Predictive thermodynamics for ionic solids and liquids

Abstract

The application of thermodynamics is simple, even if the theory may appear intimidating. We describe tools, developed over recent years, which make it easy to estimate often elusive thermodynamic parameter values, generally (but not exclusively) for ionic materials, both solid and liquid, as well as for their solid hydrates and solvates. The tools are termed volume-based thermodynamics (VBT) and thermodynamic difference rules (TDR), supplemented by the simple salt approximation (SSA) and single-ion values for volume, Vm, heat capacity, , entropy, , formation enthalpy, ΔfH°, and Gibbs formation energy, ΔfG°. These tools can be applied to provide values of thermodynamic and thermomechanical properties such as standard enthalpy of formation, ΔfH°, standard entropy, , heat capacity, Cp, Gibbs function of formation, ΔfG°, lattice potential energy, UPOT, isothermal expansion coefficient, α, and isothermal compressibility, β, and used to suggest the thermodynamic feasibility of reactions among condensed ionic phases. Because many of these methods yield results largely independent of crystal structure, they have been successfully extended to the important and developing class of ionic liquids as well as to new and hypothesised materials. Finally, these predictive methods are illustrated by application to K2SnCl6, for which known experimental results are available for comparison. A selection of applications of VBT and TDR is presented which have enabled input, usually in the form of thermodynamics, to be brought to bear on a range of topical problems. Perhaps the most significant advantage of VBT and TDR methods is their inherent simplicity in that they do not require a high level of computational expertise nor expensive high-performance computation tools - a spreadsheet will usually suffice - yet the techniques are extremely powerful and accessible to non-experts. The connection between formula unit volume, Vm, and standard thermodynamic parameters represents a major advance exploited by these techniques.

Understanding the Stability of Salt-Inclusion Phases for Nuclear Waste-forms through Volume-based Thermodynamics

“…1 , where the fitted constants k (J/K/mol/nm 3 ) and c (J/mol/K) are applied with the formula unit volume, with the constants varying as to whether the system is organic (liquid or solid) or ionic (hydrous or anhydrous). In this case we take the constants as fitted for anhydrous ionic salts 12 . …”

Absolute ion hydration enthalpies and the role of volume within hydration thermodynamics

“…In 1999 Jenkins [1][2][3][4][5][6][7] and co-workers rst introduced what they later termed VBT. The early ideas were based on work by Mallouk and Bartlett 8 and were later extended by Jenkins and coworkers to a range of applications.…”

“…21,23,[43][44][45] Further applications continue to emerge, 42 and a recent review of the main features of VBT has appeared. 7 More recently, ionic strength, I and formula unit volume, V m essential components of VBThave found application as measures of the enhanced stability brought about to Zintl structures as part of a recently emerging interpretation and new realm of crystallography in which the Zintl-Klemm [46][47][48][49] concept has been revived and extended and which now, once correctly applied, enables the prediction and rationalisation of crystal structures. 48,49 This area of crystallography is destined to yield new and exciting ideas in the future.…”

Physicochemical characterizations of novel dicyanamide-based ionic liquids applied as electrolytes for supercapacitors

“…Recently, several authors applied the empirical model of lattice energy (U POT ) of ionic solids for ILs electrolytes. [38][39][40] In this case, the coulombic interaction between ions may be deduced from the empirical equation given in eqn (5). 39,41,42 For this model, the calculation of U POT only relies on the chemical formula, the ionic charges and the molecular volume (or density).…”