Clean Solvents - American Chemical Society

Chapter 4

Viscosity and Density of 1-Alkyl-3-methylimidazolium Ionic Liquids Downloaded by STANFORD UNIV GREEN LIBR on June 10, 2012 | http://pubs.acs.org Publication Date: May 24, 2002 | doi: 10.1021/bk-2002-0819.ch004

Kenneth R. Seddon, Annegret Stark, and María-José Torres The QUILL Centre, Stranmillis Road, The Queen's University of Belfast, Belfast BT9 5 A G , United Kingdom

We report here the viscosity and density of 1-alkyl-3methylimidazolium salts of [BF ] , [PF ] , Cl , [CF SO ] and [NO ] . Viscosity decreases as a function of temperature and increases with increasing alkyl chain length, while density decreases with increasing temperature and longer alkyl chain. The viscosity data were fitted to the VFT equation. The ionic liquids were found to exhibit Newtonian behavior when isotropic, whereas they act as non-Newtonian shear-thinning materials at the liquid-crystalline mesophase temperatures. -

-

4

6

-

3

3

-

-

3

Introduction Ionic liquids increasingly gain importance as green solvents (1,2,3,4,5,6). In order to implement l-alkyl-3-methylimidazolium ionic liquids into chemical processes, viscosity and density data are of the utmost importance for chemical engineers.

34

© 2002 American Chemical Society In Clean Solvents; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

35

Downloaded by STANFORD UNIV GREEN LIBR on June 10, 2012 | http://pubs.acs.org Publication Date: May 24, 2002 | doi: 10.1021/bk-2002-0819.ch004

The dramatic effect of traces of water and chloride impurities on the viscosity and density of room-temperature ionic liquids has been reported recently (7). Thus, the results reported here are accompanied by water and chloride measurements. Prior to collecting the viscosity and density data, the ionic liquid samples were dried with heating at ca. 70 °C in vacuo for 24 h, or until a constant water content was achieved. Water contents were determined by coulometric Karl-Fischer titration (7). Chloride measurements were conducted using a chloride-selective electrode (ex: Cole-Parmer) (7).

Viscosity Viscosity was measured by a LVDV-II Brookfield Cone and Plate Viscometer (1% accuracy, 0.2% repeatability). The sample cup of the viscometer was fitted with luer and purge fittings, so that a positive current of dry dinitrogen was maintained at all times during the measurements, thus avoiding absorption of atmospheric moisture. The sample cup was jacketed with a circulating water bath that was controlled by a circulator bath Grant LTD 6G (± 0.1 °C accuracy). The viscosity data for [Qmim][BF ], [C„mim][PF ], [C mim][CF S0 ], [C„mim][N0 ] and [C mim]Cl (where n is the number of carbons on the 1-alkyl chain of the l-alkyl-3-methyhmidazolium cation), at different temperatures are reported in Tables I, II, III, IV and V. 4

3

6

n

3

3

M

Table I. Viscosity / cP of [C„mim][BF ] (n = 2 to n = 11) at different temperatures 4

T/°C

n=2

n=4

n=6

10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Water / ppm % w/w [Cl'l

110 66.5 42.9 29.4

277 154 91.4 59.1 39.6 28.0 20.4 15.5 11.9 307 0.004

620 314 177 106 67.9 45.0 31.5 22.6 16.6 108 0.05

525 0.03

«=8 897 439 240 143 88.0 58.0 39.4 28.1 20.5 80 n/a fl

w= 10

« = 11

2071 928 456 248 147 92.3 60.6 42.2 27.4 275 n/a

2082 935 473 259 152 97.4 64.9 45.5 803 n/a

fl

fl

a

n/a, these ionic liquids are water-immiscible and therefore allow for efficient extraction of chloride salts. Thus, [Cf] is below the detection limit (7).

In Clean Solvents; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

36

fl

Table II. Viscosity / cP of [C mim][PF ] (n = 2 to n = 10) at different temperatures n


T/°C 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Water / ppm

6

n=2

n=4

n= 6

n=8

23.4 17.1 13.3 n/a*

728 371 204 125 82.0 55.1 37.8 25.3 19.5 76

1434 690 363 209 125 80.3 54.0 37.6 26.8 28

1848 866 452 252 152 94.5 63.0 42.8 31.0 35

n = 10

451 253 150 96.0 62.5 43.8 n/a 6

n = 12

214 128 80.4 51.0 n/a 6

a

these ionic liquids are water-immiscible and therefore allow for efficient extraction of chloride salts. Thus, [Cf] is below the detection limit (7).

