The thermal properties of a series of twenty‐four ionic liquids (ILs) have been determined by isothermal titration calorimetry (ITC) with the aim of simulating processes involving water sorption. For eleven water‐free ILs, the molecular structures have been determined by X‐ray crystallography in the solid state, which have been used to derive the molecular volumes of the ionic components of the ILs. Moreover, the structures reveal a high prevalence of hydrogen bonding in these compounds. A relationship between the molecular volumes and the experimentally determined energies of dilution could be established. The highest energies of dilution observed in this series were obtained for the acetate‐based ILs, which underlines their potential as working fluids in sorption‐based thermal energy storage systems.

The etch mechanism of group‐III‐nitrides in hot H3PO4 has been studied and compared to KOH. The etched surfaces were investigated by AFM, SEM, TEM, EDX and the etch solutions by NMR. AlGaN is shown to be selectively etched by low temperature H3PO4 or molten KF · 2H2O. The material dependency causes much flatter etch pit angles for AlGaN over GaN as well as H3PO4 over molten KOH. 27Al‐NMR investigations prove a fast ligand exchange as well as homogenous ligands. 31P‐NMR confirms that the dissolved metal is present as AlH2PO42+ and no hydration takes place nor does water play a role in the chemical reaction, opposed to KOH etching. Hence we propose a dissolution limited etch mechanism which can explain difference etch pit angles and material dependencies, as they are the result of material dependent dissolution, assuming that the vertical etch rate is constant.

Aqueous solutions of lithium halides are state-of-the-art sorbents in sorption-based heat pump and heat storage devices, with the drawbacks of high corrosivity and limited natural resources of lithium. Halide-free ILs could be well-suited alternative sorbents and have been explored with isothermal titration calorimetry showing superior thermochemical performance. Here, we have explored the performance of selected halide-free ILs in perfusion measurements monitoring the uptake of water vapor and CO2 from an air stream with well-defined flow and humidity as a model for open sorption processes. We have determined the saturation vapor pressure of water in the corresponding water–IL mixtures and moreover determined the diffusion coefficients of molecular species in binary [EMIM]­[Ac]–water mixtures via pulsed-field-gradient nuclear magnetic resonance (PFG NMR) measurements. These, together with variation of the absorption rates with calorimetric monitoring, indicate that diffusion is approximately 3 orders of magnitude faster than absorption and that the fast absorption process is not diffusion limited for [EMIM]­[Ac] in the used concentration range (D = 5.046 ± 0.021 × 10–11 m2/s; T = 25 °C). All measurements have been carried out in comparison to LiCl which corroborates the superior performance with regard to reducing water vapor pressure, heat of dilution, and absorption rate of acetate-based ILs and [EMIM]­[Ac], in particular.

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Abstract:1,3-alkyl-imidazolium-based ionic liquids show a wide range of applications from use as a "designer solvent", in catalysis 1-3 up to material science [4][5][6] . The interaction between cation and anion can be investigated by analyzing the crystal structure.7 In our work, we synthesized various 1,3-functionalized imidazolium and choline based Ils for the use as drying agent in sorption systems. Here especially substances with organic anion such as acetate, lactate and formate were quiet interesting because they expected, among other beneficial properties, a low melting point and a strongly pronounced hygroscopic behavior. However, the purification of the synthesized compounds showed a number of previously unknown crystal structures, which allowed to examine the interaction of cations and anions could be examined. In addition, the crystal structure data provided the molecular volume of the ions, which is used to predict thermodynamic aspects. Synthesis: Scheme 1For the use in our systems, it is necessary to have liquid media, which can be pumped and preferably does not crystallize during regeneration. As a result, especially RTILs are an interesting approach to avoid this problem. The known requirements reduce the possible anions mainly to the organic ones, esp. formate, acetate, lactate and oxalate. It is understandable, that the choice of the cations results from the same considerations as for the anions. This means a relative short alkyl chain length or hydrophilic groups like alcohols or carboxylic acids attached to the 3-position in 1-methyimidazole (Scheme 1).Chart 1