Thermochemistry of ammonium based ionic liquids: Tetra-alkyl ammonium nitrates – Experiments and computations

https://doi.org/10.1016/j.jct.2012.02.035Get rights and content

Abstract

In this work data of the molar enthalpies of formation of the ionic liquids [R4N][NO3] were measured by means of combustion calorimetry. The thermal behavior of [R4N][NO3] was studied using differential scanning calorimetry. First-principles calculations of the enthalpy of formation in the gaseous phase have been performed for the ionic species using the G3(MP2) theory. Enthalpies of formation of aqueous tetra-alkyl ammonium ions ΔfHm° ([R4N]+(aq)) derived in this work are useful for prediction thermochemical properties of the ammonium based ionic liquids.

Highlights

► Gaseous enthalpies of formation of [R4N][NO3] were derived from experiment. ► We used combustion, solution, and differential scanning calorimetry. ► First-principles calculations used to test consistency of experimental data. ► Enthalpies of formation of aqueous alkylammonium cations were derived. ► The latter suggested for prediction thermochemistry of ammonium based ionic liquids.

Introduction

Ionic liquids (ILs) have attracted much attention due to widespread potential for practical applications such as heat transfer and storage medium in solar thermal energy systems as well as for many areas such as fuel cells, rechargeable batteries and “green solvents”. Quaternary ammonium cations form ILs with various anions. Because of the variety of side chains that can be connected to the central N+, many quaternary ammonium ILs could be prepared to “tune” the physical properties such as melting point, water miscibility, conductivity and viscosity by selection and modification of the anion and cation of the IL.

Tetra-alkyl ammonium ILs are one of the key compounds to clarify the structure of aqueous solution and the nature of hydrophobic solute-solvent interaction as well as for study of a thermodynamics of transfer of ions from water to other solvents [1]. Moreover, tetra-n-alkyl ammonium compounds form the clathrate hydrate crystals below ambient temperature without applying high pressures. This fact suggests the ammonium ions with long alkyl chains have a strong tendency to enable the surrounding water molecules to form the clathrate-like structure even in aqueous solution. Regarded for many years as harmful by the oil and gas industry because of their tendency to plug pipelines, clathrate hydrates are now considered propitious for various applications. They may be used for gas capture, storage, separation, and transportation, and the gas hydrates naturally found in deep seas and permafrost may provide large amounts of methane [2].

Experimental data on thermodynamic properties of ionic liquids are scarce [3], [4], [5], [6], [7], [11]. These quantities are required for the validation and development of the molecular modeling and first-principles methods towards this new class of solvents. In this work we report the measurements (oxygen combustion calorimetry) of molar enthalpy of formation, ΔfHm° (cr), of the ammonium based ionic liquids – tetra-methyl ammonium nitrate ([Me4N][NO3]); tetra-ethyl ammonium nitrate ([Et4N][NO3]), and tetra-butyl ammonium nitrate ([Bu4N][NO3]). The gaseous enthalpies of formation of the ionic liquids were obtained from first-principles calculations using the G3(MP2) method. The enthalpies of formation ΔfHm° (aq) of the aqueous cations [Me4N],+ [Et4N],+ and [Bu4N]+ were evaluated by using ΔfHm° (cr) of [R4N][NO3] measured in this work and solution enthalpies ΔsolHm° of [R4N][NO3] taken from reference [7].

Section snippets

Materials and chemicals

The samples of the ammonium based ionic liquids [R4N][NO3]: tetra-methyl ammonium nitrate [1941-24-8], tetra-ethyl ammonium nitrate [1941-26-0], and tetra-butyl ammonium nitrate [1941-27-1] used for the experiments were prepared and purified by proionic Production of Ionic Substances GmbH (Austria). Prior to using the samples were subjected to vacuum evaporation at 333 K and 10−2 mbar for more than 24 h to reduce possible traces of solvents and moisture. Purities of all samples [R4N][NO3] were

Results and discussion

Generally, the enthalpy of formation in the gaseous phase of any compound is made up of two contributions:ΔfHm°(g)=ΔfHm°(l)+ΔlgHmΔfHm°(l)=ΔfHm°(cr)+ΔcrlHmwhere ΔfHm° is the enthalpy of formation in the gas, crystalline or liquid state; ΔcrlHm is the enthalpy of sublimation, and ΔlgHm is the enthalpy of vaporization. In the current study we derived these properties from thermochemical measurements as follows.

Conclusions

Combination of combustion calorimetry, differential scanning calorimetry, and solution calorimetry with the first-principles calculations allows to obtain the broad scope of the thermodynamic data. This procedure provides indispensable data material for testing first-principles procedures and molecular dynamic simulations techniques in order to understand thermodynamic properties of ionic liquids on a molecular basis. Enthalpies of formation of aqueous tetra-alkyl ammonium ions ΔfHm° ([R4N]+(aq)

Supporting information available

Results for typical combustion experiments of the tetra-alkyl ammonium nitrate (tables S1 to S3); auxiliary properties for combustion calorimetry and data treatment (tables S4 and S5); G3(MP2) total energies at 0 K and enthalpies at 298.15 K of the molecules studied in this work (table S6); correlations of enthalpies of ionic liquids with the chain length (Figs. S1 and S2).

Acknowledgments

This work has been supported by the German Science Foundation (DFG) in the frame of the priority program SPP 1191 “Ionic Liquids”.

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