The role of the cation aliphatic side chain length in piperidinium bis(trifluoromethansulfonyl)imide ionic liquids

This paper reports on the dependence of physical-chemical and electrical properties of ionic liquids formed by N-alkyl-N-alkylpiperidinum cations and bis(trifluoromethane-sulfonyl)imide anions with respect to the length of the cation aliphatic side chain. The thermal stability as well as the density and conductivity were seen to decrease with the alkyl chain length, independently on the cation ring. The ionic conduction, in agreement with the DSC results, is correlated with the viscosity that is found to increase with the main aliphatic chain length. The alkyl chain length influences the cathodic electrochemical stability whereas no relevant difference was detected in the anodic stability. The evolution of the melting point, density and viscosity, detected for the cation having the hexyl side chain, points towards some nanoscale separation, e.g., a segregation of the long aliphatic chains into nanoscale apolar domains that are embedded into three-dimensional charged matrices generated by the anions and the charged piperidinium heads. The existence of such structural heterogeneities, predicted also by other authors, was proposed on the basis of previous Wide Angle X-ray Scattering (WAXS) measurements. (C) 2011 Elsevier Ltd. All rights reserved.