* Karl-Fischer titration was not performed, since these ionic liquids are solid at roomtemperature.

Table III. Viscosity / cP of [QmimHNOa] (n = 2 to n = 8)* at different temperatures T/°C 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Water / ppm % w/w [CI]"

n =2

n=4

n=6

n=8

33.8 26.2 18.7 14.7 11.3 8.87 n/a 1.46

542 266 144 85.0 54.1 36.9 25.9 18.9 14.6 1265 0.001

1841 804 351 190 110 68.8 45.4 31.9 23.0 441 0.047

2918 1238 563 288 159 95.8 62.0

fl

30.0 224 0.028

a

Karl-Fischer titration was not performed, since this ionic liquid is solid at roomtemperature.


37

Table IV. Viscosity / cP of [C mim][CF S0 ] (n = 2 to n = 10) at different temperatures


n

T/°C 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Water / ppm % w/w [CI]"

3

3

n=4 163 99.0 64.2 44.1 30.9 22.8 17.4 13.8 11.0 295 0.21

n=2 72.2 50.0 35.6 26.1 19.5 15.0 11.8 9.45 7.75 237 0.13

n = 10 2059 981 512 292 172 109 72.1 49.9 35.6 84 0.024

n=8 955 492 274 161 101 68.3 47.3 34.5 26.3 98 0.014

fl

Table V. Viscosity / cP of [C„mim]Cl (n = 2 to n = 8) at different temperatures n=2

T/°C 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Water / ppm

n=4 142000 40890 11000 3800 1534 697 334 182 105 1030

341 173 99.9 61.2 39.1 5046

n=6 63060 18000 6416 2543 1124 569 311 183 114 7568

n=8 117000 33070 10770 4524 1930 927 498 283 172 3072

a

Preliminary data only included here; materials with much lower water content are currently being determined.

The viscosity data obtained for [C mim][CF S0 ] and [C mim][CF S0 ] are in good agreement with previously published data (8,9). There is also a good agreement with the reported viscosity at 80 °C of [C mim][PF ] (10). The viscosity data for [C mim][BF ] are also in agreement with published data (10,11), although the data are slightly more scattered. This scatter can be due to the hygroscopic nature of [C mim][BF ] and its complete miscibility with water (7), which makes it difficult to obtain chloride and/or water-free. The viscosity 2

3

3

4

2

2

3

3

6

4

2

4


38 data for [C mim][BF ] and [C mim][PF ] reported by Suarez et al. (12) are clearly higher than those reported here. This difference can be attributed to different measurement methods and/or to the presence of impurities. 4

4

4

6

Effect of Temperature on Viscosity The effect of temperature on the viscosity of [C„mim][BF ] (n = 2 to n = 14) is displayed in Figure 1. Clearly, for n < 12, increasing temperature continuously decreases the viscosity, while [Ci mim][BF ] (•), [Ci mim][BF ] (A) and [Ci mim][BF ] (•) show a discontinuous viscosity profile. These lastmentioned ionic liquids have been reported to have a liquid crystalline mesophase (13), with a lamellar bilayer structure. Thus, the dramatic decrease of the viscosity of [Ci mim][BF ] and [C mim][BF ] at 42 °C and 86 °C, respectively, correspond to the transition from the liquid crystal phase to the isotropic disordered liquid (i.e. the clearing point). Over the temperature range studied, [Ci mim][BF ] (•) is a liquid crystal and thus shows no clearing point effect on the viscosity. The viscosity of these liquid crystals is described in more detail when discussing the effect of shear rate on the viscosity. 4


