Low-temperature heat capacities of 1-alkyl-3-methylimidazolium bis(oxalato)borate ionic liquids and the influence of anion structural characteristics on thermodynamic properties

Abstract

Two chelated orthoborate ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(oxalato)borate ([Bmim][BOB]) and 1-hexyl-3-methylimidazolium bis(oxalato)borate ([Hmim][BOB]), were prepared and characterized. Their thermodynamic properties were studied using adiabatic calorimetry and differential scanning calorimetry (DSC). The thermodynamic properties of the two ILs were evaluated and compared with each other, and then with those of other [Bmim] type ILs. The results clearly indicate that for a given cation (or anion) and at a certain temperature, the more atoms in the anion (or cation), the higher the heat capacity; the higher glass-transition temperatures of [BOB] type ILs than others are mainly caused by the higher symmetry of the orthoborate anion structure. It is suggested that a high content of strong electronegative atoms and C_n or C_nv (n = 1,2,3,…,∞) point group symmetry in the anion are favorable for the design and synthesis of room temperature ILs with a wide liquid range.

---

1. Introduction

Ionic liquids (ILs) are very promising replacements for traditional volatile organic solvents because of their unique physicochemical properties, such as low vapor pressure, non-combustibility and good solubility.^1,2 Another attractive advantage of ILs is that they are designable.^3,4 The cation and anion of ILs can be varied and combined in nearly infinite ways which lead to a quite large number of different compounds with unique desirable properties. Correspondingly, the properties of ILs including viscosity, solubility and melting point etc. could be adjusted. Therefore, understanding the relationship between the basic physical properties and the molecular structure characteristics of ILs is one of key issues for ILs' design, evaluation and successful use.

Although much effort has been made on the synthesis and application of ILs in various areas such as catalysis, electrochemistry and separation sciences, the research work on their physicochemical properties, especially thermodynamic properties is still scanty.⁵Imidazolium based ILs as one of the most popular classes has been widely investigated, and for some of them, the thermodynamic properties are available.^6–12 In 2000, A. B. McEwen et al. investigated the thermal properties of several imidazolium based ILs by using DSC and TGA/SDTA.¹⁰ After that, J. F. Brennecke et al. reported the densities, melting temperatures, glass-transition temperatures, decomposition temperatures and heat capacities for a series of 13 imidazolium based ILs.¹¹ It is found that the properties of imidazolium based ILs follow quite reasonable trends which can help to design and synthesize more useful and task-specific ILs. Our research group has paid much attention to the fundamental thermodynamic investigation of ILs, and the heat capacities, standard enthalpies of formation and thermodynamic functions of some ILs have been investigated.^13–16 Recently, the ILs based on various chelated orthoborate anions attracted much attention because of their potential electrochemistry and enantiomeric recognition application.^17,18 Moreover, they are interesting solvents for large-molecule dissolution and a good model system for a fundamental liquid-state study.¹⁹ Very recently, we have also successfully achieved a new extra-large microporous borophosphate through the use of chelated orthoborate ionic liquids as a special boron source.²⁰

As a continuation of the research work in our group on the thermodynamic properties of ILs, we prepared and characterized two chelated orthoborate ionic liquids, 1-butyl-3-methylimidazolium bis(oxalato)borate ([Bmim][BOB], CAS No. 566135-35-1) and 1-hexyl-3-methylimidazolium bis(oxalato)borate ([Hmim][BOB]). Their thermodynamic properties were studied by using adiabatic calorimetry and differential scanning calorimetry (DSC). The phase change behavior and thermodynamic properties were evaluated and compared with other imidazolium based ILs.

2. Experimental section

2.1 Materials

All reagents were purchased from commercial suppliers. 1-Methylimidazole (Industrial grade, Linhai Kaile chemical factory, China), 1-bromobutane (AR grade, Sinopharm Chemical Regent Co., China) and 1-bromohexane (AR grade, Sinopharm Chemical Regent Co., China) were distilled before use. Lithium bis(oxalato)borate (LiBOB) (Premium Battery grade, Chemetall GmbH, Germany), acetonitrile (AR grade, Tianjin Fengchuan Chemical Reagent Co., China) and dichloromethane (AR grade, Beijing Chemical Reagent Co., China) with purities ≥99.5% were used in the synthesis process without further purification.

