Fig. 6). The extrapolated value for δ 2 at 298.15K was found to be 23.70 (J\5cm-3)0.5,which was consistent with the 23.3 (J\5cm-3)0.5 value reported by Foco et al. [13].

Fig.5 Plot of δ 12/(RT)- χ 12∞/V1 versus δ 1 at 343.15K

Fig.6 Relationship between the solubility parameter of [HMIM]BF4,δ 2,and the temperature

Table 4 Solubility parameters,δ 2,of [HMIM]BF4 at various temperatures
4 Conclusions
The IGC technique is successful in determining the thermodynamic properties of [HMIM]BF4. The obtained thermodynamic parameters revealed the strength of the interaction between the selected solvents and [HMIM]BF4. According to Flory-Huggins interaction parameters,n-C6,n-C7,n-C8,n-C9,diethyl ether,tetrahydrofuran,benzene and cyclohexane are poor solvents for [HMIM]BF4. By contrast,the probes toluene,m-xylene,acetone,ethanol and methanol,dichloromethane,chloroform,tetrachloromethane,ethyl acetate and methyl acetate are excellent solvents for [HMIM]BF4. In addition,the solubility parameters δ 2 for [HMIM]BF4 were determined and the extrapolated value for δ 2 at 298.15 K was found to be 23.70 (J\5cm-3)0.5.
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