2

4

4

3

4

4

2

4

4

13

4

4

• n=2 • n=4 A

n=6

• n= 8 o n= 10 x n = 11 • n = 12 A

n = 13

• n = 14 20

40

0

80

60

100

TIC

Figure 1. Viscosity of [C mim] [BFJ (n = 2 to n = 14) vs. temperature. n

The temperature effect on the viscosity of [C„mim][PF ] (n = 2 to n = 12), [C mim][N0 ] (n = 2 to n = 8), [C„mim][CF S0 ] (n = 2 to n = 10) and [C mim]Cl (n = 2 to n = 8) is shown in Figures 2, 3, 4 and 5 respectively. 6

w

3

3

3

rt



39

20

40

60

80

100

r/°c Figure 2. Viscosity of [C mim][PF ] (n = 2 to n = 12) vs. temperature. n

20

6

40

60

80

100

T/°C Figure 3. Viscosity of [C mim][NO3] (n = 2 ton = 8) vs. temperature. n


40

• n= 2


o n= 4

• n= 8

o n = 10

| 0

20

1

1

1

40

60

80

•

1

100

T l°C Figure 4. Viscosity of [C mim][CF S0 J (n = 2 to n = 10) vs. temperature. n

3

40

3

60

T I °C Figure 5. Viscosity of [C mim]Cl (n = 2 to n = 8) vs. temperature. It should be noted that these materials have a water content > 1000 ppm. n



41

Table VI Least-squares fitted parameters for the VFT equation. Anion [BF ]" 4

N 2

In (A)

0

2

r

3

SVIO

2 5 9 6

149.4

-5.5872

0.999994

1179

148.4

-5.9607

0.999983

6

1459

142.0

-6.7329

0.999949

8

1468

144.5

-6.6429

0.999978

10 2

1496

152.9

-6.6922

0.999817

349

243.3

-3.2790

0.999034

10

4

1712

127.4

-7.2628

0.999386

31

6

1609

141.4

-6.9126

0.999989

5

8

1682

139.8

-7.0579

0.999945

11

10

1804

142.6

-7.3623

0.999941

7

2

1447

113.6

-6.5624

0.998329

23

4

1011

171.0

-5.5444

0.999977

6

6

1028

182.4

-5.5068

0.999710

26

8 2

1325

168.2

-6.3658

0.999860

22

1789

73.7

-7.0938

0.999961

5

4

1054

145.8

-5.4042

0.999933

8

8

1273

153.4

-5.7660

0.999920

11

10

1668

139.3

-6.8542

0.999982

6

2

208.3

-5.5704

0.999721

14

4

975 2232

163.0

42

1680

173.4

-9.4849 -7.0795

0.999667

6

0.999955

15

8

1896

167.3

-7.4999

0.999813

32

[NO J3

[CF3SO3]"

cr

a

T /K

996

4

[PF«r

k/K

Fit standard error, parameters)]

8 = [ ! ( / « (rj)

20

-ln ^ ) /(number of points-number of fitted 2

c&XcA

expt

172


42 The viscosity vs. temperature data were least-squares-fitted to the VogelFulcher-Tarnmann (VFT) empirical equation (14,15,16), \n(rj) =k/(T-T ) + Vi ln(7) + ln(A) (where T is the absolute temperature), characteristic of glassforming liquids. The values of the fitted parameters k, T and A are displayed in Table VI. The empirical parameter T is usually referred to as the "ideal glass transition temperature" and it has been given theoretical significance both through the free volume theory of Cohen and Turnbull (17) and the configurational entropy approach of Adam and Gibbs (18). Below T the material exists in a state of closest random packing of the molecules or ions as an equilibrium glass with zero mobility (19). Experimentally, this thermodynamic equilibrium cannot be attained, and the experimental glass transition, T , is observed instead, as a sudden change in properties such as heat capacity, volume or viscosity. Thus, for the VFT parameter T to represent a theoretically meaningful ideal glass transition temperature, it has to be lower than the observed T . The values of T obtained in this study for [C„mim][BF ] and [C„mim][PF ] are all lower than the reported T values for such systems (13,20) by 50-60 °C. For smaller temperature ranges, the data may also be fitted to the Arrhenius equation. However, the analysis of the residual errors, r and 8 showed that the VFT equation describes the temperature behavior more accurately, and is thus more versatile. The kinematic and absolute viscosity of 1,3-dialkylimidazolium chloroaluminate ionic liquids (21) (temperature range -10 to 95 °C) and for [C mim]Br-AlBr (22) (temperature range ca. 25-100 °C) has been reported previously to follow non-Arrhenius behavior. In both studies, the viscosity was found to fit the VFT equation better than the Arrhenius equation. The nonlinearity of the Arrhenius plots of the viscosity of [C mim][CF S03] (8,9), [C mim][CF S0 ], [C mim][Tf N] (where [Tf N]" is [bis(trifluoromethylsulfonyl)amide]) and [C mim][Tf N] (8) (temperature range 5 to 85 °C) has also been reported. On the other hand, Arrhenius behavior has been reported for the absolute viscosity of A^-alkylpyridinium chloroaluminate (23) ionic liquids (temperature range 25 to 75 °C) and for the kinematic viscosity of [C mim][BF ] (77) (temperature range 20 to 100 °C). 0