2.2 Preparation and characterization of [Bmim][BOB] and [Hmim][BOB] ILs

1-Methylimidazole (1 mol) was reacted with an excess of 1-bromobutane or 1-bromohexane (1.1 mol) in a round-bottomed flask under a nitrogen atmosphere at 70 °C for two days to prepare [Bmim][Br] or [Hmim][Br]. The product [Bmim][Br] was purified by recrystallization using acetonitrile as a solvent several times, and [Hmim]Br was purified by passing an active carbon and Al₂O₃ packed column. For preparation of chelated orthoborate ILs, an equal molar amount of [Bmim][Br] or [Hmim][Br] and LiBOB were used to conduct to an anion metathesis procedure as described elsewhere.¹⁹ The insolubility of lithium bromide in dichloromethane is sufficient enough to purify the orthoborate ILs by solvent extraction. After filtering of the LiBr, the dichloromethane phase containing the final product was extracted by water several times until the test for trace of bromide using an AgNO₃ solution showed no more cloudiness. Dichloromethane was removed under reduced pressure by using a rotary evaporator, and the product was finally dried under high vacuum at 70 °C for 48 h. The reaction scheme was shown in Scheme 1.

Scheme 1 Preparation scheme of [Bmim]/[Hmim][BOB] ILs.

The bromide and orthoborate products were characterized by a Bruker DRX 400 NMR spectrometer, and the ¹H, ¹³C and ¹¹B NMR spectra of orthoborate ILs were presented in Fig. S1–S6 in the ESI.† Their corresponding chemical shifts were as following:

[Bmim][BOB]: NMR in DMSO-d₆: ¹H (400 MHz) δ = 9.102 (s, 1H), 7.761 (s, 1H), 7.697 (s, 1H), 4.192–4.156 (q, 2H, ΔJ = 7.2 Hz), 3.867 (s, 3H), 1.816–1.742 (m, 2H, ΔJ = 7.2 Hz), 1.320–1.227 (m, 2H, ΔJ = 7.2 Hz), 0.926–0.889 ppm (t, 3H, ΔJ = 7.2 Hz). ¹³C (100.6 MHz) δ = 158.228, 136.538, 123.645, 122.280, 48.556, 35.778, 31.385, 18.802, 13.255 ppm. ¹¹B (128.3 MHz) δ =7.591 ppm, with H₃BO₃ as external.

[Hmim][BOB]: NMR in DMSO-d₆: ¹H (400 MHz) δ = 9.101 (s, 1H), 7.725 (s, 1H), 7.664 (s, 1H), 4.213–4.176 (t, 2H, ΔJ = 7.4 Hz), 3.907 (s, 3H), 1.831–1.798 (m, 2H, ΔJ = 6.8 Hz), 1.266 (s, 3H), 0.831 ppm (s, 3H). ¹³C (100.6 MHz) δ = 158.386, 136.605, 123.619, 122.241, 49.015, 35.806, 30.599, 29.467, 25.242, 21.900, 13.666 ppm. ¹¹B (128.3 MHz) δ = 7.608 ppm, with H₃BO₃ as external.

2.3 Adiabatic calorimetry

Heat capacity measurements were carried out in a high-precision automated adiabatic calorimeter described in detail elsewhere.^21–23 To verify the reliability of the adiabatic calorimeter, the molar heat capacities for Standard Reference Material 720, Synthetic Sapphire (α-Al₂O₃) were measured. The deviations of our experimental results from the recommended values by NIST²⁴ were within ±0.1% in the temperature range of 80–400 K.

2.4 Differential scanning calorimetry (DSC)

The DSC measurements were carried out using a Netzsch differential scanning calorimeter (model: DSC 204) under N₂ with a flow rate of 25 ml min⁻¹. The samples were first cooled with a rate of 10 K min⁻¹ from 300 K to 173 K, and then heated with the same rate from 173 K to 290 K.

3. Results and discussion

3.1 Low-temperature molar heat capacity

The experimental molar heat capacities of two ILs measured using an adiabatic calorimeter over the experimental temperature range (from 82 to 398 K) are listed in Table 1 and shown in Fig. 1. As seen in Fig. 1, glass transition behavior was obviously observed from 230 to 244 K for [Bmim][BOB], and from 230 to 241 K for [Hmim][BOB]. No other thermal anomaly or phase transition took place in the solid phases and liquid phases for both of them.

Fig. 1 Experimental molar heat capacity C_p,m of [Bmim][BOB] and [Hmim][BOB] as a function of temperature.

