Understanding the phase and solvation behavior of fluorinated ionic liquids

Abstract

Fluorinated ionic liquids (FILs) are defined as molecules having fluorinated tags equal to or longer than 4 carbon atoms in the anion and/or cation structures. They present nanosegregated domains making them 3-in-1 solvents with exceptional properties, including an attractive solubility power. This work is an important contribution towards understanding the current research on the phase behavior of FILs, which may be used as task-specific materials for industrial applications. An overview of the main works published in the last two decades is presented, concerning gas solubility in FILs, the application of membranes to improve the gas absorption in FILs, and the use of modeling approaches to ease the application of FILs in gas capture and separation processes, with emphasis on the relationship between the structural properties and their performance. Contributions concerning the liquid-liquid and solid-liquid equilibria behavior of FILs, including the liquid-liquid equilibria (LLE) of FILs in water and perfluoroalkanes, and the solid-liquid equilibria (SLE) of solid FILs in water and mixtures of FILs are also presented. Regarding the absorption of gases in FILs, a careful analysis of the published works reveals that: (1) an optimal density of fluorine atoms in FILs structure is required to positively impact the absorption of different gases, (2) the functionalization of membranes can be also a useful method to improve the performance in separation processes, and (3) modeling tools can ease the screening of the features that promote the absorption of gases by FILs. The study of FILs LLE showed a rich phase behavior with water and perfluoroalkanes and the enhanced surfactant power of FILs, which is highly dependent on the length of the hydrogenated and fluorinated side chains. Finally, studying the SLE of FILs mixtures allowed the formation of deep eutectic systems that enlarges the applicability of FILs.