(367b) The Viscosity of Ether-Functionalized Ionic Liquids

The development of low viscous and highly conductive ionic liquids is always being driven by the ambition to improve their performance in related fields, like heterogeneous catalysis and supporting electrolyte. Generally, a low-viscosity ionic liquid is characteristic of well-delocalized positive and negative charges, low-symmetry ions and highly flexible side chains, for example ether groups. Recently, hole theory indicated that: 1) the ions in ionic liquids cannot pack closely and lots of holes may come into existence; 2) the bulk volume consists of the inherent volume and the total volume of holes; and 3) a large amount of total hole volume is of advantage to a high rate of mass transfer and low viscosity.^[1,2]

In this study, 12 ether-functionalized ionic liquids and their alkyl counterparts short of an alkoxy O atom were prepared and characterized to determine the reason for the alkoxy chain effect on viscosity, as shown in Figure 1, based on the ether chains’ contributions to viscosity (η_O/η), the activation energy of viscosity (ΔE_η) and the total free volume (ΔFV_O), which was calculated from density and surface tension [FV = 2N × 0.6791(kT/γ)^3/2].^[2] For example at 25 ^oC, when the C₃H₇- group in [C₃H₇-MPyrr]NTf₂ was replaced by C₂H₅OCH₂-, the viscosity and its activation energy would be reduced from 62.4 cP and 28.83 kJ mol^–1 to 38.7 cP and 25.29 kJ mol^–1 respectively, yielding η_O/η = 0.620 (38.7/62.4) and ΔE_η = -3.54 kJ mol^–1 (25.29 - 28.83) respectively, accompanied by ΔFV_O = 4.76 mL mol^–1, while the total free volume in [C₂H₅OCH₂-MPyrr]NTf₂ is 32.25 ml mol^–1. Although the obtained η_O/η, ΔE_η and ΔFV_O for the 12 pairs of ionic liquids are highly variable, both η_O/η and ΔE_η show a decreasing dependence on ΔFV_O. As a result, the alkoxy chain effect on reducing viscosity was ascribed to the ability of the highly flexible alkoxy chains to increase the total free volume for the convenience of mass transfer. In order to maximize the effect on decreasing viscosity, the alkoxy group was supposed to contain a rod-like alkyl tail and a spacer length as short as only one methylene between O atom and the cationic N atom, and be in a non-hydrogen-bonding environment.

[1] A. P. Abbott, ChemPhysChem, 2004, 5, 1242-1246.

[2] J. Z. Yang, X. M. Lu, J. S. Gui and W. G. Xu, Green Chem., 2004, 6, 541-543

[3] Z. J. Chen, T. Xue, J. M. Lee, Rsc Adv., 2012, 2, 10564-10574.