Table 1 Experimental molar heat capacities of [Bmim][BOB] and [Hmim][BOB]

T/K	C p,m/J K^−1 mol^−1	T/K	C p,m/J K^−1 mol^−1	T/K	C p,m/J K^−1 mol^−1
[Bmim][BOB]
82.260	162.68	187.494	291.06	294.720	551.30
84.253	164.80	189.427	293.99	296.704	548.95
86.175	167.39	191.350	296.21	298.689	549.07
88.114	169.92	193.303	298.74	300.668	550.11
90.038	172.86	195.291	300.98	302.649	550.60
91.957	175.54	197.267	303.12	304.624	550.99
93.888	178.05	199.233	305.52	306.599	551.46
95.810	180.61	201.627	309.09	308.573	552.38
97.724	182.92	204.037	312.02	310.543	552.45
99.653	185.50	206.020	315.00	312.509	553.28
102.331	189.45	208.000	317.55	314.473	555.34
104.994	193.24	209.971	320.03	316.435	556.40
106.920	195.26	211.938	321.68	318.394	557.61
108.859	197.53	213.896	323.98	320.352	557.15
110.775	199.69	215.852	326.20	322.303	558.16
112.702	202.09	217.801	329.42	324.256	559.28
114.642	204.87	219.806	332.92	326.204	560.50
116.562	207.20	221.863	335.96	328.152	560.98
118.493	209.88	223.919	339.45	330.096	561.78
120.441	211.92	225.971	342.57	332.092	563.39
122.365	214.00	228.020	345.96	334.140	564.45
124.304	216.71	230.056	350.39	336.185	564.63
126.259	218.92	232.074	361.82	338.224	566.77
128.193	220.92	234.141	361.55	340.266	568.03
130.108	223.20	236.241	370.61	342.302	568.32
132.039	225.31	238.241	394.03	344.335	569.25
133.982	227.87	240.030	473.56	346.365	570.84
135.911	230.25	242.083	525.22	348.392	571.96
137.852	232.95	244.303	528.69	350.415	572.99
139.814	235.14	246.290	528.83	352.437	573.08
141.756	235.90	248.334	528.83	354.455	573.61
143.679	238.11	250.372	529.19	356.471	574.41
145.590	241.90	252.415	528.85	358.482	576.20
147.521	244.43	254.450	529.21	360.492	578.27
149.463	246.62	256.486	530.58	362.499	579.53
151.952	248.86	258.520	531.80	364.503	579.76
154.404	251.85	260.553	531.94	366.499	580.19
156.317	254.53	262.579	533.59	368.490	582.08
158.267	256.13	264.608	534.57	370.480	582.13
160.198	258.45	266.632	534.39	372.463	583.83
162.118	261.49	268.653	536.90	374.456	585.44
164.072	264.24	270.677	537.26	376.445	582.92
166.057	266.60	272.694	537.44	378.428	585.20
168.031	267.94	274.709	538.78	380.417	587.00
169.988	270.01	276.721	539.56	382.395	587.98
171.937	272.91	278.733	540.89	384.374	589.30
173.874	275.19	280.744	542.47	386.346	588.94
175.798	276.91	282.758	542.58	388.317	590.79
177.710	279.16	284.767	542.56	390.289	592.57
179.656	281.05	286.744	556.25	392.256	593.07
181.633	283.48	288.759	544.97	394.216	591.44
183.597	286.34	290.758	549.07	396.203	596.40
185.552	288.38	292.740	551.92	398.239	598.12
[Hmim][BOB]
83.600	182.21	189.062	322.00	295.453	599.83
85.596	183.90	191.025	324.67	297.481	597.88
87.516	186.71	192.971	326.98	299.507	598.99
89.457	189.85	194.912	327.94	301.535	596.30
91.381	193.51	196.837	330.20	303.549	600.05
93.300	196.77	198.798	335.70	305.575	602.38
95.230	199.44	200.798	338.30	307.594	602.99
97.152	202.40	203.219	340.82	309.613	601.75
99.065	205.16	205.598	344.23	311.626	602.02
101.761	208.48	207.552	346.98	313.636	604.40
104.455	212.11	209.557	350.94	315.649	604.33
106.367	214.75	211.562	352.93	317.655	603.88
108.290	217.82	213.555	354.70	319.658	606.29
110.230	219.95	215.545	358.27	321.610	640.72
112.140	223.45	217.532	362.00	323.726	617.23
114.069	227.71	219.513	365.99	325.774	609.87
116.013	229.97	221.547	370.10	327.761	612.50
117.933	231.26	223.636	373.98	329.752	616.75
119.867	233.90	225.718	380.16	331.740	616.17
121.815	236.46	227.796	385.72	333.722	616.98
123.744	238.84	229.856	391.45	335.705	618.71
125.651	241.78	231.870	409.86	337.684	619.09
127.577	243.64	233.889	420.76	339.662	619.27
129.516	246.01	235.815	468.47	341.634	620.48
131.439	248.76	237.524	551.66	343.604	622.85
133.381	250.67	239.332	575.78	345.576	622.52
135.333	253.60	241.650	583.30	347.540	622.03
137.275	256.53	243.967	580.69	349.501	625.75
139.197	258.79	245.941	578.80	351.463	625.38
141.135	261.75	247.914	580.61	353.411	627.88
143.092	264.39	249.889	580.96	355.416	631.33
145.034	265.58	251.862	580.26	357.462	629.85
146.958	267.04	253.834	580.79	359.497	633.48
148.867	269.40	255.802	582.26	361.528	634.99
150.796	271.85	257.774	582.42	363.537	635.85
153.301	275.44	259.743	578.44	365.564	635.68
155.779	277.28	261.707	578.20	367.598	638.71
157.704	280.92	263.669	582.16	369.627	637.66
159.672	284.18	265.633	583.67	371.643	638.92
161.624	286.08	267.595	582.63	373.654	643.64
163.562	288.93	269.549	583.95	375.664	642.97
165.490	291.82	271.508	582.90	377.621	641.54
167.453	292.54	273.457	583.71	379.623	644.09
169.446	294.62	275.407	585.61	381.640	645.41
171.429	296.99	277.352	588.66	383.614	645.21
173.395	300.83	279.359	587.14	385.578	647.82
175.356	303.92	281.352	587.85	387.530	650.01
177.305	306.21	283.350	589.18	389.526	648.42
179.241	308.79	285.339	592.75	391.513	647.81
181.168	310.79	287.330	594.64	393.448	650.54
183.125	314.07	289.371	594.05	395.446	655.72
185.118	316.44	291.402	595.56	397.436	656.71
187.098	318.06	293.426	600.80

