Chemistry

Salt Solution

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Science  06 Oct 2006:
Vol. 314, Issue 5796, pp. 19
DOI: 10.1126/science.314.5796.19b

Ionic liquids (ILs), or more specifically cation-anion pairs that form a stable fluid near room temperature, are playing an increasingly practical role as chemical reaction solvents, electrolytes, and heat-transfer media on scales that range from the laboratory bench to industrial manufacturing processes. Their advantages include miniscule vapor pressure, high polarity, and relative inertness. However, efforts to fine-tune IL properties by structural modification are hindered by a limited understanding of why specific cation-anion combinations melt at such low temperature.

Krossing et al. have developed a simple predictive framework for calculating the melting point of a given IL with knowledge of the dielectric constant, or conversely estimating dielectric properties from the melting point. Their method involves computing the free energy of fusion using a thermodynamic cycle that adds the lattice energy (required to move ions from the lattice to the gas phase) to the compensating stabilization energy arising from naked ion solvation in a dielectric medium. Enthalpic and entropic parameters are calculated using a combination of quantum chemical methods and volume-based-thermodynamics approximations. The modeling scheme proved highly effective in reproducing experimental data for 14 ionic liquids composed of the most commonly used cations (substituted imidazolium, pyrrolidinium, pyridinium, and ammonium) and anions [BF4, PF6, CF3SO3, and (CF3SO2)2N]. — JSY

J. Am. Chem. Soc. 128, 10.1021/ja0619612 (2006).

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