2.1. Thermal Transitions and Density
The thermal behavior of the four liquids is characterized by DSC measurements (
Figure 2). All the studied liquids (see curves in
Figure 2) display upon cooling the typical feature of a glass transition, and the relative transition temperatures [T
G] are reported in
Table 1.
The glass transition temperatures displayed by the phosphonium-based ILs are lower than those of their ammonium analogues, showing little difference (in the order of 6%). An even smaller difference is observed between liquids with the same cation but a different anion, because the two considered anions are both rigid; as such, no contribution coming from the different flexibilities is observable, in contrast to different series of ammonium of phosphonium ILs previously reported [
33,
34], which listed anions with different flexibilities. Upon heating, the ammonium-based ILs revert from the glassy to the liquid state (
Figure 2), while the two phosphonium samples undergo additional cold crystallization and subsequent melting upon further heating.
The temperatures observed for the cold crystallization and the melting of the studied samples are reported in
Table 1 for comparison. It is worth noting that the [P2225][TCM] sample displays an exothermic peak just before melting, which is likely due to the occurrence of a solid–solid phase transition, as already observed in other similar systems [
33]. Indeed, it is widely accepted that ILs can display different polymorphs in their crystal state, and that these phases can also depend upon the crystallization conditions [
33,
37].
The temperature-dependent density values were also measured for all four ILs; the data and the parameters obtained from the best linear fit are reported in the
Supplementary Materials, Tables S1 and S2. The values obtained at room temperature are reported in
Table 1 as a reference. These data indicate that the liquids with the [DCA]
− anion have higher density than their [TCM]
− analogues, while the liquids with the phosphonium cation have a higher density than those with the ammonium analogues. This behavior is in agreement with similar results obtained for phosphonium and ammonium ILs with a rigid anion, such as [B(CN)
4]
− [
33], while it is the opposite to what is observed in systems where the quaternary cations are combined with flexible fluorinated anions. Indeed, in the latter case, the flexible phosphonium ionic liquids showed a decrease in density compared to the more rigid ammonium analogues as a result of the larger free volume [
33].
2.2. Viscosity
The viscosity
of ionic liquids is a central limitation for mass and heat transport. For the vast majority of practical applications, low viscosities are desirable to improve performance. For the presently studied ILs, the viscosities as well as the Vogel–Fulcher–Tammann (VFT) fitting parameters (Equation (1)) and Angell’s strength factor for temperature-dependent viscosity are given in
Table 2 and plotted in
Figure 3. The experimental values and the activation energies for the viscous flow according to the Arrhenius equation (Equation (2)) at 25 °C are given in the
Supplementary Materials (Table S3).
At 25 °C, the viscosities for the ammonium ionic liquids are higher than those of the phosphonium analogues for a given cation, while the [DCA]
– samples have significantly higher viscosities than the [TCM]
– ionic liquids. The trend of the higher viscosity of ammonium ionic liquids compared to the isostructural phosphonium is a quite general finding, and this has also been observed for the bis(trifluoromethanesulfonyl)imide anion [NTf
2]
– (also termed [TFSI]
–), with both comparably small [
38,
39] and comparably large cations [
40], as well as for the tetracyanoborate [B(CN)
4]
− anion [
33]. This is rationalized by the stronger interactions in the ammonium samples [
35] resulting from the lower flexibility of the ions [
41]. Lower viscosities of [TCM]
−-based ionic liquids compared to [DCA]
−-based analogues have also been observed for the 1-buty-1-methyl-pyrrolidinium cation [
42]. For samples with the 1-ethyl-3-methyl imidazolium cation, only minor differences in viscosity for the two anions have been found [
43].