---

In order to fit the heat capacity data well to a polynomial equation, the temperature T was replaced by the reduced temperature X which is defined as:

X = [T − 0.5(T_max + T_min)]/[0.5(T_max − T_min)] (1)

where T is the thermodynamic temperature, T_max and T_min are the maximum and the minimum of the temperatures in the experimental temperature range. Then, −1 ≤ X ≤ 1.

The values of the experimental heat capacities can be fitted to the following polynomial equations with the least square method:

For [Bmim][BOB], before the glass transition (82–230 K),

C_p,m/(J K⁻¹ mol⁻¹) = 253.937 + 87.233X − 0.752X² + 1.271X³ + 2.181X⁴ + 5.415X⁵ (2)

where the reduced temperature X = (T − 156)/74. The above equation is valid from 82 to 230 K. The correlation coefficient of the fitted curve is R² = 0.99992, the standard deviation calculated from fitted data is ±0.2%.

After the glass transition (244–398 K),

C_p,m/(J K⁻¹ mol⁻¹) = 557.853 + 33.803X + 16.311X² + 3.391X³ − 38.341X⁴ − 0.604X⁵ + 27.596X⁶ − 2.181X⁷ (3)

where the reduced temperature X = (T − 321)/77. The above equation is valid from 244 to 398 K. The correlation coefficient of the fitted curve is R² = 0.99927, the standard deviation calculated from fitted data is ±0.1%.

For [Hmim][BOB], before the glass transition (83–230 K),

C_p,m/(J K⁻¹ mol⁻¹) = 279.346 + 89.957X + 11.183X² + 18.811X³ − 32.367X⁴ − 26.454X⁵ + 28.079X⁶ + 23.536X⁷ (4)

where reduced temperature X = (T − 156.5)/73.5. The above equation is valid from 83 to 230 K. The correlation coefficient of the fitted curve is R² = 0.99986, the standard deviation calculated from fitted data is ±0.3%.

After the glass transition (241–398 K),

C_p,m/(J K⁻¹ mol⁻¹) = 607.245 + 38.353X + 9.484X² + 37.313X³ − 8.442X⁴ − 83.323X⁵ + 9.964X⁶ + 44.552X⁷ (5)

where reduced temperature X = (T − 319.5)/78.5. The above equation is valid from 241 to 398 K. The correlation coefficient of the fitted curve is R² = 0.99772, the standard deviation calculated from fitted data is ±0.2%.

