A Closer Look: Excess Properties of Binary Ionic Liquid Mixtures

in the excess properties. To understand underlying structural changes and the resulting effects on their properties, further detailed investigations of physicochem-ical properties are necessary.

3.4 A Closer Look: Excess Properties of Binary Ionic Liquid Mixtures

A more detailed look at the excess properties leads to a better understanding about reorientation processes occurring in a mixture. The excess property (e.g., excess molar volume) is the difference between the experimental value of a given mixture and the ideal value [(5) for the excess molar volumeV_E]. ForV_E, positive values indicate that the mixture requires more space than the ideal mixture, indicating less effective packing, the loss of interaction, or the occurrence of repulsive forces:

VE ¼ Vm,exp Vm,id (5)

The meta-analysis of the combined literature and this new data on excess molar volumesV_Eleads to the following conclusions (Table3):

1. V_E are generally small (<0.1%), indicating little structural rearrangements occurring upon addition of a second ionic liquid. Somewhat larger deviations (0.1–0.5%) are obtained if the cations or anions are very unlike.

2. V_Eare often positive, indicating that the packing is less efficient in the binary mixture than expected from the molar volumes of the neat ionic liquids. For example,V_Eincreases in binary ionic liquid mixtures consisting of a common anion ([NTf₂]^) if the alkyl chain length difference between two 1-alkyl-3-methylimidazolium substituents becomes larger [106], or the cation types are very different ([C6,6,6,14P][NTf2]/[C3mpyr][NTf2]) [117]. In some instances, polar and apolar domains can form, as witnessed in a combined molecular dynamics/ATR-IR spectroscopic study on the example of [C₃mpyr][BF₄]/

[C₈mpyr][BF₄] binary mixtures [111].V_Ealso increases for mixtures with very

“unlike” anions, e.g., [C₂mim][OAc]/[C₂mim][CF₃CO₂] [108] mixtures show a maximum deviation of 0.3%. Also, in the study of [C₄mim][PF₆]/[C₄mim][BF₄], [C₄mim][PF₆]/[C₄mim][NTf₂], and [C₄mim][BF₄]/[C₄mim][NTf₂], the devia-tion found for the [BF₄]^/[NTf₂]^ mixture was largest [106]. Similarly, [C6mim][PF6]/[C6mim][Cl] binary mixtures showed larger deviations of VE

than [C₆mim][BF₄]/[C₆mim][Cl] [110].

Table 3 Meta-analysis of the literature on the excess molar volume (V_E) and the deviation of the viscosity data from ideal mixing behavior (Δη)

Examples of positiveVE Deviation^a Examples of negativeΔη Deviation [C₄mim][BF₄]/[C₄mim][PF₆]

Examples of positive (Δη) Deviation [C₄mim][BF₄]/[C₄mim]

aNote that due to the low experimental error of density measurements (generally<0.2%) and the conversion to molar volumes, these deviations are indeed statistically significant

bThis work

However, negative V_E were found occasionally (Table 3), indicating more efficient packing. How this relates to the structure is not yet obvious from the little data available.

In certain instances, unsymmetrical and sometimes sinusoidal functions were observed, e.g., with increasing amounts of [C₂mim][SCN] in [C₂mim][Cl], with Δmax¼ 0.11 cm³mol^1atx([C₂mim][SCN])¼ 0.5 and Δmin¼ 0.08 cm³mol^1 at x([C₂mim][SCN])¼ 0.85 (at 70^C), with a maximum deviation of 0.08%

[84]. Such behavior has also been found for binary ionic liquid mixtures such as [C₈mim][BF₄]/[C₈mim][Cl], with negativeV_Eat low chloride ratios [110].

3. V_Eincreases in magnitude with increasing temperature [110,112,117].

Although fundamentally, ideal and excess quantities are only defined for ther-modynamic properties, this concept has been extended to other properties. Figure13 shows typical deviations from ideal viscosity of various binary ionic liquid mixtures at 25^C.

The meta-analysis of the viscosity data revealed the following trends:

1. Most mixtures investigated to date show large negative deviation from ideality indicating reduced friction, i.e., the addition of the second ionic liquid acts as a lubricant on the first (see Table 3). In many cases, the functions are not symmetrical, and may even be sinusoidal (e.g., for [C₂mim][OAc]/[C₂mim]

[NTf2] and [C2mim][OAc]/[C2mim][CH3SO3]).

