Ether tails make a large difference for the structural dynamics of imidazolium-based ionic liquids

Abstract

Ionic Liquids are often characterized by multiple length scales and these tend to be well separated. As an example, for 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide apolar pockets or domains are separated by a characteristic distance that is different from that of positive-negative charge alternation along a charge network. Accordingly, these structural features have their own well separated signatures in the reciprocal space structure function. Each of these well separated structural features of prototypical ionic liquids relaxes on its own time scale and we have explored this in a set of recent articles. The overarching goal of the current work is to explore what happens when the structural motifs are still present, but the length scale difference between these is small. Specifically, for ionic liquids containing ether tail functionalities such as 1-(2-(2-ethoxyethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, the separation between cationic head subspecies intercalated by anions (or anions sharing a cationic head counterion) is not that different from that of cationic heads or anions intercalated by tail regions and the time scale for relaxation of these structural motifs are also similar. For the ether-tail containing system, the slowest liquid relaxation scale associated with tail region-charge network correlations is about an order of magnitude faster than for its alkyl-tail isoelectronic analog.