Ionic liquids (IL) have important applications in separations, energy storage, and tribology. The temperature dependent morphologies of ILs represent important physical processes for applications as they control IL physicochemical properties. Most of the molecular ions which make up ILs are bulky and asymmetric, their crystallization processes are complex and multifaceted. Furthermore, addition of solvents to ILs can be a direct method to tune properties, which significantly alters the IL crystallization pathways and sometimes creates ‘solvate’ systems, akin to eutectic mixtures, with unexpected properties. This work contributes to valuable understanding of these processes. Phase and structural changes in butyltrimethylammonium bis(trifluoromethylsulfonyl)imide [N1114][TFSI] have been characterized as a function of temperature using, differential scanning calorimetry (DSC), infrared spectroscopy (IR), and X-ray Diffraction (XRD). Phase transitions of [N1114][TFSI] include, cold crystallization, solid-solid, and multiple melting transitions, which all involve IL structure change. Analyses show that specific phase transitions in [N1114][TFSI] change in temperature, heat flow, and structure as a function of sample cooling and heating rates. Moving forward, since most ILs are hygroscopic, many IL applications involve significant amounts of water. Our group has collected DSC and IR measurements of butyl-imidazolium based ILs diluted with varying amounts of water and acetonitrile (as an aprotic control). Our data show when pure 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] is cooled no crystallization is observed, but as the water concentration is increased in [BMIM][PF6] two separate crystallizations appear (1st crystallization: attributed to water, 2nd crystallization: not previously characterized in [BMIM][PF6]), which is indicative of structural changes in [BMIM][PF6]. 1-butyl-3-methylimidazolium triflate [BMIM][OTf] was used to compare the IL imidazolium cation with a different anion. Unlike the pure [BMIM][PF6], pure [BMIM][OTf] shows a crystallization upon cooling, but with the addition of water a second crystallization occurs, which releases much less enthalpy than the crystallization of water. Based on this the water might be complexing with [BMIM][OTf], but not [BMIM][PF6]. Analysis of IL structure variation due to temperature and water concentration have proven complex but will strengthen the molecular understanding of this fluid class.

Thermal and spectroscopic analysis of ionic liquid solvent mixtures

2019 Fall National Meeting

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