What causes the low viscosity of ether-functionalized ionic liquids? Its dependence on the increase of free volume

Abstract

A series of 12 ether-functionalized room temperature ionic liquids (RTILs) and their alkyl counterparts (short of an oxygen atom) were prepared and characterized to investigate the reason for the alkoxy chain effect on decreasing viscosity and how to best apply it. In addition to the ability of the alkoxy chains to decrease viscosity (η_O/η, as low as 0.594) and its activation energy (ΔE_η, as low as -4.14 kJ mol⁻¹), they were also found to be able to increase the total free volume (ΔFV_O, up to 4.93 mL mol⁻¹), by using density and surface tension. Furthermore, both the obtained η_O/η and ΔE_η values for the 12 pairs of RTILs show a strong decreasing dependence on ΔFV_O. Accordingly, the reason for the alkoxy chain effect on decreasing viscosity was proposed to be due to the ability of the highly flexible alkoxy chains to increase the total free volume, which offers the convenience of transport for the adjacent molecules. To maximize this effect, the alkoxy groups were supposed to contain a rod-like alkyl tail and a short –CH₂– spacer between the ether O atom and the cationic N atom, and be in an environment without hydrogen bond donors. Generally, the ether-functionalized pyrrolidinium and ammonium RTILs possess high conductivity (up to 4.97 mS cm⁻¹ at 25 °C) and wide electrochemical windows (∼5.75 V), indicating their significant promise for electrochemical applications.