Effect of the Dodecanoate Anion on Thermal Stability and Decomposition Mechanism of Mono- and Dicationic Ionic Liquids

Abstract

This work presents the synthesis of mono- and dicationic ionic liquids (ILs) that combine the cations 1-butyl-3-methylimidazolium ([C₄MIM]⁺), 1-decyl-3-methylimidazolium ([C₁₀MIM]⁺), 1,4-bis(3-methylimidazolium-1-yl)butane ([C₄(MIM)₂]²⁺), and 1,10-bis(3-methylimidazolium-1-yl)decane ([C₁₀(MIM)₂]²⁺) with the anion dodecanoate ([C₁₁COO]⁻), along with a study into their thermal stability and mechanism for thermal decomposition. Thermal stability was investigated using the Kissinger–Akahira–Sunose (KAS) isoconversional method to determine the isoconversional activation energies (E_α) and compensation effect to calculate the pre-exponential factor (ln A_α). The results showed that there was no significant difference in the thermal stabilities between the ILs, with all compounds being thermally stable up to 450 K. The thermal decomposition mechanism was analyzed using nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS), and thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy (TGA-FTIR). The main decomposition pathways were nucleophilic substitution at the lateral or spacer chain and the methyl group.