3.2 Thermodynamic parameters of glass transition

It can be seen from Fig. 1 that the heat capacity jumps, corresponding to the glass transition of the two ILs, took place in the range from 230 to 244 K for [Bmim][BOB], and from 230 to 241 K for [Hmim][BOB]. The glass-transition temperatures, 237.18 and 235.75 K, respectively, were taken from the average temperature of the starting and end points of the heat capacity jumps. The molar enthalpies, ΔH_g, and entropies, ΔS_g, of their glass transition were determined by the following equations:²⁵where Q is the total energy introduced into the calorimeter during the course of glass transition; T_i is a temperature point slightly lower than the temperature of the starting glass transition, and T_f is the temperature a little higher than the temperature of the ending glass transition; T_g is the glass-transition temperature; H_cell is the average heat capacity of the empty calorimeter; C_p(s) is the heat capacity of the sample in solid region at temperature (T_i + T_g)/2; C_p(l) is the heat capacity of the sample in liquid region at temperature (T_g + T_f)/2; n is the mole number of the sample. The results of the calculation were ΔH_g = 0.989 kJ mol⁻¹ and ΔS_g = 4.17 J K⁻¹ mol⁻¹ for [Bmim][BOB], and ΔH_g = 1.841 kJ mol⁻¹ and ΔS_g = 7.809 J K⁻¹ mol⁻¹ for [Hmim][BOB].

3.3 Thermodynamic functions

The thermodynamic functions (H_T − H_298.15) and (S_T − S_298.15) of the two ILs relative to the reference temperature 298.15 K were calculated in the experimental temperature range with an interval of 5 K, by using the polynomial equations of heat capacity and thermodynamic relationships as follows:

For [Bmim][BOB], before the glass transition,

after the glass transition,where T_i is the temperature at which the solid–liquid phase transition started; T_f is the temperature at which the solid–liquid phase transition ended; T_g is the temperature of glass transition; ΔH_g is the molar enthalpy of glass transition.

For [Hmim][BOB], the calculation of thermodynamic functions is the same as those of [Bmim][BOB]. The thermodynamic functions (H_T − H_298.15) and (S_T − S_298.15) of the two ILs are listed in Table 2.

Table 2 Thermodynamic functions of [Bmim][BOB] and [Hmim][BOB]

T/K	C p,m/J K^−1 mol^−1	H T − H298.15/kJ mol^−1	ST − S298.15/J K^−1mol^−1	C p,m/J K^−1 mol^−1	H T − H298.15/kJ mol^−1	ST − S298.15/J K^−1mol^−1
	[Bmim][BOB]			[Hmim][BOB]
80	158.42	−67.89	−357.39	175.50	−76.65	−406.50
85	165.87	−67.07	−347.55	183.55	−75.75	−395.59
90	172.96	−66.23	−337.87	191.25	−74.81	−384.88
95	179.75	−65.35	−328.34	198.75	−73.84	−374.35
100	186.31	−64.43	−318.95	206.12	−72.83	−363.97
105	192.69	−63.48	−309.71	213.37	−71.78	−353.74
110	198.94	−62.50	−300.60	220.49	−70.69	−343.64
115	205.08	−61.49	−291.62	227.44	−69.57	−333.68
120	211.15	−60.45	−282.76	234.20	−68.42	−323.85
125	217.17	−59.38	−274.02	240.77	−67.23	−314.15
130	223.14	−58.28	−265.38	247.14	−66.01	−304.59
135	229.10	−57.15	−256.85	253.35	−64.76	−295.15
140	235.03	−55.99	−248.41	259.44	−63.48	−285.83
145	240.95	−54.80	−240.06	265.46	−62.16	−276.63
150	246.86	−53.58	−231.79	271.46	−60.82	−267.53
155	252.76	−52.33	−223.60	277.51	−59.45	−258.53
160	258.65	−51.05	−215.49	283.66	−58.05	−249.62
165	264.54	−49.75	−207.44	289.92	−56.61	−240.79
170	270.43	−48.41	−199.45	296.32	−55.15	−232.04
175	276.32	−47.04	−191.53	302.85	−53.65	−223.35
180	282.24	−45.64	−183.66	309.48	−52.12	−214.72
185	288.19	−44.22	−175.85	316.17	−50.55	−206.15
190	294.19	−42.76	−168.08	322.88	−48.96	−197.63
195	300.28	−41.28	−160.36	329.59	−47.33	−189.16
200	306.48	−39.76	−152.68	336.32	−45.66	−180.74
205	312.85	−38.21	−145.03	343.13	−43.96	−172.35
210	319.43	−36.63	−137.41	350.24	−42.23	−164.00
215	326.28	−35.02	−129.82	357.98	−40.46	−155.67
220	333.48	−33.37	−122.24	366.91	−38.65	−147.34
225	341.12	−31.68	−114.66	377.88	−36.79	−138.97
230	Glass transition			Glass transition
235
240
245	528.80	−28.58	−101.38	581.18	−31.13	−114.94
250	528.78	−25.93	−91.12	580.77	−28.23	−103.21
255	530.00	−23.29	−81.03	580.60	−25.32	−91.73
260	531.96	−20.63	−71.11	580.92	−22.42	−80.46
265	534.31	−17.97	−61.33	581.83	−19.51	−69.38
270	536.78	−15.29	−51.71	583.31	−16.60	−58.48
275	539.23	−12.60	−42.22	585.25	−13.68	−47.75
280	541.57	−9.90	−32.87	587.54	−10.75	−37.18
285	543.75	−7.18	−23.65	590.04	−7.80	−26.76
290	545.79	−4.46	−14.56	592.63	−4.85	−16.47
295	547.71	−1.73	−5.59	595.23	−1.88	−6.33
298.15	548.89	0.00	0.00	596.84	0.00	0.00
300	549.58	1.02	3.26	597.78	1.11	3.69
305	551.43	3.77	12.01	600.26	4.10	13.59
310	553.33	6.53	20.64	602.68	7.11	23.36
315	555.31	9.30	29.17	605.07	10.13	33.02
320	557.42	12.08	37.60	607.49	13.16	42.57
325	559.65	14.88	45.93	609.99	16.20	52.01
330	562.02	17.68	54.17	612.63	19.26	61.34
335	564.52	20.50	62.32	615.44	22.33	70.58
340	567.10	23.33	70.38	618.44	25.41	79.73
345	569.74	26.17	78.35	621.62	28.51	88.79
350	572.38	29.02	86.24	624.93	31.63	97.76
355	574.98	31.89	94.06	628.31	34.76	106.64
360	577.50	34.77	101.80	631.66	37.91	115.45
365	579.92	37.67	109.46	634.88	41.08	124.18
370	582.23	40.57	117.05	637.89	44.26	132.83
375	584.47	43.49	124.57	640.64	47.46	141.41
380	586.73	46.42	132.03	643.17	50.67	149.91
385	589.13	49.36	139.42	645.64	53.89	158.34
390	591.89	52.31	146.75	648.42	57.12	166.69
395	595.30	55.28	154.02	652.11	60.37	174.98
400	599.77	58.26	161.23	657.68	63.65	183.20

