(Keynote) Ionic Liquids for Improved Separations
Authors: Noble Richard , Gin Douglas 
Published: February 2020

Synthetic membranes are important for many separation applications, such as the separation and purification of atmospheric gases for medical and industrial use, and salt removal from seawater in regions lacking fresh water sources. Unlike mesh filters used to separate large solid particles from a fluid (i.e., via phase difference), membranes used to separate molecular species from one another (i.e., a chemical separation) are required to perform a more difficult process, given the size scale and level of mixing of the components.

Membranes perform chemical separations generally based on their morphology (i.e., whether they are dense or porous). Dense (i.e., nonporous) membranes operate by the solution-diffusion (S-D) mechanism in which one molecular species is separated from another by the difference in their abilities to be dissolved in the membrane material and their ability to diffuse through it. The keys to designing porous membrane materials for chemical separations are the ability to generate uniform pores with the correct size on the molecular level and that are continuous across the membrane. If there is a distribution of pore sizes, most of the molecules will pass through the largest pores accessible, thereby compromising selectivity. Because of the benefits of membrane-based chemical separations over other methods (e.g., smaller device footprints, lower energy use compared to distillation), there has been recent interest in applying membranes to new separation problems, as well as improving membrane materials used in existing separations. Consequently, there is a need for better membrane materials in general (i.e., with better selectivity, better productivity, longer life and operational stability, increased operating temperature range, use in chemically challenging environments, and the ability to adjust materials properties and performance for a given application).

We have focused our research on novel material platforms that provide some advantages compared to conventional membranes. One approach is to use room-temperature ionic liquids (RTILs) in various morphologies as membranes. RTILs have unique physiochemical properties, such as negligible vapor pressure, high thermal stability, and intrinsic solubility for certain gases, which make them unique in terms of organic liquids and solvents. In comparison to conventional polymeric membranes, RTILs perform gas separations due to solubility differences. They can also be converted to polymerizable molecules, thus allowing promising RTILs to be polymerized into solid-state materials. Thus, they can be converted to various morphologies as membranes while maintaining the inherent selectivity of the material. These materials have all the advantages listed earlier. They can be prepared as polymer films, composite structures with ionic liquid within the structure and gels. The wide range of morphologies and chemical structures possible for RTILs allows for designing materials that not only have the desired physicochemical properties but also the mechanical properties needed to produce viable membranes. We have prepared polymerized RTIL-based membranes that can be used for selective gas separations, as membrane separators in electrochemical devices, as catalytic membranes, and as “breathable” barrier materials against chemical warfare agents and toxic industrial chemicals.