T. Endo et al. / Chemical Physics 472 (2016) 128–134
133
Table 2
3.4. Plausible factors contributing small p for CO diffusion in SiILs
Cation and anion volumes. The ion structures used for calculations are shown in
Figure S1. Free volume fractions were derived from Eq. (9). Data of [BMIm][TFSI] and
[EMIm][TFSI] are also listed, the density of which were taken from the Refs. [58,59].
The solute diffusivities in SiILs are not comparable to those in
silicone oil. However, particularly for CO, the diffusivity in SiILs is
slightly larger than in AlILs. This is the case of the results with a
variety of ILs and at various temperatures with ILs constituting
the same anion and the cation ring structure. This indicates the
presence of the difference in factors that control the small solute
diffusion between SiILs and AlILs. Since utilizing the GW theory,
which takes into account the solute–solvent size ratio, does not
eliminate the difference in diffusion, it is reasonable to attribute
this faster diffusion to other molecular-level properties.
As we first expected, there are three possible factors, that is,
flexibility, interaction energy and free volume. The high flexibility
and large free volume originate from the siloxane (or silyl) struc-
ture [6,8–9,14–15]. The change in intermolecular interaction is
caused by change in ionic nature of the SiIL ions [15–16,29]. Here-
after, this discussion is framed in terms of these three factors. As
previously suggested by Shirota et al., the flexibility of the silyl
and siloxane groups affect the physical properties of ILs [15–16].
Later, Niedermeyer et al. confirmed the high flexibility of the
SiOSiIm cation compared to the BMIm cation through DFT calcula-
tions [29]. They obtained potential energy surfaces of the rotations
along each bond angle in the side chain and found lower activation
energies than the butyl group rotation for the same cation ring
structure. They also discovered rotational coupling within the
Si–O–Si chain, which facilitated the movement of surrounding ions
and molecules. The flexible side chain can decrease the local
viscosity of solutes and consequently enhance solute diffusivities,
as explained by the SE equation.
IL
Cation volume/Å3 Anion volume/Å3 Free volume
fraction
[propylSiIm][FSI]
[propylSiIm][TFSI]
[propylSiIm][BETI]
[propylSiIm][PF6]
[SiOSiIm][FSI]
[SiOSiIm][TFSI]
[SiOSiIm][BETI]
[SiOSiIm][PF6]
[propylSiPyrr][FSI]
[propylSiPyrr][TFSI]
[propylSiPyrr][BETI] 219.98 9.19
[propylSiPyrr][PF6]
[SiOSiPyrr][FSI]
[SiOSiPyrr][TFSI]
[SiOSiPyrr][BETI]
[SiOSiPyrr][PF6]
[P1,3][TFSI]
210.36 6.21
210.36 6.21
210.36 6.21
210.36 6.21
246.90 13.93
246.90 13.93
246.90 13.93
246.90 13.93
219.98 9.19
219.98 9.19
100.40 4.77
149.32 7.00
197.05 8.46
69.93 6.01
100.40 4.77
149.32 7.00
197.05 8.46
69.93 6.01
100.40 4.77
149.32 7.00
197.05 8.46
69.93 6.01
100.40 4.77
149.32 7.00
197.05 8.46
69.93 6.01
149.32 7.00
149.32 7.00
149.32 7.00
149.32 7.00
149.32 7.00
0.382 0.016
0.403 0.016
0.408 0.015
0.383 0.019
0.405 0.025
0.397 0.024
0.414 0.022
0.393 0.019
–
0.399 0.019
0.406 0.018
–
0.384 0.021
0.399 0.020
–
219.98 9.19
258.07 11.22
258.07 11.22
258.07 11.22
258.07 11.22
143.78 5.20
158.83 9.81
144.31 4.76
112.67 6.30
141.63 9.66
–
0.391 0.018
0.387 0.024
0.394 0.017
0.387 0.022
0.389 0.025
[P1,4][TFSI]
[BMIm][TFSI]
[EMIm][TFSI]
[AEIm][TFSI]
tendency can also be seen in the difference of the anion, where
larger (flexible) anions seem to possess larger ffv values, while it
results in undesirably higher viscosity (Table 1).