---

3.4 Thermal property studies by DSC

The DSC curves of [Bmim][BOB] and [Hmim][BOB] are shown in Fig. 2. It should be noted that the scan began from 300 K cooling down to 173 K, followed by heating from 173 to 290 K. For [Bmim][BOB], one exothermic peak due to the freezing was observed at 196.3 K during the cooling process, and one endothermic peak due to glass transition appeared at 240.8 K when heating. The DSC curve of [Hmim][BOB] was similar to that of [Bmim][BOB], but the positions of exothermic and endothermic peaks moved to 207.4 and 238.0 K, respectively. Their glass-transition temperatures determined from DSC measurements are 238.8 and 235.4 K which are in agreement with the values observed from adiabatic calorimetry. The 238.8 K of T_g for [Bmim][BOB] is 5 K lower than the T_g value reported by Angell et al.¹⁹ This may be due to their faster heating rate (20 K min⁻¹) used in the DSC measurements.

Fig. 2 DSC curves measured under high purity nitrogen. H_f is the heat flow. The inset shows the detailed exothermic peaks of the two ILs.

In general, three types of thermal behavior are observed for the imidazolium based ionic liquids.¹¹ The first type has a distinct freezing point on cooling and a melting point on heating. The second type shows only glass transition with neither melting nor freezing points. The third one is characterized by cold crystallization. Upon heating, the compound passes from the glass to a sub-cooled liquid phase, and then cold crystallization occurs, followed by melting transition. In our cases, the thermal behavior of [Bmim][BOB] and [Hmim][BOB] is similar to the second type except for the additional appearance of freezing points during cooling. Cold crystallization occurs after the glass phase formation during cooling which is accompanied by exothermic behavior, as seen in the inset of Fig. 2. The molar enthalpy for the crystallization of the two ILs calculated through integration of the area of exothermic peaks is 37.7 J mol⁻¹ and 57.1 J mol⁻¹, respectively. No corresponding endothermic peak was observed during the heating process which might be due to the heat energy required for this phase transition being much smaller than that offered by the DSC equipment.

3.5 Estimation and comparison of thermodynamic properties for imidazolium based ionic liquids

Although the data reported in literature for heat capacity values of ILs are rather scarce, some heat capacities at specific temperature for imidazolium based ILs are available, which are listed in Table 3. Analyzing the data in Table 3 indicates that at a certain temperature, the more atoms in the anion, the higher the molar heat capacity of the ILs.¹¹ The molar heat capacity of [Hmim][BOB] is higher than that of [Bmim][BOB] at the same temperature due to the additional two –CH₂groups in its cation. The higher molar heat capacity of [Bmim][dca] contrasting to [Bmim][BF₄] at the same temperature is because that the [dca] anion has chemical triple bonds which can store more energy.

