Cation effects on the properties of halloysite-confined bis(trifluoromethylsulfonyl)imide based ionic liquids

Abstract

Four types of ionic liquids (ILs) of [X]TFSI ([X]⁺ is a cation such as 1-butyl-3-methylimidazolium BMIm⁺, 1-butyl-1-methylpyrrolidinium BMPyrr⁺, 1-butyl-1-methylpiperidinium BMPip⁺ and methyltrioctylammonium MOc₃Am⁺ and TFSI⁻ is the bis(trifluoromethylsulfonyl)imide anion) were confined in halloysite nanoclay (Hal) at an excess ionic liquid concentration (IL : Hal ∼55 : 45 wt%) and studied by X-ray diffraction, TG, DSC analysis and FTIR spectroscopy. It was found that the physicochemical properties of ILs trapped by halloysite at maximum loading are similar to those of bulk ILs and change depending on the cation type and size. The cold crystallization temperature (T_cc) and melting point (T_m) of the crystalline mesophase in confined BMIm⁺ and BMPyrr⁺ ionic liquids are higher than in the bulk ones, while in the amorphous BMPyrr⁺ mesophase, the T_cc and T_m values decrease by 9.7 and 14.2 °C, respectively. Confined BMPip⁺ and MOc₃Am⁺ only have the glass transition temperature (T_g), which increases by 1.5 and 8.0 °C, respectively, compared to bulk ILs. The onset decomposition temperature (T_d) decreases by 106.5, 40.7, 19.0 and 7.7 °C in BMIm⁺, BMPip⁺, BMPyrr⁺ and MOc₃Am⁺, respectively. The changes in the properties are explained by the cation and anion interaction with halloysite, as well as by the transformation of the ionic liquid structure. It is found that in this case the amount of the TFSI⁻ anion trans-conformer increases in the following order: BMIm⁺ > BMPyrr⁺ ∼ BMPip⁺ >> MOc₃Am⁺.