Transport properties and ionicity of phosphonium ionic liquids

Abstract

Ionic liquids (ILs) are promising electrolytes and many efforts have been made in basic scientific research as well as in applied research. In this contribution, we synthesised a variety of partly novel phosphonium ILs with different anions as well as with different compositions and lengths of the side chains of the cations. We measured a variety of their important transport properties such as viscosity, conductivity and diffusivity by means of stress-controlled rheology, impedance spectroscopy and PFGSTE NMR diffusometry. The results are analysed with respect to different models for derivation from ideal behaviours such as the ionicity and the (fractional) Walden rule depending on their molecular structure. These models are well established in the literature and are herein applied to rarely investigated but promising phosphonium ILs, with a particular emphasis placed on the effect of ether side chains. In comparison, the models show a qualitative correlation but distinct deviation in the quantification especially in the temperature dependent values and with other IL systems. These results aim to facilitate a better understanding of the IL properties depending on the molecular composition and by this way help to choose the ILs with optimal properties for practical applications.

Multiple Ether‐Functionalized Phosphonium Ionic Liquids as Highly Fluid Electrolytes

“…[66] The IL [P1112O2][NTf 2 ] has a slightly lower molar, and even specific, conductivity than [P(2O2) 3 (which is 1.18 S cm 2 mol À 1 ), but above the one for [P4442O2] [NTf 2 ] (which is 0.35 S cm 2 mol À 1 ). [23] This supports the observation that [P1112O2][NTf 2 ] is the one exception of the rule that smaller cations are preferable in otherwise structurally similar subgroups. Furthermore, the potential of multiple ether functionalization is impressively demonstrated once again.…”

“…The region where these ILs are located is similar to other results for phosphonium ionic liquids, especially those with [a] 5.11 × 10 À 9 5.11 × 10 À 9 [P(2O2) 3 single ether functionalization. [23] There is, however, one curious outcome visible in this data set. The structurally analogous pairs with and without ether functionalization are located on common straight lines to a high degree, with the functionalized systems being shifted towards higher ionic conductivity and lower viscosity.…”

“…Since I W 0 and log C are directly connected, only the former quantity is discussed here. [23] This trend is again not followed by the trimethyl-phosphine based cations. All Walden ionicities are slightly higher for the ether analogues of the pairs with [PRRRn] cations, with the exception again being [P111R][NTf 2 ].…”

“…, although the mass of the latter is larger by a factor of 1.7. Interestingly, the very small ionic liquid [P1115][NTf 2 ] also shows a viscosity comparable to [P(2O2) 3 [23] and [P111(1O1)][NTf 2 ] (around 43 mPa s). [11] The actual optimum size of the underlying phosphine in this respect appears to be the triethylphosphine, although smaller cations are generally preferable.…”

“…[20][21][22] It has also been observed that the fluidity of phosphonium ionic liquids is greatly enhanced upon introduction of ether side chains. [23][24][25][26][27][28] Although phosphonium cations might react with basic anions containing oxygen at elevated temperatures, this is usually not a problem with weakly coordinating anions. [29,30] Additionally, the good performance of phosphonium ionic liquids at room temperature makes them suitable for applications like batteries where the operation temperature is limited anyway.…”