Physicochemical Properties of Ether-Functionalized Ionic Liquids: Understanding Their Irregular Variations with the Ether Chain Length

Ether groups are well-known for their unique contribution to low viscosity and high conductivity, and hence ether-functionalized ionic liquids (ILs) have been widely studied and successfully employed in various applications. However, the ether chain length effect on physicochemical properties is complex and still lacks a systematic study. In this study, an attempt was made to provide a thorough understanding for such complexities. A series of ILs functionalized with various ether groups (C_mOC_n–, n, m = 1, 2, or 3) were synthesized and characterized, and their properties with irregular variations along the ether chain length were recorded and systematically analyzed. Generally, the irregular variations are mediated by three interrelated factors: the C_m– tail length, the −C_n– spacer length, and hydrogen bonding interaction. For example, though ROC₂– is bulkier by a CH₂ unit than ROC₁–, ROC₂-based imidazolium ILs are less viscous and more conductive than ROC₁–-based analogues, since ROC₁– is apt to form intermolecular rather than the five-membered-ring intramolecular hydrogen bonding with the imidazolium ring H atoms, while for ROC₂– the six-membered-ring intramolecular hydrogen bonding comes into prominence.