engleski

Ionic transport in supramolecular ionic liquid gels prepared with bis(amino alcohol)oxamides as gelators

Supramolecular ionogels prepared by the gelation of ionic liquids (ILs) using low-molecular-weight (LMW) gelators are becoming increasingly important as they exhibit a unique combination of properties which make them a superior alternative to traditional polymeric ionogels. In these materials all physical properties of the parent IL (i.e. negligible vapor pressure, nonflammability, good electrochemical and thermal stability, high ionic conductivity) are kept preserved and additionally enriched with a solid-like consistency, self- healing capability and thermoreversible gelation property controlled by their sol–gel transition temperature. In this contribution, we report on a series of novel ionogels of three ILs containing imidazolium/pyrrolidinium cations and tetrafluoroborate/bis(trifluoromethylsulfonyl)imid e anions prepared using (S, S)-bis(amino alcohol)oxamides as LMW gelators. Based on the correlations between structures of the gelators and ILs it can be inferred that the steric effect, which depends on the size and flexibility of the amino alcohol substituents on the oxamide unit, plays a key role in the gelation process. All investigated supramolecular ionogels retain high ionic conductivity and ion diffusion coefficients of their parent IL, even for high gelator concentrations. Interestingly, at low temperatures, an enhancement of the ionic conductivity was observed for ionogels of (i) 1-butyl-3-methylimidazolium tetrafluoroborate and (ii) 1-butyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide. While in the former case the positive effect of gelation on the ionic transport can be attributed to specific interactions between ionic species and gelator molecules in the latter case it can be related to the inhibited crystallization of IL. In contrast to ionic transport, mechanical strength of the ionogels shows a wider variation depending on the type and concentration of the oxamide gelator. Among all the ionogels, 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 wide temperature range (−70 °C to 80 °C), high conductivity (0.0032 (Ω cm)−1 at 20 °C) and high mechanical strength (G′ = 278 kPa at 20 °C).

ionic liquid gels ; ionic conductivity ; mechanical properties

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