Supramolecular ionogels prepared with bis(amino alcohol)oxamides as gelators: ionic transport and mechanical properties

Santic A, Brinkkötter M, Portada T, Frkanec L, Cramer C, Schönhoff M and Mogus-Milankovic A

Abstract

Supramolecular ionogels composed of an ionic liquid (IL) immobilized in a network of self-assembled lowmolecularweight molecules have been attracting considerable interest due to their applicability as smartelectrolytes for various electrochemical applications. Despite considerable scientific effort in this field,the design of a mechanically and thermally stable yet highly conductive supramolecular ionogels stillremains a challenge. In this article, we report on a series of novel ionogels of three ILs containingdifferent cations (imidazolium/pyrrolidinium) and anions (tetrafluoroborate/bis(trifluoromethylsulfonyl)imide) prepared using (S,S)-bis(amino alcohol)oxamides as gelators. The gelation behaviour of theoxamide compound depends strongly on the structural features of amino alcohol substituents. Amongthem, (S,S)-bis(valinol)oxamide (capable of gelling all three ILs) and (S,S)-bis(phenylalaninol)oxamide(capable of gelling ILs based on bis(trifluoromethylsulfonyl)imide with a concentration as low asz0.2 wt%) are highly efficient. All investigated supramolecular ionogels retain the high ionic conductivityand ion diffusion coefficients of their parent IL, even for high gelator concentrations. Further, at lowtemperatures we observe an enhancement of the ionic conductivity in ionogels of (i) 1-butyl-3-methylimidazolium tetrafluoroborate which can be attributed to specific interactions between ionicspecies and gelator molecules and (ii) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide dueto inhibited crystallization. In contrast to ionic transport, mechanical strength of the ionogels showsa wider variation depending on the type and concentration of the oxamide gelator. Among all theionogels, that of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide prepared with 1 wt%(S,S)-bis(phenylalaninol)oxamide exhibits the best performance: optical transparency, stability over a widetemperature range, high conductivity and high mechanical strength. The results presented here revealthe versatile nature of bis(amino alcohol)oxamides as gelators and their high potential for preparingfunctionalized IL-based materials.