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Handbook of Solvents - George Wypych - ChemTech - Ventech!

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21.2 Ionic liquids 1477<br />

According to this study, the viscosity <strong>of</strong> ionic liquids is mainly controlled by hydrogen<br />

bonding, van der Waals forces, molecular weight, and mobility.<br />

In respect to the cation structure, alkyl lengthening or fluorination make the salt more<br />

viscous. Also the reduction <strong>of</strong> freedom <strong>of</strong> rotation (from butyl to isobutyl) increased the viscosity.<br />

Unexpectedly, methylation at the C2(H) <strong>of</strong> the imidazolium ring increased viscosity<br />

even if it suppressed the position most likely to hydrogen bond with the anion. 158<br />

In respect to the anion, from TfO - to NfO - and from TA - to HB - , the increase <strong>of</strong> van der<br />

Waals attraction dominates over the H-bonding decrease due to the better charge<br />

delocalization. However, from TfO - to Tf2N - , the almost complete suppression <strong>of</strong> H-bonding<br />

seems to influence more than the increase in Van der Waals attractions smaller anion<br />

weight <strong>of</strong> TfO - . On the other hand, comparing TA - and AcO - , the H-bonding strength <strong>of</strong><br />

AcO - is more important than its smaller size. The salts with lower viscosity are TA - , with<br />

minimal anion weight and moderate basicity, and Tf2N - , with minimal basicity and moderate<br />

anion weight. 158<br />

Table 21.2.11. Viscosity <strong>of</strong> some ionic liquids at 25 o C. For the binary mixtures the ratio<br />

means the proportion <strong>of</strong> AlX3 to ImX or TMSuX<br />

Cation Anion η, cps at 25 o C<br />

[C2-py] Br/AlCl3 (ratio 2:1) 22.5 155<br />

Br/AlBr3 (ratio 2:1) ~50 159<br />

[C2-mim] Cl/AlCl3 (ratio 2:1) 13.5 156<br />

Br/AlBr3 (ratio 2:1) 31 157<br />

BF4 37.7 (22 o C) 160<br />

Triflate (CF3SO3) 43 161<br />

Mesylate (CH3SO3) 160 161<br />

[C4-mim] BF4 233 (30 o C) 162<br />

PF6 312 (30 o C) 162<br />

Trimethylsulfonium Cl/AlCl3 (ratio 2) 39.3 163<br />

Br/AlCl3 (ratio 2) 54.9 163<br />

Br/AlBr3 (ratio 2) 138 163<br />

Clearly, many structural parameters affect the viscosity <strong>of</strong> ionic liquids and a more exhaustive<br />

study is needed to rationalize the different trends and to establish a correlation<br />

model for prediction.<br />

Finally, there are some scattered data on viscosity <strong>of</strong> other ionic liquids which are<br />

summarized in Table 21.2.11, together with the most relevant data from the studies cited<br />

above.<br />

As it was mentioned above, care must be taken when comparing or using these data as<br />

different measuring techniques can yield different viscosity results and furthermore, our recent<br />

investigations (to be published shortly) have shown that the presence <strong>of</strong> traces <strong>of</strong> water<br />

or other impurities like Cl - can have a dramatic effect on viscosity.

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