ahmadino
lighter, and more symmetric anions leading to
more viscous RTILs.
36,37 However, bis(perfluoroethylsulfonyl)imide anion, [BETI], leads to an increase of viscosity
by
∼100 mPa s as compared with its smaller [NTf2] anionicparent,
32,34 indicating that other parameters need to beconsidered. Also, the relative basicity of the anions and their
ability to form hydrogen bonds or to allow van der Waals
attractions have a pronounced effect on viscosity.
31,32 Thefluorinated anions such as [BF
4], [PF6], or [BETI] formviscous ionic liquids due to the formation of hydrogen bonds
or to increased van der Waals forces.
31,32,34 The weakly basic[NTf
2] anion exhibits an extensive charge delocalizationwithin the S
-N-S backbone,38 which decreases interactionsin these salts, accounting for their low viscosity.
31,32,39However, it is worth outlining that charge delocalization is
a significant factor but is certainly not the only critical factor
because a charge delocalization is also observed with
[BETI].
40Cations also influence the viscosities of the RTILs. For
any of the cation types, increasing the length of alkyl
substituents results in increasing the viscosities because of
stronger van der Waals interactions between larger cations.
31,36,41,42
The increase in viscosity for a series of 3-alkyl-1-methyl-imidazolium [NTf
2] salts was found to be linearwith the increase of the number of
-CH2 groups, whereas amore complex behavior was associated with the corresponding
[PF
6] series.36 Thus, the correlation of the physicochemicalproperties of RTILs with their structures implements a
subtle balance between electrostatic and inductive forces.
41Branching of the alkyl chains in 1-alkyl-3-imidazolim
RTILs reduces the viscosity.
43 In contrast, the introductionof a methyl group at the 2-position of the cation to form
1-alkyl-2,3-dimethyl-imidazolium induces an increase of the
viscosity (Table 2). This result appears counterintuitive since
the additional methyl group eliminates the possibility of
forming a strong hydrogen-bonded interaction between the
cation and the anion. From this H-bond suppression, a weaker
cation
-anion interaction is expected, leading to a significantdecrease of the viscosity. Indeed, calculations performed on
[BMIm][Cl] and [BMMIm][Cl] salts have demonstrated that
the strength of the cation
-anion interaction became weakerin the [BMMIm][Cl] as compared with [BMIm][Cl], but
[BMMIm][Cl] exhibits a higher viscosity than [BMIm][Cl].
44Two different main factors were demonstrated to contribute
to an increase of the viscosity.
44 First, a significant decreaseof entropy in the methylated salt has been evidenced due to
a restricted number of ion-pair conformers that could be
formed.
44 Such reduced ion-pair configurational variationsresult in more order within the ionic liquid. Second, the butyl
chain rotation is inhibited because of steric repulsion from
the methyl group,
44 and this has also been demonstrated toparticipate to an increase of the molecular arrangement.
When the molecular order is high within the ionic liquid,
the viscosity is increased.
44A significant decrease of viscosity is generally observed
in ionic liquids as the temperature increases.
30,45 An Arrhenius-type relationship (Guzman
-Andrade law, η ) A eEa/(
RT), where Ea is the activation energy for viscous flow) wasfollowed by most ionic liquids, while some others were better
fitted with the Vogel
-Tamman-Fulcher (VTF) equation (η)
η0 eB/(T-T0), where η0 (mPa s), B (K), and T0 (K) areconstants). Those RTILs that obeyed the Arrhenius law
typically contained asymmetric cations without functional
groups in the alkyl chains. The VTF model accurately
described the RTILs containing small, symmetrical cations
with low molar mass.
45 However, it is worth indicating thatthe two behaviors are linked since the Arrhenius-type
relationship is simply the higher temperature limit of the VTF
equation.
As a general conclusion, it can be said that electrostatic
interactions dominate the characteristic physicochemical
properties of RTILs and distinguish them from common
organic solvents. But, by consideration of RTILs of similar
structures, the differences in the viscosity are mainly
influenced by hydrogen bonding and van der Waals
interactions.
31,413.2. Conductivity
For any electrochemical process, the conductivity is a
property of primary importance and the conductivity of the
RTILs has been reviewed on several occasions, for example
refs 20, 46, and 47. Being composed entirely of ions, RTILs
are supposed to be among the most concentrated electrolytic
fluids with many charge carriers per unit volume. When these
charge carriers are mobile, very high conductivities are
possible. Room-temperature ionic liquids exhibit conductivities
in the broad range 0.1
-20 mS cm-1.20,31,48 Rather highconductivities of the order of 10 mS cm
-1 can be found inthe imidazolium family.
49 Quaternary ammonium ionicliquids are always characterized by lower conductivities; 2
mS cm
-1 is thus the highest conductivity found for a N,Ndialkyl-pyrrolidinium [NTf
2] salt.48,50–52 It is worth mentioningthat ionic liquids incorporating the tricyanomethanide
anion and the dicyanamide anion possess some of the highest
conductivities (up to 36 mS cm
-1).53–55 But these valuesare considerably lower than those of concentrated aqueous
electrolytes (for example, the aqueous KOH solution (29.4
wt %) applied in alkaline battery is 540 mS cm
-1).The high viscosity of the RTILs has a major impact on
the conductivities because the conductivity is inversely linked
to the viscosity.
32,37,41,51 The less viscous [NTf2] salts usuallyexhibit among the highest conductivities.
48 Although acorrelation between viscosity and conductivity is generally
observed, the viscosity alone cannot account for the conductivity
behavior. For instance, [EMIm][OTf] and
[BMIm][NTf
2] display similar viscosities and densities, buttheir conductivities differ by a factor of 2 (Table 3).
31 Manyothers factors contribute to the conductivity. Besides the
effects of the ion size,
31,32,35,56 of the anionic chargedelocalization,
31,56 and of the RTILs’ density,32,35,41 aggregationand correlated ionic motions have to be stressed.
34,37,39,57Transport properties and conductivities in a family of
dialkylimidazolium RTILs were examined by varying the
alkyl chain or the anion.
37,56 Increasing the length of thealkyl chains results in a higher viscosity and a lower
conductivity. Increasing the size of the anions lowered the
viscosity, but the conductivities, directly measured, were
similar for any of the anions. However, conductivities
calculated from the diffusion coefficients of the anions and
the cations gave higher conductivities for the less viscous
salts (larger anion).
37 The inconsistency between calculatedconductivities from diffusion measurements and measured
conductivities was attributed to correlated ion motion or the
diffusion of neutral species or both. Strong ionic association
(ion pairing) was found for the more viscous salts. While
the diffusion coefficients of such neutral species or ion pairs
can be measured, they do not contribute to conductivity due
to their lack of net ionic charge. In addition to the formation
Electrochemical Reactivity in RTILs Chemical Reviews, 2008, Vol. 108, No. 7
2241
