4.2 T HERMOPHYSICAL PROPERTIES
4.2.4 Thermal stability
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The measured refractive indices for the present ILs were correlated as a function of temperature T using the following form of equation [94, 182]:
4-5 The coefficients A2 and A3 are estimated using the method of least squares and are listed in Table A- 36 together with the standard deviations (SD). The standard deviations were calculated using equation 4-2.
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50 100 150 200 250 300 350 400 450 500
0 20 40 60 80 100
Weight loss %
T/C
[C2CN Him]Br [C2CN Him]Cl [C2CN Him]DOSS [C2CN Him]DDS [C2CN Him]TFMS [C2CN Him]BS [C2CN Him]SBA
Fig 4-68 Thermogravimetric traces of [C2CN Him]-based ILs
Also the Td of ILs with incorporation of a CN group decreased remarkably in comparison with the corresponding ILs without a CN group (Td of [C4Mim]Cl, [C6Mim]Cl and [C8Mim]Cl is (254, 253 and 243) C respectively) [92]. However, the incorporation of a CN group made the ILs more reactive than the ILs without a CN group [106].
The decomposition temperatures of the present nitrile functionalized ILs incorporating sulfonate –based anions are high compared with that reported for other nitrile functionalized ILs; for [C3CN Mim]Cl the Td is (254.9) C [106], as a result of replacing the basic, reactive chloride with more stable anions. The thermal stability of the TFMS containing ILs is the highest due to the extremely stable anion (Fig 4-68).
This might be due to the high chemical and thermal stability of the C-F bond. The thermogravimetric traces of the ILs incorporating DDS, SBA, BS and TFMS, Cl and Br anions are shown in Fig 4-69 to Fig 4-72. Moreover, the thermal stability of the ILs with DOSS, DDS, SBA, BS and TFMS anions were high in comparison with the corresponding ILs with bromide anion which due to the high reactivity of the bromide anion.
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The start and onset temperatures of the studied ILs are presented in Table 4-2.
Thermal stability studies of the dual functionalized imidazolium-based ILs incorporating nitrile functionality with allyl, benzyl or 2-hydroxyethyl functionalities shows that these ILs begin to decompose at temperatures lower than that for their analogous incorporating only nitrile functionality (Fig 4-67). However, the incorporation of a two functional group made the ILs more reactive than the ILs with one functional group. The results show that the thermogravimetric decreases of the benzyl substituted imidazolium compound took place at much higher temperatures than those of the other imidazolium-based dual functionalized ILs, revealing the relatively high thermal stability of the benzyl-substituted imidazolium-based compound. The π-electrons in the benzyl group might be associated with the d orbital interactions. These interactions seem likely to give a significant increase in the bond strength, thereby resulting in the high thermal stability. Similar results have been reported [205].
50 100 150 200 250 300 350 400 450 500
0 20 40 60 80 100
Weight loss %
T/C
[C2CN Bim]DDS [C2CN Him]DDS [C2CN Oim]DDS [C2CN Dim]DDS [C2CN Him]SBA [C2CN Oim]SBA [C2CN Dim]SBA
Fig 4-69 Thermogravimetric traces of [C2CN Cnim]DDS and [C2CN Cnim]SBA ILs
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50 100 150 200 250 300 350 400 450 500 550 600
0 20 40 60 80 100
Weight loss %
T/C
[C2CN Bim]BS [C2CN Him]BS [C2CN Oim]BS [C2CN Bim]TFMS [C2CN Him]TFMS [C2CN Oim]TFMS
Fig 4-70 Thermogravimetric traces of [C2CN Cnim]BS and [C2CN Cnim]TFMS ILs
50 100 150 200 250 300 350 400 450 500 550 600 650
0 20 40 60 80 100
Weight loss %
T /C
[C2CN Bim]Cl [C2CN Him]Cl [C2CN Oim]Cl [P8,8,8,14]Cl [P8,8,8C6P8,8,8]Cl2 [P8,8,8C10P8,8,8]Cl2
Fig 4-71 Thermogravimetric traces of [C2CN Cnim]Cl ILs
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50 100 150 200 250 300 350 400
0 20 40 60 80 100
Weight loss %
T/C
[C2CN Bim]Br [C2CN Him]Br [C2CN Oim]Br [C2CN Dim]Br
Fig 4-72 Thermogravimetric traces of [C2CN Cnim]Br
Decomposition of imidazolium salts is attributed to decomposition of cation as facilitated by the anion which itself becomes transformed. Pyrolysis of imidazolium salts yielded volatile degradation products such as 1-substituted imidazoles and alkylated anions and the degradation pathway was identified as elimination of the N- substituent, which is essentially the reverse substitution synthesis reaction.
Nucleophilicity of anion and the ability of cation to undergo alkyl migration or elimination reaction have a significant influence on thermal stability of ionic liquids [123].
The start and onset temperatures of the phosphonium-based ILs are also affected slightly by the size of the alkyl chain of the cation (Fig 4-67). The decomposition temperatures of the present phosphonium-based ILs are lower compared to other phosphonium ILs with short alkyl chain, for [P2,2,2,8]NTf2 and [P2,2,2,12]NTf2 are 400 and 380 C respectively [113]. Decomposition temperatures of the DCILs are affected slightly by the spacer alkyl chain length; the decomposition temperature decreases as the spacer alkyl chain length increases as reported by Jared L. et al. [51] for DCILs.
This might be due to the increase of the reactivity with the increase of the alkyl chain length. The measured decomposition temperatures values of the present ILs are comparable to other DCILs reported by Guiqin, Y. et al. [50]. The TGA data reported
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showed only a dynamic property of IL where the IL is heated for a short period of time, which is not representative for the application in which longer heating time is required [206].
Table 4-2 Start and onset temperatures of the studied ILs Cations
Anions
[C2CN Bim] [C2CN Him] [C2CN Oim] [C2CN Dim]
Tstart
(C) Tonset
(C) Tstart
(C) Tonset
(C) Tstart
(C) Tonset
(C) Tstart
(C) Tonset
(C)
Br 218 253 211 245 194 240 191 238
Cl 205 247 194 239 191 229 - -
DOSS 291 307 263 293 257 274 250 261
DDS 279 297 265 285 238 259 235 246
SBA - - 268 302 249 288 255 273
BS 298 312 271 308 256 291 - -
TFMS 304 328 288 316 261 296 - -
[C2CN Ayim] [C2CN Bzim] [C2CN Heim]
DOSS 199 228 205 241 203 219 - -
[P6,6,6,14 ] [P8,8,8,14 ] [P8,8,8 C6 P8,8,8] [P8,8,8 C10 P8,8,8]
Cl - - 236 297 378 395 354 381
DOSS 294 368 283 341 389 446 371 426
As general the residue of the TGA results might be due to the impurities in the studied RTILs. Most of the ionic liquids synthesized are fully water-miscible at room temperature, which makes it difficult to free them from bromide and chloride impurities. Clearly, all ionic liquids made by metathesis from the corresponding sodium salt have high halide contents. The sources of impurities in RTILs were studied and it was concluded that the high halide content is due to unreacted starting material [100].
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