Solubility of Water in Aprotic Heterocyclic Anion (AHA) Ionic Liquids

Abstract

The hydrophobic−hydrophilic character of a series of 15 tetraalkylphosphonium ionic liquids with aprotic heterocyclic anions (AHAs) was studied by measuring their liquid−liquid equilibrium with water. Specifically, the solubility of water in the ionic-liquid-rich phase was determined at room temperature by Karl Fischer titration. The effect of the tetra-alkyl-phosphonium cation was studied by varying the alkyl chain length; as expected, hydrophobicity increases with increasing alkyl chain length. The effect of the anion was studied by pairing the tetra-alkyl-phosphonium cation with different AHAs and by adding different substituents on the anion. The anion nature, basicity, and electron withdrawing effect of different substituents proved to have a significant impact on the IL's hydrophobicity. Changes in the alkyl chain length of the cation and the nature of the anion are able to tune the solubility of water in the ILs over an order of magnitude.

Carbon Dioxide Chemisorption by Ammonium and Phosphonium Ionic Liquids: Quantum Chemistry Calculations

“…The previous experimental works ,, revealed and confirmed an outstanding CO 2 sorption potential of the aprotic heterocyclic anions paired with quaternary cations. In particular, the Brennecke group made a significant impact on this field by introducing a separate class of liquid-state compounds.…”

“…In the present work, we have the hypotheses that were inspired by the previously published experimental analyses. , While the CO 2 chemisorptions by the cation and the anion are likely rather independent chemical processes, the deprotonation of the α-carbon atom of the quaternary cation and grafting of the CO 2 molecule are sequential. No experimental insights are helpful in deciding which of them takes place first.…”

“…Understanding these peculiar geometries is essential to assessing the feasibility of different quaternary cations for the CO 2 chemisorption. The long alkyl chains, such as those selected by Brennecke and co-workers, ,,, are important to modulate the phase behavior of the resulting RTILs, but they also increase the viscosity. High viscosity complicates the processing of RTILs in the experiment.…”

“…Room-temperature ionic liquids (RTILs) represent a huge-sized class of organic–inorganic chemical compounds that are liquid under standard conditions. − The liquid state of the prospective CO 2 sorbent is a definite advantage since it allows the gas molecules to spontaneously diffuse to the location where chemisorption takes place. Furthermore, RTILs exhibit negligible volatility even in their pure state thanks to a strong cation–anion attraction.…”

“…The most promising RTILs in the context of CO 2 sorption are composed of bulky organic cations and chemically reactive anions. ,, The aprotic heterocyclic anions (AHAs) represent an interesting family of organic structures with a set of potential applications in different fields of chemical engineering due to their strong affinity to the proton. This feature of AHAs can arguably convert certain thermodynamically forbidden reactions into favorable ones.…”