Table 3 Heat capacities at around 298 and 323 K, and glass-transition temperatures of 1-alkyl-3-methylimidazolium based ionic liquids^a

Ionic liquid	Molecular weight	Cp(298 K)/J mol^−1 K^−1	C p(323 K)/J mol^−1 K^−1	T g /K	Anion structures	Point group
a The data of other [Bmim] type ILs are taken from ref. 11 except that the Tg value of 243.8 K for [Bmim][BOB] from ref. 19 The data obtained in our work is presented in bold.
[Bmim][Cl]	174.6709	322.7	333.7	204	Cl
[Bmim][dca]	205.2598	364.6	370.0	183		C 2v
[Bmim][Br]	219.1222	316.7	323.6	223	Br ^−
[Bmim][BF4]	226.0228	351.5	358	188		T d
[Bmim][PF6]	284.1824	397.6	405.1	197		O h
[Bmim][BOB]	326.0672	549.068 (at 298.69 K)	559.276 (at 324.26 K)	238.8		D 2d
				243.8
[Hmim][BOB]	354.1203	597.881 (at 297.48 K)	610.325 (at 323.73 K)	235.4
[Bmim][Tf2N]	419.3644	536.3	543.9	187		C 2v
[Bmim][methide]	550.4380	782.8	802.4	208		C 3v

---

The glass-transition temperatures of some [Bmim] based ILs are listed in Table 3. No simple correlation between glass-transition temperatures and the molar volume of anions can be found though it is well known that the glass-transition temperature increases with increasing of ion size.²⁶ Another generally accepted viewpoint is that the decrease in ion symmetry and increase in the number of atoms with strong electronegativity in anion will help obtain desirable ILs with a wide liquid temperature range.^27,28 Therefore, the structural symmetry of the anion or cation has a significant impact on the physicochemical properties of ILs. Checking the symmetry of anions in Table 3 finds that [dca], [Tf₂N] and [methide] anions have the symmetry of C_2v, C_2v and C_3v point group, respectively, which are the only three anions with dipole moments among those compounds listed in Table 3.^29,30 The strong electronegative atoms, F, S or O, in these ILs can better disperse the negative charges of anions, which may contribute to the lower melting point or glass-transition temperature. For a given cation, when the anions have comparable capacities of negative charge delocalization, the glass-transition temperature increases with increasing of anion size. Based on this assumption, higher T_g of [BOB] type ILs can be easily explained. Although the content of O atoms in [BOB] anion is high which may help to delocalize the negative charge, its D_2d symmetry without dipole moment counteracts this effect, which leads to a higher T_g. In terms of the above analysis, we suggest to design and synthesize the ILs with anions containing strong electro negative atoms, and with the symmetry of C_n or C_nv (n = 1,2,3,…,∞) point groups (only compounds with the above mentioned structural symmetries have dipole moments.²⁹) so that the ILs have widened liquid ranges.

Conclusions

1-Butyl-3-methylimidazolium bis(oxalato)borate ([Bmim][BOB]) and 1-hexyl-3-methylimidazolium bis(oxalato)borate ([Hmim][BOB]) were prepared and studied by using adiabatic calorimetry and differential scanning calorimetry. Their thermodynamic properties and phase change behavior were compared with other series of [Bmim] based ionic liquids. The results indicate that their heat capacities increase with increasing of the atom number of the anion. The glass-transition temperature of ILs is normally affected by the ion size and the charge dispersion degree. The high content of strong electro negative atoms and C_n or C_nv (n = 1,2,3,…,∞) point group symmetry in the anion are recommended for the design room temperature ILs with a wide liquid temperature range.

Acknowledgements

We are thankful for financial support from the National Natural Science Foundation of China under grant NSFC No. 20901076 and 21073189, the Scientific Research Foundation for Returned Scholars from the Ministry of Education of China, and the Natural Science Foundation of Liaoning Province, China under grant No.20092174. A new ionic liquid database, recently launched by our research group, aims to collect comprehensive and up-to-date information on ionic liquids. The database—Delph-IL—is searchable by compounds or properties, and classifies data sets using quality indicators (Novionic, delph-IL, www.delphil.net, 2010).