0

0


0

g

0

g

0

4

6

g

2

2

3

2

4

3

3

2

2

4

2

3

2

2

4

Effect of the alkyl chain length The viscosity of the l-alkyl-3-methylimidazolium ionic liquids was found to increase with increasing alkyl chain length, although this increase is not linear (see Figure 6). The increasing viscosity of ionic liquids with longer alkyl chain lengths has been reported previously (8,21,23). Longer alkyl chain lengths in the l-alkyl-3-methylimidazolium cation not only lead to heavier and bulkier ions, but also give rise to increasing Van der Waals attractions between the aliphatic alkyl chains (8). Van der Waals forces occur increasingly, for n > 4, since the ionic headgroup has little or no electronwithdrawing effect on that part of the alkyl chain. In the extreme case, with n>12, liquid crystal phases are formed with a microphase separation of the hydrophilic ionic headgroups and the hydrophobic alkyl chains. This scenario is consistent with the interdigitated layered structure observed in the solid state for long chain ionic liquid like [C mim][PF ] (20). 12

6



43

0

J

,

,

,

(

2

4

6 n

8

10

Figure 6. Viscosity at 30 °C of [C mim][NOJ (D) and [C mim][CF S0 ](0) vs alkyl chain length. n

n

3

3

Effect of the shear rate The effect of shear rate (the velocity gradient within the flowing liquid) on viscosity indicates whether a liquid is Newtonian or non-Newtonian. Newtonian liquids display a viscosity which is independent of the shear rate (24). The viscosity of [C„mim][BF ] (n = 4 to n = 8) and [C„mim][PF ] (n = 4 to n = 12) ionic liquids were tested for Newtonian behavior by measuring the viscosity at different shear rates. The same experiment was performed on [Ci mim][BF ] at 40 °C, in the liquid crystal phase, and at 65 °C, when the ionic liquid is an isotropic liquid. For [C„mim][BF ] (n = 4 to n = 8) and [C„mim][PF ] (n = 4 to n = 12) and for [Ci mim][BF ] at 65 °C, the viscosity values at the different shear rates were constant within experimental error for all these ionic liquids. The results for [Ci mim][BF ] at 65 °C are displayed in Figure 7 (top). In contrast, Figure 7 (bottom) shows how the viscosity of the liquid crystalline [Ci mim][BF ] decreases as the shear rate increases. This is typical nonNewtonian shear-thinning behavior and reflects the breaking up of the layered structure of this liquid crystal by the shearing action. Furthermore, when the shear rate was reduced, the viscosity values were lower than the ones obtained on the increasing shear rate cycle. This indicates that the structure of the liquid crystal cannot be totally recovered, at least on the time scales used (1 h for the whole experiment, 330 sec between speed increases / decreases). 4

6

2

4

2

2

2

4

6

4

4

4



44

0

0.2

0.4

0.6

0.8

r/s"

1

Figure 7. Viscosity of [C mim][BFJ at 65 °C (top) and 40 °C (bottom) vs. shear rate. The upper lines in both graphs correspond to increasing shear rate (i.e.fromlow shear rates to high shear rates) and the lower lines correspond to decreasing shear rate (i.e. from high shear rates to low shear rates) 12

Density 3

Density was measured using calibrated 10 cm density bottles, with an experimental error of ± 0.0008 g cm" . The density bottles were immersed in a water bath (Grant LTD 6G, ±0.1 °C accuracy). The density data for [C„mim][BF ], [C„mim][PF ], [C„mim][N0 ] and [C„mim]Cl as a function of temperatures are reported in Tables VII, VIII, IX, and X. Chloride and water contents are the same as reported in the viscosity tables (Tables I, II, III and V). The density data obtained for [C mim][BF ] agree well with previously reported data (11), while the data for [C mim][BF ] and [C mim][PF ] are again slightly lower than that reported by Suarez et al. (12). 3