Although there have been no reports regarding the interaction
between neutral solutes and ions of SiILs, electron density calcula-
tions and Raman-induced Kerr effect spectroscopy revealed a
decrease in cation–anion interactions [15–16]. Niedermeyer et al.
estimated the cation–anion interactions of the SiOSiIm and BMIm
cations with the chloride anion in the gas phase [29]. They con-
cluded that the interaction energies are nearly identical; however,
a slight decrease was reported as 367.23 kJ molꢀ1 for SiOSiIm/Cl
and 377.99 kJ molꢀ1 for BMIm/Cl. The typical ion–ion interaction
4. Conclusion
We synthesized sixteen ILs containing Si or Si–O–Si groups, in
which the diffusion coefficients of CO, DPA, and DPCP were mea-
sured using the TG method, with the exception of the four SiILs
that are a solid at room temperature. Although the diffusivities in
SiILs at room temperature are not as high as in silicone oil at the
same T/g, a slight enhancement with the solute molecules, partic-
ularly CO, was observed compared with that in conventional ILs
and their mixtures with molecular solvents. Comparing the previ-
ous results in the PhIL system, the solute diffusivity is lower in the
SiILs. However, the difference in the deviation parameter p indi-
cates that CO diffusion eventually becomes faster in the SiILs in
potential is expressed as Uion—ion ¼ ꢀqþqꢀ=4
p
e0err2d, where q is
the charge, D0 and Dr are the dielectric constants in the gas phase
and medium, respectively, and rd is the distance between two ions.
The interaction of an ion and either a permanent or induced dipole
low T/g region. The similarity of the value p between the silicone
has a very similar form, where dipole moment (
D is used instead of
oils and the SiILs demonstrates that the latter inherits the charac-
teristic solute-diffusion property from the former. Measurements
at various temperatures confirmed faster solute diffusion in SiILs
than in AlIL system even though the gap is less prominent. These
findings strongly imply that the factors that control the solute dif-
fusion differ between these types of ILs. There are three possible
factors that can explain the faster solute diffusion, i.e., flexibility,
interaction, and free volume fraction. Previous results combined
with our estimation of free volume fraction imply that all the fac-
tors contribute to the difference in the diffusion.
charge. Based on this primitive approach, perhaps slightly weaker
solute–solvent interactions in SiILs compared to AlILs is responsi-
ble for their different values of p.
Regardless of the silyl or siloxane containing side chain, free
volume in solvents is an important contributor to the diffusivity,
as classically presented in the Cohen–Turnbull equation [57]. How-
ever, unlike the other two factors, free volume has not been inves-
tigated in SiILs. Here, we derived the free volume fraction defined
as
VFU ꢀ Vions
In this study, we took a different approach to designing ILs that
were applicable as reaction media and electrolytes, which involves
high deviation from the SE equation, while not lowering the viscos-
ity. While this concept was successfully realized, the viscosity
properties became worse in SiILs, lowering the solute diffusivity.
The slight diffusion enhancement observed for CO in SiILs was
not able to overcome the increase in viscosity; therefore, the solute
diffusivity of the SiILs were lower than those of AlILs at the same
temperature. This is due to the bulkiness of Si or Si–O–Si groups
with short chain length which obscures the three above mentioned
factors. Berg et al. investigated rotational diffusion of anthracene in
polydimethylsiloxanes (silicone oil) with different molecular
ff
¼
ð9Þ
v
VFU
where VFU is the formula unit volume obtained from molecular
weight divided by density and Vions is the sum of van der Waals
volumes of the cation and anion. These results are summarized in
Table 2. The free volume fractions of SiILs are generally somewhat
larger than in AlILs, which can also explain the faster solute diffu-
sion in SiILs. It should be noted that other trends can also be seen
in Table 2. SiILs with a siloxane group may display larger ffv than
that with silane groups. This implies that high ffv ILs are obtained
if a longer Si–O–Si chain is introduced on the IL cation. This