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

Abstract

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 (CmOCn–, 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 Cm– tail length, the −Cn– 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.