Effect of Switching the Length of Alkyl Chains on Electric Double
Layer Structure and Differential Capacitance at the Electrode Interface
of Quaternary Ammonium-Based Ionic Liquids Studied Using Molecular
Dynamics Simulation
Electric
double-layer structure at the electrode interface has been studied
by using molecular dynamics simulation on four quaternary ammonium-based
ionic liquids (QaILs) to investigate the effect of switching the alkyl
chain length of the Qa cation. These four QaILs are composed of a
common anion, bis(trifluoromethanesulfonyl)- amide (TFSA–) and different cations: butyltrimethylammonium (N1114+, k = 1),
dibutyldimethylammonium (N1144+, k = 2), tributylmethylammonium
(N1444+, k = 3), and tetrabutylammonium (N4444+, k = 4), where k represents the number of butyl chains. The difference
in k affects the potential dependence for the composition
of the first ionic layer and the orientation of butyl chains in the
layer. For the case of k = 1, 2, 3, the butyl chains
parallel to the interface increases as the potential becomes negative,
but further negative potential results in the increase in perpendicular
ones. In the case of k = 1, all the cations in the
first ionic layer show the perpendicular orientation at the negative
potentials, forming a honeycomb lattice consisting of only cations.
On the other hand, in the case of k = 4, no change
in orientation has been observed due to the geometrical restrictions.
The difference in k also affects the differential
capacitance. The potential dependence of differential capacitance
shows bell shape for the smaller two (k = 1, 2) and
camel shape for the larger two (k = 3, 4). The camel
shape for larger IL cations agrees with the prediction from the mean-field
lattice gas model and recent experimental results. The differential
capacitance at negative potentials deviated to the values higher than
the model prediction and the discrepancy becomes greater for smaller k. The results indicate that the potential dependence of
ionic orientation significantly affects the differential capacitance.
Even for k = 4, which does not show the orientational
change, the discrepancy has been observed, indicating that not only
the orientational change but also the densification of ions in the
first ionic layer are the factors we should take into account beyond
the lattice gas model.