The temperature-dependent viscosity reveals that the order of magnitude of the viscosity remains the same in the experimental temperature range. Due to the temperature dependence of the activation energy for the viscous flow, fitting with the phenomenological VFT equation needs to be applied, as shown in
Figure 3. The temperature dependence of transport properties
for glass-forming materials, such as ionic liquids, can be quantified by Angell’s strength parameter
(
). Therefore, low values of
indicate a highly temperature-dependent activation energy, which is found for so-called ‘fragile’ liquids, while ‘strong’ liquids, on the contrary, have a constant activation energy and high values of
[
44]. For the investigated ammonium and phosphonium ionic liquids, the fragility of the ammonium samples with the same anion is higher than for the phosphonium samples, and the samples with the [TCM]
− anion are more fragile than the [DCA]
−-based ones. All of these ionic liquids with cyano-based anions are highly fragile. For comparison, the
of [P2225][NTf2] has been given as 6.32 [
45], while the commonly used cations 1-butyl-3-methylimidazolium and 1-buty-1-methylpyrrolidinium with the [DCA]
− anion have been reported to have
values of 7.24 and 3.24 [
41,
45].
2.3. Conductivity and Walden Plot
The conductivity of ionic liquids is a central quality for their use as electrolytes in electrochemical applications. The obtained values for the molar conductivity
, the VFT fitting parameters for the temperature-dependent molar conductivity, and Angell’s strength factor are reported in
Table 3 and plotted in
Figure 4a. Experimental values for the specific conductivity, and calculated values for the molar conductivity as well as activation energy at 25 °C according to the Arrhenius equation, are given in the
Supplementary Materials (Table S4, Table S5 and Table S6 respectively)
The molar conductivity at 25 °C shows the exact inverse order of the viscosity, so the values of the [P2225]+ samples are higher than those of the [N2225]+ samples for the same anion, and the [TCM]− anions give higher conductivities than the [DCA]− samples. The order of the molar conductivity is maintained upon increasing the temperature, with the samples [N2225][DCA] and [P2225][DCA] exchanging their positions around 60 °C. This is the result of the different temperature dependence of the two [DCA] samples. The values for Angell’s strength parameter for the molar conductivity are very similar to the ones obtained for the viscosity , showing higher fragilities for the [N2225]+ cation and for the [TCM]− anion.
This behavior can be rationalized by the fact that two transport properties are interrelated by the Walden equation
, with
being a fractional exponent that obtains values close to unit [
46,
47]. The linear relationship of the two transport properties is illustrated by the Walden plot (
Figure 4b). The values for the exponent
range from 0.92 to 0.99, and are thus in the range commonly found for ionic liquids [
46,
47]. As all samples are very close to the bisection in the Walden plot (often termed the ‘ideal KCl line’), they are classified as ‘good ionic liquids.’ Although this classification is somewhat arbitrary [
48], it still allows for comparison with other ionic liquids and electrolytes. For instance, 1-alkyl-1-methylpyrrolidinium cations with the [DCA]
− anions have been reported to show values closer to the Walden bisection than other ionic liquids [
49].
2.4. Self-Diffusion Coefficients
Compared to the macroscopic properties (viscosity and conductivity), the self-diffusion coefficients give insight into the dynamics on the molecular scale. The cation self-diffusion coefficients
, as well as the VFT fitting parameters and Angell’s strength parameters for the
-dependence of the cation-self diffusion coefficients, are given in
Table 4. The values are plotted in
Figure 5a. The experimental values and Arrhenius activation energies at 25 °C are given in the
Supplementary Materials (Table S7, Table S8 and Table S9 respectively).
At ambient temperature, the values of the cation self-diffusion coefficients have the same order as the molar conductivity and are inverse to the viscosity. Consequently, the [P2225]+ samples have larger self-diffusion coefficients than the [N2225]+ samples, and the cations combined with [TCM]− diffuse faster than the ones paired with [DCA]. This order of [P2225][TCM] > [N2225][TCM] > [P2225][DCA] > [N2225][DCA] is maintained in the investigated temperature range, with the two [DCA]− samples exchanging their positions at approximately 65 °C. This crossing of the values for the self-diffusion coefficients at a particular temperature is similar to the behavior of the molar conductivity. By investigating the self-diffusion coefficients, the macroscopic transport behavior can be explained by the larger increase in self-diffusion for the cation of [P2225][DCA] with temperature.
