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

Lee, J. M. - Presenter, Nanyang Technological University
Chen, Z., Nanyang Technological University
Xue, T., Nanyang Technological University

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 (ηO/η), the activation energy of viscosity (ΔEη) and the total free volume (ΔFVO), 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 ηO/η = 0.620 (38.7/62.4) and ΔEη = -3.54 kJ mol–1 (25.29 - 28.83) respectively, accompanied by ΔFVO = 4.76 mL mol–1, while the total free volume in [C2H5OCH2-MPyrr]NTf2 is 32.25 ml mol–1. Although the obtained ηO/η, ΔEη and ΔFVO for the 12 pairs of ionic liquids are highly variable, both ηO/η and ΔEη show a decreasing dependence on ΔFVO. 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.