6.2.2.1. Infinite dilution activity coefficients in
imidazolium-based FILs
An examination of figures in Appendix G gives an insight into
how IDACs values are affected by the presence of a particular cation in the
structure of the imidazolium-based FILs . It appears that for the same anion,
limiting activity coefficients of n-alkanes (Figures G-1 to G-4), alk-1- enes
(Figures G-15 to G-18), alk-1-ynes (Figures G-29 to G-32), cycloalkanes
(Figures G-38 to G-41), alkan-1-ols (Figures G-51 to G-54), alkylbenzenes
(Figures G-64 to G-66) and ket-2- ones (Figures G-75 to G-76) decrease with
increasing alkyl chain length of the IL cation.
It is observed that for n-alkanes (Figures G-5 to G-8),
alk-1-enes (Figures G-19 to G-22), cycloalkanes (Figures G-42 to G-45) and
ket-2-ones (Figures G-77 to G-79), infinite dilution activity coefficients
decrease when the anion is changed in the following order: [BF4]-
> [TFA]- > [TfO]- > [SbF6]- ;
[PF6]- > [Tf2N] - . This is the general trend as in some cases
this is not true (Figures G-44 and G-78)
No such clear hierarchy is displayed by mixtures involving
alk-1-ynes (Figures G-33 to G-35) and alkan-1-ols (Figures G-55 to G-58).
However, for all investigated FILs, the smallest activity coefficient values
are obtained with [Tf2N] - anion.
6.2.2.2. Infinite dilution activity coefficients in
phosphonium-based FILs
The available experimental data allow examination of the
effects of the anion only, since all the phosphonium-based fluorinated ionic
liquids studied so far have in common the [3C6C14P] + cation.
According to the plots represented in Figures G-9, G-23 and G-46, G-71
and G-80, the infinite dilution activity coefficients of all classes of
solutes, except alcohols and alk-1-ynes,
decrease when anions are changed in the following order:
[PF6]- > [BF4] - > [Tf2N]- >
[(C2F5)3PF3]-. The following hierarchy is observed for alcohols:
[PF6]- > [Tf2N]-; [(C2F5)3PF3] - >
[BF4]- (See Figure G-59). And for alk-1-ynes: [PF6]- >
[Tf2N] - > [BF4]- > [(C2F5)3PF3]- . (Figure
G-36)
6.2.2.3. Infinite dilution activity coefficients in
ammonium-based FILs
There are no data for a reliable description of the effect of
the anion as only two ammonium-based fluorinated ionic liquids have been
investigated in the literature. They have in common the [Tf2N]-
anion. For n-alkanes (Figure G-10), alkan-1-ols (Figure G-60), alkylbenzenes
(Figure G-72) and ket-2-ones (Figure G-81), limiting activity coefficients
decrease with increasing alkyl chain length of the ammonium-based FILs. The
opposite trend is observed only for cycloalkanes (Figure G-47).
6.2.2.4. Infinite dilution activity coefficients in
pyrrolidinium-based FILs
Under the same anion, the limiting activity coefficients of
n-alkanes (Figure G-12) and alk-1- enes (Figure G-26) decrease with the
increasing alkyl chain length of pyrrolidinium-based fluorinated ionic liquids.
There are no literature data related to alk-1-ynes, cycloalkanes, alkan1-ols
and alkylbenzenes. Limiting activity coefficients of n-alkanes (Figure G-13),
alk-1-enes (Figure G-27) and cycloalkanes (Figure G-49) decrease when the anion
is changed from [TfO]- to [Tf2N] - . The reverse is observed for
alkan-1-ols. (Figure G-62). Data for other classes of solutes are not
available.
6.3. Limiting selectivity and capacity of fluorinated
ionic liquids
The theory of Prausnitz and Anderson (1961) explains the
separation mechanism of a mixture of hydrocarbons using a polar separation
agent. It is also true for ionic liquids. To act as an effective entrainer or
extractant, an ionic liquid has to interact differently with the mixed
components. This occurs when one of the components is saturated whereas the
other is not. The ionic liquid solvent polarises the non-saturated component.
The former interacts more strongly with the solvent and is carried along as the
bottom product. Selectivity values depend on the relative polarisability of the
two components to be separated. This explains in part the trends of selectivity
and capacity values described in this section and depicted by Figures
H-1through H21 in Appendix H. Solvent capacity is, the numerical amount of
solute removed from the mixture by the extracting solvent. Selectivity normally
decreases with temperature and often follows a different trend from solvent
capacity. The compromise between selectivity and capacity is quantified as the
Performance Index, P.I., which is the arithmetic product of these two
properties. For each separation problem discussed in this work, the effect of
structure on the
separation ability of ionic liquids is reflected by the
hierarchies given in each of the following sections. When analyzing the trends
of selectivity and capacity, it is essential to incorporate in addition to
polarisability, the shape and the size of species, as well as hydrogen bonding
potential within ionic liquids.
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