The fragility of the cation self-diffusion coefficients with temperature is similar to the other transport properties. Thus, the samples with the ammonium cation are more fragile than the isostructural phosphonium analogues, and the tricyanomethanide samples have lower values than the dicyanamide ionic liquids with the same cation. The absolute values for Angell’s strength parameter are higher for the cation self-diffusion coefficients than for the macroscopic transport properties (viscosity and molar conductivity).
The self-diffusion coefficients are related to the viscosity by the Stokes–Einstein relation
with
being a fractional exponent close to unity. The linear relationship between the cation self-diffusion coefficients and the viscosity is shown in
Figure 5b. The values for the exponent
u range from 0.98 to 1.06.
2.5. DMA Measurements
The low-frequency mechanical spectroscopy experiments performed by DMA enable measurement of the mechanical modulus of the ionic liquids and its variation during the main phase transitions occurring by varying the temperature in both the liquid and the solid states.
Figure 6 reports the DMA spectra (modulus, M, and tan δ) of the [P2225][DCA], [P2225][TCM], [N2225][DCA], and [N2225][TCM] samples. The storage modulus is plotted as the relative variation with respect to the value measured around room temperature, because it is not possible to separate the contribution of the ILs from that of the pocket, which is considered as a background [
10,
11].
The spectra measured upon cooling of the four samples are qualitatively similar because they display the same features. In particular, all the samples show the occurrence of a thermally activated relaxation process, which appears at a different temperature for each of them: for a vibration frequency of 1 Hz, it is detectable around 240 K for the two [DCA]-containing ILs, at 210 K for [P2225][TCM], and at 225 K for [N2225][TCM].
Indeed, at these temperatures, the tan δ curve shows a peak; its maximum shifts to a higher temperature with increasing frequency, and, concomitantly, the modulus curve displays a step.
Upon further cooling, all the samples show an intense stiffening of the modulus and an intense peak in tanδ, which displays a limited shift with the frequency. These last features indicate the occurrence of the glass transition, which is detected around 190 K for [P2225][DCA], around 200 K for [N2225][DCA], at 180 K for [P2225][TCM], and around 200 K for [N2225][TCM]. The obtained temperature values are in agreement with those previously obtained by DSC and further confirm that all the liquids undergo a transition to a glass phase.
To obtain information about the dynamic process giving rise to the observed relaxation peak, the data measured at the three frequencies were fitted for each sample using Equation (7), which is appropriate for jumps in an asymmetrical potential well with asymmetry ΔE, and assuming for the relaxation time (τ) a Vogel–Fulcher–Tammann-type (VFT) temperature dependence (Equation (6) or (1)). This model is the same as previously used to fit similar relaxation processes found in the liquid phase of other ILs [
9,
10,
11,
12]. The values of the best-fit parameters are reported in
Table 5. The values obtained for the
B and
T0 parameters are comparable with those obtained from the VFT fitting of the transport properties and of the diffusion coefficient. In the present case,
B represents the activation energy measured in K. In all cases, the asymmetry Δ
E was found to be zero. This last fact is consistent with the observation that in the spectra, the intensity of the relaxation peaks after background subtraction does not increase with the frequency. In particular, the background shift was slightly higher for the curve measured at 1 Hz. Overall, for all the samples, the value obtained for Δ
E indicates that the relaxation involves two sites with the same energy. An evaluation of the energy barrier that the relaxing unit has to overcome to go from one configuration to the other one is easily obtained by the
B parameter.
