Controlling Gas Absorption in Liquids through Manipulation of Free Volume

Abstract

The capture, storage, and transport of gases by liquids is integral to many biological, environmental, and industrial processes. The density of gas molecules dissolved within a liquid in equilibrium with a bulk gas phase is determined in part by the structure of the liquid—specifically, by thermally-induced density fluctuations that can create empty space or free volume and lower the energetic penalty for gas absorption. Understanding the relationship between the molecular-scale structure of a liquid and its ability to accommodate gas molecules is critical to the design of solvents with high gas capacities. However, it is challenging to study and control the free volume within liquids. The work presented in this dissertation describes two broad efforts aimed at manipulating free volume in liquids to control gas absorption. Chapter One introduces gas absorption in liquids and describes the role of free volume. In addition, the nascent field of porous liquids is discussed to illustrate how liquids can be created with a much greater fraction of gas-accessible free volume than is present in conventional solvents. In Chapters Two, Three, and Four, I discuss our efforts to probe and manipulate free volume in ionic liquids. Using small-angle X-ray scattering experiments, which directly probe density fluctuations in a liquid, we demonstrate that there is a correlation between O2 absorption capacity and void size distribution in a series of ionic liquids. We then develop a generalizable synthesis of highly fluorinated ionic liquids to explore trends in ionic liquid properties as a function of fluorocarbon chain length, with a particular focus on gas solubility. Lastly, we attempt to control the structure—and thus, gas absorption capacity—of ionic liquids by incorporating metal–ligand coordination bonds, resulting in the first synthesis of a fluorinated metal-containing ionic liquid. Chapters Five, Six, Seven, and Eight explore the interplay between microporous materials—such as zeolites or metal–organic frameworks—and liquid water. These efforts aimed to understand the role that permanent free volume in micropore networks has on the structure and gas capacity of water. First, a thermodynamic strategy is presented that allows for dramatic increases in the amount of gas molecules that can be transported through aqueous solutions. This strategy relies on microporous materials with hydrophobic internal pore surfaces that exclude liquid water and hydrophilic external surfaces that allow nanocrystals to form uniform, stable dispersions in a variety of aqueous solutions. In an effort to expand the number of microporous materials that can be used to form aqueous porous liquids, we investigated the specific structural and chemical factors that allow microporous materials to have hydrophobic enough pore networks to prevent water intrusion. Next, I describe the potential for aqueous porous liquids to act as sorbents for acid-gas separations. Finally, I discuss our exploration of the role of pore size, shape, and chemistry on gas solubility of water confined within hydrophilic microporous materials, elucidating how certain pore structures prevent confined water from reaching its bulk density and cause substantial enhancements in gas solubility.