2. With regards to the cation, the deviation from ideal appears to be larger if the alkyl substituents are more “unlike.” Hence, the deviation is less for [C₄mim]

[BF₄]/[C₆mim][BF₄] binary mixtures than for [C₂mim][BF₄]/[C₆mim][BF₄] [116]. With regards to the effect of the anion, it is not possible to extract a Fig. 13 Deviation from ideal viscosity of binary ionic liquid mixtures at 25^C

trend from the data (Table3). However, it is clear that both the cation and the anion combinations can be responsible for reduced friction.

3. The viscosity of binary mixtures approaches the ideal behavior with increasing temperature [112; Stark A, Ramzan M (2012) unpublished work]. However, for sinusoidal functions such as [C₂mim][OAc]/[C₂mim][CH₃SO₃] (see Fig. 13), this trend is not clearly discernible.

Overall, the available viscosity data does not allow one to draw a conclusive picture of the structural changes that lead (in most instances) to a lower viscosity than predicted. Combined conductivity measurements could show if this “lubrica-tion” is connected to increased ion conductivity or not. Only a few conductivity studies have been carried out with binary ionic liquid mixtures. It is known that, like the viscosity, the conductivity is not always linearly dependent on the composition of the binary ionic liquid mixture [113,117]. Every et al. [127], who studied binary ionic liquid mixtures of [C₂mim][CF₃SO₃]/[C₂mim][NTf₂] as early as 2000, found by combining results from density, conductivity, and diffusion measurements that the enormous increase of conductivity in binary mixtures of about 40% (determined at 90^C) must be due to a suppression of ion aggregation, i.e., a lower ordering than in the respective pure ionic liquids. A conductivity increasing effect (albeit of only 4%) was also found for [C₂mim][(CN)₂N]/[C₂mim][BF₄] at 25^C [109]. One could expect that if mixtures existed which possess lower than ideal mixture viscosity, their conductivity might be higher, which could be beneficially exploited in elec-trochemical applications. However, one study showed that in the case of [C₈mim]

[BF₄]/[C₈mim][Cl] mixtures, both the viscosity and the molar conductivity deviated negatively from ideality, meaning that although the friction within these binary mixtures is less than expected, the conductivity does not increase accord-ingly. This could indicate stable ion pair formation or nano-segregation leading to both reduced ion mobility and lower interactions with the neighboring aggregates, hence reducing the viscosity [110]. Clearly, more detailed experiments are required before conclusions can be drawn.

Extreme deviations from ideal mixing behavior indicate de-mixing phenomena [117], as witnessed by the fact that ionic liquids are not necessarily miscible over the full molar range of compositions. For example, the corresponding chloride salt precipitates from dry mixtures of [C₄mim][Cl]/[C₄mim][BF₄], [C₄mim]

[Cl]/[C₄mim][NO₃] [128], or [C₂mim][Cl]/[C₂mim][SCN] [84]. In 2006, Arce et al. reported the mutual immiscibility of binary ionic liquid mixtures based on trihexyltetradecylphosphonium ([C_6,6,6,14P]⁺) and [C_nmim]⁺ (where n < 6) chlorides or bis(trifluoromethanesulfonyl)imides. While for the [C_6,6,6,14P][NTf₂]/

[C₂mim][NTf₂] (and in later work [C_6,6,6,14P][NTf₂]/[C₂py][NTf₂] [129] and [C_6,6,6,14P][NTf₂]/[C₃mpyr][NTf₂] [117]) binary mixture, a strong temperature dependence was found, the solubility of the corresponding [C_6,6,6,14P]

[Cl]/[C_nmim][Cl] mixtures was less affected by the temperature, and decreased with increasing alkyl chain on the imidazolium cation. The analysis of the compo-sition of the phases showed that preferential ion association occurs via hydrogen bonding. This preferential interaction leads to even higher separation selectivities in

liquid–liquid systems, when two anion- and cation types are used, e.g., [C_6,6,6,14P]⁺ is coextracted with the [NTf₂]^, while [C₂mim]⁺ is preferentially extracted with [CH₃SO₃]^[130].