For all the samples, the obtained energy barriers are rather small, ranging between 452 K (3.6 kJmol
−1) obtained for the [N2225][TCM] and 1271 K (10.6 kJmol
−1) obtained for [P2225][DCA]. These values are lower than those usually reported by DMA analyses on other ILs with flexible anions, but closer to that obtained for 1-butyl-3-methyl imidazolium tetracyanoborate [C
4C
1im][B(CN)
4]. Similarly, the relaxation time is in the order of tenths of microseconds and is much larger than those obtained for other ILs. Again, it is close to that obtained for [C
4C
1im][B(CN)
4] [
9], as its dynamics are dominated by a mechanism of an intermolecular nature. As previously stated, the obtained
B parameters are in agreement with those obtained for the VFT fitting of other quantitiessuch as conductivity, viscosity, and cation self-diffusion coefficient. In particular, in all cases, the corresponding energy barrier values obtained for [N2225][DCA] and [P2225][TCM] are very close, while the highest and lowest energies are displayed by [P2225][DCA] and [N2225][TCM], respectively. It must be pointed out that these energy barrier values imply a temperature dependence and are obviously different to those obtained at a certain temperature by a local approximation applying the Arrhenius low, which is not valid to describe the energy behavior in the whole temperature range.
The values obtained for τ
0 are indicative of a diffusive process, as already reported for some ionic liquids, where the structural relaxation is not dominated by anion flexibility [
9,
10,
11,
12]. Moreover, the obtained energy values are quite small, ranging between 3.6 and 10.6 kJmol
−1, and suggest that the mechanism dominating the observed dynamics is likely of an intermolecular nature [
9].
Indeed, contrarily to what is observed in other quaternary ILs with non-flexible anions [
12], in the presently studied samples, the presence of rigid anions does not induce the occurrence of at least partial solidification of the samples upon cooling, and this allows the observation of a relaxation process in the supercooled liquid phase. The relaxations observed in these cases, moreover, present parameters very close to those reported by other transport techniques, such as viscosity, conductivity, and self-diffusion coefficients. These observations suggest that in the present case, all these techniques report the same dynamic process. In particular, the similarity between DMA and viscosity VFT analysis parameters indicates that the two techniques provide consistent results regarding the diffusive process dominating the viscous flow, even though they measure a different quantity in different conditions. In fact, as previously stated, the stress applied on the samples during DMA experiments is not a pure shear stress [
10], thus allowing the detection of relaxations that are not necessarily observed by applying a pure shear deformation, as in the case of classic shear viscosity measurements. Moreover, it is worth noting that, similarly to what was already observed in the case of [C
4C
1im][B(CN)
4], when the local dynamics are not dominated by the anion flexibility, the DMA data provide indications about the diffusive processes involved in the transport properties. In particular, previous mechanical spectroscopy measurements on ILs with imidazolium cations and anions with different flexibility [
9] showed that the fast structural reorganization of flexible anions on a local level affects the dynamics and results in the observation of a relaxation strongly affected by the anion flexibility, which can provide alternative pathways for relaxation on an intermediate timescale. In this framework, the occurrence of translational motion by means of hopping processes (as suggested by the model used for the DMA data) is possibly coupled to the rotational motions and to the transport properties.
2.6. Ab Initio Simulations
Throughout the experimental methods used in this work, activation energies tend to be higher for [DCA]
− ionic liquids compared to [TCM]
− ionic liquids—this was found to be the case for DMA, viscosity, conductivity, and diffusion data. This observation inspired us to perform additional ab initio calculations to gain insight into plausible underlying molecular mechanisms. Specifically, we calculated ion pair complexation energies in the gas phase at two levels of theory (
Table 6). For the sake of simplicity, the tetramethyl substituted cations were used as model systems.
The difference in complexation energy between the pairs with [N1111]/[P1111] cations but the same anion is negligible. However, the complexation energies for pairs with the same cation but different anions are much more negative for [DCA]
− compared to [TCM]
−. Specifically, [DCA]
− complexes are between 8.2 and 8.8 kcal/mol more stable than [TCM]
− complexes. This corresponds to 34 and 37 kJ/mol, or approximately 14 times RT (14 × R × 298 ≈ 35 kJ/mol). The largest part of this stabilizing energy stems from electrostatic interactions (see also
Table 6).Thus, it seems plausible that the larger activation energy in [DCM]
−-based ionic liquids is simply a result of the smaller size of this anion and the resulting higher charge density that dominates the intermolecular interactions and, thus, the structural relaxation.