Angewandte
Chemie
applied to the 23 ILs investigated in this study, the predicted
physical properties are in excellent agreement with the
experimentally determined values (Figure 3; for plots of
conductivity and density, see the Supporting Information).
currently assessing other physical properties of ILs in depend-
ence on Vm, for example, surface tension, refractive index, and
heat capacity.
Received: March 2, 2007
Published online: June 14, 2007
Keywords: ionic liquids · molecular volumes ·
.
physical properties · predictive methods · sustainable chemistry
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Figure 3. Experimental versus predicted viscosities for all ionic liquids
in the current study.
The experimental and predicted viscosities, conductivities,
and densities are highly correlated in all cases (R2 = 0.9976,
0.9871, and 0.9702, respectively) and the standard error of the
estimate (sest) statistics, which give an estimate of the average
absolute error of the predictions, are also very good (26 cP,
0.2mScmÀ1, and 27 gdmÀ3 respectively). As the viscosity data
covers a very large range of values (from 17 to 2181 cP) the sest
value is not a good estimate of the error in the predictions at
the extremes of this range. In this case the average of the
percentage errors for the predictions (9%) may also be used
to assess the accuracy of the predictions.
The predictions herein show that these empirical relation-
ships are powerful tools which enable the prediction of
fundamental physical properties of ILs for some of the most
common anion types. However, a final caveat must be taken
into account: the above correlations were derived from highly
pure ILs that are liquid at the temperature at which measure-
ments were made (20–228C). They are not applicable to
impure ILs or materials that have significant long-range order
at this temperature, that is, liquid-crystalline or solid phases.
For example, [C2MIM][PF6] melts at 58.38C (synthesized and
measured in the Dyson laboratory). If the above equations
are used to predict the properties of this solid salt (Vm =
0.265 nm3) under ambient conditions they predict a fairly
fluid material (h = 111 cP), which is clearly not the case. Thus,
when investigating previously unknown ILs it may be useful
to combine this technique with a method to predict the
melting point of the IL.[10,12,13]
Extrapolation of the Vm correlations to different temper-
ature regimes is in progress by analysis of temperature-
dependent physical data for a variety of ILs. Moreover, we are
Angew. Chem. Int. Ed. 2007, 46, 5384 –5388
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