References

1. R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers, J. Am. Chem. Soc., 2002, 124, 4974–4975.
2. T. Welton, Chem. Rev., 1999, 99, 2071–2083.
3. A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver, D. C. Forbes and J. H. Davis, J. Am. Chem. Soc., 2002, 124, 5962–5963.
4. J. H. Davis, Chem. Lett., 2004, 33, 1072–1077.
5. S. J. Zhang, N. Sun, X. Z. He, X. M. Lu and X. P. Zhang, J. Phys. Chem. Ref. Data, 2006, 35, 1475–1517.
6. Y. A. Sanmamed, P. Navia, D. Gonzalez-Salgado, J. Troncoso and L. Romani, J. Chem. Eng. Data, 2010, 55, 600–604.
7. R. Souda, J. Phys. Chem. B, 2008, 112, 15349–15354.
8. A. Muhammad, M. I. A. Mutalib, C. D. Wilfred, T. Murugesan and A. Shafeeq, J. Chem. Thermodyn., 2008, 40, 1433–1438.
9. H. Tokuda, K. Hayamizu, K. Ishii, M. Susan and M. Watanabe, J. Phys. Chem. B, 2005, 109, 6103–6110.
10. H. L. Ngo, K. LeCompte, L. Hargens and A. B. McEwen, Thermochim. Acta, 2000, 357–358, 97–102.
11. C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. Aki and J. F. Brennecke, J. Chem. Eng. Data, 2004, 49, 954–964.
12. Y. Shimizu, Y. Ohte, Y. Yamamura, K. Saito and T. Atake, J. Phys. Chem. B, 2006, 110, 13970–13975.
13. Z. H. Zhang, W. Guan, J. Z. Yang, Z. C. Tan and L. X. Sun, Acta Physico-Chim. Sinica, 2004, 20, 1469–1471.
14. Z. H. Zhang, Z. C. Tan, Y. S. Li and L. X. Sun, J. Therm. Anal. Calorim., 2006, 85, 551–557.
15. Z. H. Zhang, Z. C. Tan, L. X. Sun, J. Z. Yang, X. C. Lv and Q. Shi, Thermochim. Acta, 2006, 447, 141–146.
16. B. Tong, Q. S. Liu, Z. C. Tan and U. Welz-Biermann, J. Phys. Chem. A, 2010, 114, 3782–3787.
17. H. Bruglachner, S. Jordan, M. Schmidt, W. Geissler, A. Schwake, J. Barthel, B. E. Conway and H. J. Cores, J. New Mater. Electrochem. Syst., 2006, 9, 209–220.
18. S. F. Yu, S. Lindeman and C. D. Tran, J. Org. Chem., 2008, 73, 2576–2591.
19. W. Xu, L. M. Wang, R. A. Nieman and C. A. Angell, J. Phys. Chem. B, 2003, 107, 11749–11756.
20. M. Yang, F. F. Xu, Q. S. Liu, P. F. Yan, X. M. Liu, C. Wang and U. Welz-Biermann, Dalton Trans., 2010, 39, 10571–10573.
21. Z. C. Tan, G. Y. Sun, Y. Sun, A. X. Yin, W. B. Wang, J. C. Ye and L. X. Zhou, J. Therm. Anal., 1995, 45, 59–67.
22. Z. C. Tan, L. X. Sun, S. H. Meng, L. Li, F. Xu, P. Yu, B. P. Liu and J. B. Zhang, J. Chem. Thermodyn., 2002, 34, 1417–1429.
23. Z. C. Tan, Q. Shi, B. P. Liu and H. T. Zhang, J. Therm. Anal. Calorim., 2008, 92, 367–374.
24. D. G. Archer, J. Phys. Chem. Ref. Data, 1993, 22, 1441–1453.
25. Z. C. Tan, B. Xue, S. W. Lu, S. H. Meng, X. H. Yuan and Y. J. Song, J. Therm. Anal. Calorim., 2000, 63, 297–308.
26. W. Xu, E. I. Cooper and C. A. Angell, J. Phys. Chem. B, 2003, 107, 6170–6178.
27. Z. B. Zhou, H. Matsumoto and K. Tatsumi, Chem.–Eur. J., 2005, 11, 752–766.
28. P. Wasserschaid, in Ionic Liquids in Synthesis, second ed., Wiley–VCH, Weinheim, Germany, second edn, 2007.
29. G. D. Zhou and L. Y. Duan, in The Basis of Structural Chemistry, Peking University Publisher, China, forth edn, 2008.
30. K. V. Reddy, in Symmetry and Spectroscopy of Molecules, New Age Science, Second edn, 2010.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C and ¹¹B NMR spectra and TG/DTG curves of [Bmim][BOB] and [Hmim][BOB] ILs. See DOI: 10.1039/c0cp01744b

---