4

6

3

2

4

4

4

4

6


45

3

Table VII. Density / g cm" of [C„miml(BF l (« = 2 to n = 10) at different temperatures


4

T/°C 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

n=2 1.2479 1.2401 1.2334 1.2262 1:2181 1.2102 1.2050 1.1972

n= 6 1.1531 1.1455 1.1382 1.1323 1.1248 1.1156 1.1095 1.1031

n=4 1.2077 1.2017 1.1947 1.1885 1.1807 1.1748 1.1676 1.1603

n=8 1.1095 1.1001 1.0933 1.0868 1.0801 1.0730 1.0657 1.0574

n = 10 1.0723 1.0670 1.0603 1.0533 1.0461 1.0395 1.0330 1.0272

3

Table VIII. Density / g cm" of [C„mim][PF J (n = 4 to n = 8) at different temperatures 6

77 °C 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 b

C

n=4 1.3727 1.3626 1.3565 1.3473 1.3359 1.3285 1.3225 1.3126 d

e

n= 6 1.3044 1.2920 1.2798° 1.2681 1.2570 1.2521* 1.2389 1.2296 c

n=8 1.2345 1.2220 1.2124 1.2013*" 1.1929* 1.187/ 1.1753 1.1676

/

37.0 °C; 67.0 °C; 82.0 °C; 55.0 ° C ; 64.0 ° C ; 7 2 . 0 °C

3

Table IX. Density / g cm" of [C„mimHN03] (n = 4 and n = 6) at different temperatures T/°C 20.0 30.0 40.0 50.0

n=4 1.1574 1.1497 1.1435 1.1372

n=6 1.1185 1.1144 1.1078 1.1014

T/°C 60 70 80 90

n=4 1.1309 1.1239 1.1167 1.1120

n=6 1.0951 1.0892 1.0820 1.0769


46 3

Table X. Density / g cm" of [C„mim]Cl'' (h = 6 and n = 8) at different temperatures

T/°C

n= 6

25.0 40.0 60.0

n=8 1.0124 1.0074 0.9999

1.0338 1.0241 1.0135

a


Preliminary data only included here; materials with much lower water content are currently being determined

The decrease in the density of ionic liquids with increasing temperature can be fitted to linear equations of the form p = a + b (F-60), where T is the temperature in °C, a is the density at the arbitrary temperature, 60 °C, in g cm' , and b is the temperature coefficient in g cm" K* . The fitting values obtained for a and b for density of all the studied ionic liquids are tabulated in Table XI. The trends are displayed graphically in Figure 8 for [C„mim][BF ] (n = 2 to n = 10). The effect of the alkyl chain length on the density of ionic liquids is also linear (see Figure 9). The decrease of density with increasing size was reported previously for the 1,3-dialkylimidazolium chloroaluminate ionic liquids by Wilkes and co-workers (21), and the Af-alkylpyridinium chloroaluminate ionic liquids by Hussey and co-workers (23). The same feature is reflected in the density data reported by Bonhote et al. (8). 3

3

1

4

3

3

Table X L Least-squares fitted parameters a I g cm" and big cm' K'

Anion [BF ]" 4

[PF r 6

[N0 ]" 3

cr

n 2

4

a

-b/ia

1.2187 1.1811 1.1242 1.0796 1.0466 1.3381 1.2596 1.1960 1.1306 1.0951 1.0132 0.9999

4 6 8 10 4 6 8 4 6 6 8

7.2359 7.6229 7.2090 7.1304 6.6034 8.5275 10.2938 9.2302 6.5239 6.1377 5.7756 3.6033

2

r

0.9987 0.9992 0.9982 0.9979 0.9989 0.9962 0.9921 0.9928 0.9985 0.9980 0.9973 0.9990

a

1

4

s /io-

1 5 8 8 6 14 25 21 6 7 7 3

2

° Fit standard error, 8 =[S(p i -p ) /(number of points-number of fitted parameters)] ca cd

expt



47

Figure 8. Density of [C mim][BFJ (n = 2 to η = 10) vs. temperature. n

Figure 9. Density of [C mim][BF ] (n = 2 to η = 10) (Ο) and [C mim][PFJ (n=2 to η = 8) (Π) at 40 °C vs. alkyl chain length. n

