Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.2 Ionic liquids 1475
( )
ln = K / T − T + lnT + lnA
η 1
η 0 2
η
[21.2.5]
where:
Aη is a scaling factor
T0 is the “ideal transition temperature” and
Kη is a constant characteristic of the material
can be used for such fluids, with Kη being the formal analogue of the Arrhenius activation
energy. The parameter T0 has been given a quasi-theoretical significance by the free volume
theory of Cohen and Turnbull. According to this theory, T0 represents the temperature at
which the free volume of the liquid disappears and liquid transport becomes impossible.
155,157 Alternatively, it has been interpreted by Adams and Gibbs as the temperature at
which the configuration entropy of the supercooled liquid becomes zero. 155,157
The fitted parameters for the VFT equation are displayed in Table 21.2.8 for the
1,3-dialkylimidazolium chloride-aluminium chloride systems.
Table 21.2.8. Fitted parameters for the VFT equation for some AlCl 3 systems 156
Melt Mol ratio T0, K Kη,K -lnAη
[C1-mim]Cl- AlCl3 1:2
2:1
109.1
194.8
1024
811
5.444
4.739
[C 3-mim]Cl- AlCl 3
[C 4-mim]Cl- AlCl 3
[C 4-bim]Cl- AlCl 3
1:1 141.5 879 5.157
1:2 160.0 608 4.351
2:1 185.0 958 5.208
1:1 184.5 484 3.819
1:2 154.8 666 4.537
2:1 165.4 1261 5.793
1:1 156.5 792 4.811
1:2 150.7 750 4.765
These systems show a strong dependence of the composition for the basic range, with
an increase in viscosity as the aluminum trichloride mole fraction decreases. In the acidic
range the viscosity seems almost independent of the composition, nevertheless showing a
slight decrease with increasing aluminum trichloride mole fraction. However it was also observed
that increased viscosities where associated with longer alkyl chain lengths.
In the study of the 1-methyl-3-ethylimidazolium bromide - aluminum bromide systems,
the VFT parameters for the different compositions were fitted as a function of X
(AlBr 3 mole fraction) to cubic polynomial equations of the form
()
2
F x = c + c X + c X + c X
0 1 2
in neutral and basic melts and to second order equations of the form
()
1
F x = c + c X + c X
0 1
− −2
2
3
3
[21.2.6]
[21.2.7]
in acidic melts. The values generated for these constants are displayed in Table 21.2.9.