dc.title: Ionic liquid-based membranes for light paraffin and CO₂ separations dc.description.abstract: Gas separation membranes can offer significant advantages over traditional separation technologies, such as distillation, including substantially lower energy consumption, modular design, and scalability with surface area rather than volume. Membranes for gas separations are broadly classified as either size-selective or solubility-selective. Size-selective membranes separate species primarily based on differences in diffusivity, whereas solubility-selective (i.e., reverse-selective) membranes rely mainly on differences in condensability, such that more condensable gases exhibit higher permeability than less condensable species. Membrane technology is now widely used in industrial applications including natural gas processing, air separation, and carbon dioxide (CO₂) removal. However, decades of research on polymeric gas separation materials have revealed an inherent tradeoff between permeability and selectivity, commonly described by the permeability-selectivity upper bound. Overcoming this limitation through the design of new materials remains a central challenge in membrane science. This dissertation explores the design and characterization of solubility-selective, ionic liquid (IL)-based membranes targeted toward light paraffin and CO₂ separations. Ionic liquids are nonvolatile molten salts with melting points below 100 °C that exhibit high solubility for condensable gases, including hydrocarbons and CO₂, making them attractive candidates for new membrane materials that target these species. The first portion of this work focuses on improving selectivity for light paraffin separations. IL-based membranes containing significant hydrocarbon character were synthesized and evaluated in both supported ionic liquid membrane (SILM) and poly(ionic liquid) (PIL) membrane formats. These studies identified key molecular design parameters governing gas sorption, transport, and membrane stability, establishing guiding principles for the development of IL-based membranes for light paraffin separations. While the materials investigated here do not yet surpass the selectivity of industrial polydimethylsiloxane (PDMS), they provide important fundamental insights into IL-based membrane design. The second portion of this dissertation addresses CO₂ separations relevant to post-combustion carbon capture, natural gas sweetening, and CO₂-intensive industrial processes. Technoeconomic analyses indicate that increasing membrane permeance has a greater impact on process cost than increasing selectivity, since required membrane area scales inversely with flux. Guided by this principle, a new class of highly permeable poly(1,3-dioxolane)-containing terpolymers was characterized for their gas transport behavior, with performance linked to polymer structure and ether oxygen content. To further enhance CO₂ permeance, polymer/IL blend membranes incorporating highly CO₂-philic ILs were prepared, yielding membranes with both high permeability and useful selectivity. These results reinforce the potential of IL-based membranes as a platform for CO₂ separation technologies.