4

n


48


Conclusions We have found suitable equations to predict the viscosity and density of an array of ionic liquids as a function of temperature and alkyl chain length. With regards to temperature, the viscosity data fit the VFT equation for glass forming liquids. Lengthening the alkyl chain results in an increased viscosity. Liquidcrystalline ionic liquids show a substantially higher viscosity than isotropic ionic liquids and behave as non-Newtonian shear-thinning materials. Density decreases linearly with both alkyl chain length and temperature. Because viscosity and density behave in a regular and predictable manner as a function of chain length and temperature, these properties can be easily modeled for chemical engineering purposes.

Acknowledgements We (AS and M-J T) would like to thank the European Union for the funding of this study which was conducted within the BRITE-EURAM III framework (Contract no BRPR-CT97-043, Project no BE96-3745), and the EPSRC and Royal Academy of Engineering (KRS) for funding a clean technology fellowship. We are also indebted to Dr. B. Ellis (BP Amoco), Dr. J.D. Holbrey, and to the Dept. of Chemical Engineering of the Queen's University of Belfast. We also wish to thank Dr. R. Wareing (Elementis) for the supply of imidazolium chlorides.

References 1. 2. 3. 4. 5. 6. 7. 8.

Rooney, D. W.; Seddon, K. R. In Handbook of solvents; Wypych, G., Ed.; William Andrew: Toronto, 2000, pp 1459-1484. Earle, M. J.; Seddon, K. R. PureAppl. Chem. 2000, 72, xxxx-xxxx. Earle, M. J.; Seddon, K. R. In Clean Solvents; Abraham, M . and Moens, L., Ed.; ACS Symp. Ser., 2000, pp in press. Welton, T. Chem. Rev. 1999, 99, 2071-2083. Freemantle, M . Chem. & Eng. News 2000, 78, 15th May, 37-50. Holbrey, J. D.; Seddon, K. R. Clean Prod. Proc. 1999, 1, 223-236. Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 12, 2275-2287. Bonhote, P.; Dias, A. P.; Armand, M . ; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168-1178.


49 9.

10. 11. 12.


13. 14. 15. 16. 17. 18. 19. 20. 21.

22. 23. 24.

Cooper, E. I.; O'Sullivan, E. M . In New, stable, ambient-temperature molten salts; ; Electrochem. Soc.: Pennington, NJ, 1992; Vol. 92, pp 386396. McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. J. Electrochem. Soc. 1999, 146, 1687-1695. Fuller, J.; Carlin, R.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881-3886. Suarez, P. A. Z.; Einlof, S.; Dullius, J. E. L.; de Souza, R. F.; Dupont, J. J. Chim. Phys. 1998, 95, 1626-1639. Holbrey, J. D., Seddon, K.R. J. Chem. Soc., Dalton Trans. 1999, 21332139. Tammann, G.; Hesse, W. Z. anorg. allg. Chem. 1926, 156, 245-257. Fulcher, G. S. J. Am. Ceram. Soc. 1925, 8, 339-355. Vogel, H. Phys. Z. 1921, 22, 645-646. Cohen, M. H.; Turnbull, D. J. Chem. Phys. 1961, 34, 120-125. Adam, G.; Gibbs, J. H. J. Chem. Phys. 1965, 43, 139-146. Angell, C. A.; Moynihan, C. T. In Molten Salts; Mamantov, G., Ed.; Marcel Dekker: New York, 1969, pp 315-375. Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627-2636. Fannin, A. A.; Floreani, D. A.; King, L. A.; Landers, J. S.; Piersma, B. J.; Stech, D. J.; Vaughn, R. L.; Wilkes, J. S.; Williams, J. L. J. Phys. Chem. 1984, 88, 2614-2621. Sanders, J. R.; Ward, E. H.; Hussey, C. L. J. Electrochem. Soc. 1986, 133, 325-330. Carpio, R. A.; King, L. A.; Lindstrom, R. E.; Nardi, J. C.; Hussey, C. L. J. Electrochem. Soc. 1979, 126, 1644-1650. Barnes, H. A.; Howard, A. An introduction to rheology; Elsevier: Amsterdam, 1989.


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