Going Full Circle: Phase‐Transition Thermodynamics of Ionic Liquids

Abstract

We present the full enthalpic phase transition cycle for ionic liquids (ILs) as examples of non-classical salts. The cycle was closed for the lattice, solvation, dissociation, and vaporization enthalpies of 30 different ILs, relying on as much experimental data as was available. High-quality dissociation enthalpies were calculated at the G3 MP2 level. From the cycle, we could establish, for the first time, the lattice and solvation enthalpies of ILs with imidazolium ions. For vaporization, lattice, and dissociation enthalpies, we also developed new prediction methods in the course of our investigations. Here, as only single-ion values need to be calculated and the tedious optimization of an ion pair can be circumvented, the computational time is short. For the vaporization enthalpy, a very simple approach was found, using a surface term and the calculated enthalpic correction to the total gas-phase energy. For the lattice enthalpy, the most important constituent proved to be the calculated conductor-like screening model (COSMO) solvation enthalpy in the ideal electric conductor. A similar model was developed for the dissociation enthalpy. According to our assessment, the typical error of the lattice enthalpy would be 9.4 kJ mol(-1), which is less than half the deviation we get when using the (optimized) Kapustinskii equation or the recent volume-based thermodynamics (VBT) theory. In contrast, the non-optimized VBT formula gives lattice enthalpies 20 to 140 kJ mol(-1) lower than the ones we assessed in the cycle, because of the insufficient description of dispersive interactions. Our findings show that quantum-chemical calculations can greatly improve the VBT approaches, which were parameterized for simple, inorganic salts with ideally point-shaped charges. In conclusion, we suggest the term "augmented VBT", or "aVBT", to describe this kind of theoretical approach.

Effect of Synthesis Parameters on the Electrochemical Properties of High‐Surface‐Area Mesoporous Titanium Oxide with Polypyrrole Nanowires in the Pores

“…Consequently, the use of quantities derived from the crystal structure to explain bulk properties is highly desired. This has been confirmed by the success of the molecular volume14 ( V m ) in the description and prediction of IL properties 1,15–19…”

Ionic Liquids of Cationic Sandwich Complexes

“…[Co(C 5 H 4 Et) 2 ][N(SO 2 C 4 F 9 ) 2 ] ( 13 d ) exhibited complicated transitions, including a re‐entrant transition and a transition into a glassy crystal (see the Supporting Information, Figure S4). Many salts exhibited small enthalpies and entropies of melting ( ΔH m and Δ S m , respectively Tables 1–3) compared with usual crystals of ILs27 because orientationally disordered phases appear at high temperatures when Δ S m <21 J K −1 mol −1 28…”

Is Universal, Simple Melting Point Prediction Possible?

“…The addition of 15.6 kJ mol À1 is necessary because of the chemical potential difference of a 1 mol L À1 solution and a 1 bar gas to which each ion contributes the same amount. [100] The entropy difference was calculated according to Equation (12):…”