Thursday, February 19, 2015
Engineering Research Center (ENGRC) 490, Tempe campus [map]
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 two 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 polymers, they perform gas separations due to solubility differences. They can be converted to polymerizable molecules. This allows promising RTILs to be prepared as membranes. 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 allow for finding materials that not only have the desired physicochemical properties but also the mechanical properties needed to produce viable membranes.
A second approach is the development of polymers with ordered, interconnected, sub-1-nm size pores for molecular size separation. These nanostructured polymers are made by cross-linking lyotropic liquid crystal (LLC) assemblies formed by polymerizable surfactants in the presence of water. Supported LLC polymer membranes with a type I bicontinuous cubic (QI) architecture possess a 3-D interconnected water pore system with a uniform pore size of ca. 0.75 nm. These LLC membranes are able to cleanly size-exclude hydrated salt ions and a variety of small organic solutes from water (i.e., water desalination) with good permeabilities. The water desalination/nanofiltration performance of this LLC polymer membrane is superior to that of conventional nanofiltration membranes, and close to that of commercial reverse-osmosis membranes. The relationship between the nanoscale architecture of these polymerized LLC assemblies and their separation will be discussed.