Handbook of Solvents - George Wypych - ChemTech - Ventech!

1474 D.W. Rooney, K.R. Seddon
pumped easily. In addition it is desired for the fluid to have only small changes in viscosity
through the normal operating temperature range.
In general, the published data on the viscosity of ionic liquids is scarce. Most of this
published literature on ionic liquids viscosity deals with the first generation ionic liquids.
The viscosity of any fluid is highly dependent on both the measuring technique used and the
purity of the samples. Given this difficulty the reported values in the literature are often neither
comparable or reproducible.
Once again, the first studies of the viscosity where related to the development of ionic
liquids as nonaqueous battery electrolytes. Hussey et al. 155 reported the viscosity of several
N-alkylpyridinium chloroaluminate salts over a temperature range of 25 to 75 o C where they
observed an increase in viscosity with alkyl chain length and concluded that the temperature
dependence of the viscosity could be fitted to the Arrhenius type equation, i.e.,
( )
η= η0 exp E η / RT [21.2.3]
where:
Eη is the energy of activation for viscous flow
R is the gas constant
T is the absolute temperature
This conclusion was supported by the linearity of the ln η vs. 1/T plots for the studied
temperature range. The above equation can be modified to give;
ln = A + B / T [21.2.4]
η η η
where:
Aη =lnη0 Bη =Eη/R The fitted parameters, along with the values of Eη, appear in Table 21.2.7.
Table 21.2.7. Parameters for Arrhenius equations for viscosity
Melt Mol ratio - Aη Eη
[NC1-py]Cl -AlCl3 1:2 4.801 4.639
[NC2-py]Cl -AlCl3 1:2 4.355 4.276
[NC3-py]Cl -AlCl3 1:2 4.734 4.523
[NC4-py]Cl -AlCl3 1:2 5.314 4.950
[NC2-py]Br -AlCl3 1:2 5.575 5.136
The viscosities of the 1,3-dialkylimidazoilium aluminium chloride 156
and
1-methyl-3-ethylimidazolium aluminium bromide 157 ionic liquids have also been reported
for different compositions and temperatures. For both the chloroaluminate and
bromoaluminate ionic liquids the temperature dependence was found not to have an
Arrhenious type curve, with non-linear plots of lnηvs. 1/T. In these studies the temperature
range used was wider than that of the N-alkylpyridinium. This non-Arrhenius behavior is
characteristic of glass forming melts. Here the three parameter Vogel-Tammann-Fulcher
(VFT) equation: