One of the aims of this research was to investigate the role of the anion in the
appearance of the XRD patterns of ILs. Of particular interest is the role of dis- persion interactions between anions and alkyl chains and their influence on other
structural features in XRD patterns of ILs. There is unpublished computational evidence from within the group concerning the study of cluster formation in ILs
that suggests dispersion interactions between the alkyl chains and anions can oc- cur and this affects the nature of the structuring of the IL. Dispersion interactions
involve the interaction of dipoles between molecules, although they exclude the in- teraction between two permanent dipoles (Keesom forces). Dispersion interactions
include the interaction of a permanent dipole with an induced dipole, however the symmetry of most IL anions means they do not have permanent dipole moments,
despite often having asymmetric electron distributions. The exception in this case is the thiocyanate anion, which has a dipole moment. What effect the addition of
the permanent dipole-induced dipole interactions will have on the total of the dis- persion interactions of the thiocyanate ILs is not clear, given that the other anions
will have dispersion interactions with the alkyl chains solely composed of instan- taneous dipole-instantaneous dipole interactions. The [NTf2]− anion has a dipole
moment when the trifluoromethyl groups are cis, but no dipole moment when the trifluoromethyl groups are trans. The trans conformer is the lower in energy of the
two, so it is likely to be in this conformer.
Dispersion interactions are more prominent in molecules or ions with more electrons; the instantaneous dipoles occur because of spontaneous asymmetry in the distribu-
tion of electrons in their orbitals, thus when there are more electrons instantaneous dipoles occur more regularly. However it is not simply a function of the number of
electrons; three of the anions in this study, bromide, nitrate and thiocyanate, have similar numbers of electrons (36, 32 and 30 respectively). The size of the orbital also
has an effect; larger orbitals have a greater tendency to have instantaneous dipole effects (that is, they are more polarizible). So while the electron counts of the three
anions are similar, the nitrate anion has electrons in 2p orbitals which are small, the thiocyanate has a electrons spread over 2p and 3p orbitals and the bromide has
electrons in 4p orbitals, thus nitrate is not particularly prone to dispersion inter- actions, while thiocyanate is and bromide even more so. The [NTf2]− anion is an
interesting case; it is large and has many electrons but it is not particularly soft.
It is possible to examine these interactions using X-Ray diffraction because the alkyl chains in an IL tend to be clustered into a non-polar domain164. The formation of
a non-polar domain gives rise to a peak in the integration of the XRD pattern at
around 0.25˚A−1 caused by correlations between the alkyl chains perpendicular to the chain axis. This has been dubbed Peak 4 in this thesis. This peak occurs
because of scattering from alkyl chain-alkyl chain repeat structures. If the anions engage in dispersion interactions with the chains this would be expected to disrupt
the non-polar domain.
Figure 4.22 shows a stack plot of [C8C1im]+ ILs. Figure 4.23 shows a stack plot
of [C6C6im]+ ILs. The peaks intensities, with the exception of Cl− and Br− are
comparable within this data set and give some indication of the extent of structuring.
As can be seen in the figures, [C8C1im][NO3] (Fig. 4.22) and [C6C6im][NO3] (Fig.
4.23) both have very intense diffraction peaks. As Peak 4, around 0.24˚A−1 (4.2˚A), is solely made up of alkyl chain-alkyl chain interactions, differences in the correlation
ability of the anions can be ignored.
So it is safe to conclude that the nitrate systems are exhibiting more structuring of
the alkyl chain region than other ILs. While the intensity has not been corrected for IL density, which will affect the apparent extent of structuring, this effect will
be relatively small as IL densities vary over a relatively small range.
Peak 4 is more intense in [C6C6im][Y] than it is in the corresponding [C8C1im][Y].
It would be premature to immediately conclude that the [C6C6im]+ ILs have more
structuring, this is because each cation now has two alkyl chains capable of producing
a diffraction signal; it would be expected that this would approximately double the total scattering intensity.
Figure 4.22: Stack of Integrations of X-Ray Diffraction patterns of [C8C1im][Y] ILs
Repeating Structure (˚A)
Ionic Liquid Peak 1 Peak 2 Ratio
[C6C6im][NO3] 34.0 16.4 2.07
[C6C6im][NTf2] 28.8 14.2 2.03
[C6C6im][SCN] 31.3 15.6 2.01
[C6C6im]Cl 30.3 15.4 1.97
[C6C6im]Br 45.8 17.2 2.7
Table 4.9: Peak Table of [C6C6im][Y] ILs
role in disrupting the non-polar domain. For the [C8C1im]+ ILs the extent of alkyl
chain-alkyl chain correlation follows the order [NO3]−>> [NTf2]−≈ Cl− > Br−>[SCN]−.
For the [C6C6im]+ILs it follows the order [NO3]−>> [NTf2]−≈ Cl−≈ [SCN]−>Br−.
This trend shows that anions capable of dispersion interactions exhibit a significant
ability to interrupt the alkyl chain-alkyl chain interactions of the cations. This is a remarkable result given that dispersion interactions are typically believed to be
insignificant in short chain systems. MD simulations on the position of the anion relative to the cation typically show longer alkyl chains pushing the anion away161,
this seems to agree with the results of Russina et al.58 who found the intensity of Peak 3, which arises from nearest neighbour spacings, becomes less intense as the
alkyl chain length is increased.
The precise mechanism for this interruption is unclear; if the anion was disrupting the non-polar domain by positioning itself between the alkyl chains there would be a
significant change in the nearest neighbour peak and it would also affect the position of the pre-peak. In the case of [C8C1im][SCN] there is no Peak 4 present, however
there is still a strong Peak 2 or pre-peak.