Heats of Dissolution of Several Imidazolium-Based Ionic Liquids into Low Dielectric Media

Abstract

Ionic liquids (ILs) are salts that are liquid at or near room temperature. They have unique properties since they are completely composed of ions and therefore have many potential applications. This thesis research focuses on the properties of ILs in low polarity solvent, chloroform (CHCl3). The motivation for this research was to further confirm the presence of particularly stable, long-lived ion pairs for several different imidazolium-based ILs dissolved in chloroform, as evidenced in prior research by the presence of two resonance sets in 1H NMR spectra.1 Specifically, it was shown that the variation of experimental conditions such as temperature and concentration changed the relative intensities of the two sets of resonances in the 1H NMR spectrum for the particular IL 1-ethyl-3-methylimidazolium bis(triflyl)amide ([C2mim][NTf2]), indicating an equilibrium between freely dissolved ions and ion pairs. From this discovery, it became relevant and of interest to measure the heats of dissolution of [C2mim][NTf2], which are useful to assess the thermodynamic stability of the ion pair formation. For comparison, the heats of dissolution were also obtained for the structurally similar ILs 1-hexyl-3-methylimidazolium bis(triflyl)amide ([C6mim][NTf2]) and 1-hexyl-3-methylimidazolium tris(triflyl)methide ([C6mim][MeTf3]). These ILs serve as comparisons for the effect of imidazolium chain length and anion. An RC1 calorimeter was used to obtain the heats of dissolution measurements of three imidazolium-based ILs using adiabatic controls. The dissolution of [C2mim][NTf2], [C6mim][NTf2], and [C6mim][MeTf3] were observed to be exothermic, releasing a small amount of heat into the solution, varying from -2.5 kJ/mol for [C6mim][MeTf3] up to -5.5 kJ/mol for [C2mim][NTf2]. To assess the reliability and accuracy of the measurements, the effect of different potential contaminants were examined, dissolved gas and solvent stabilizers (amylenes). It was determined that these potential contaminants did not affect the heats of dissolution. Heats of dissolution measurements were found to be lower for [C2mim][NTf2] when dissolving into 96:4 CHCl3:acetone solvent as compared to pure CHCl3. Using Hess’ Law, the change in enthalpy for the equilibrium between the freely dissolved ions and ion pairs in CHCl3 was determined to be between -1.0 and -2.1 kJ/mol IL for each of the temperatures investigated in this study, the largest change in enthalpy being at the lower temperatures.