Understanding the factors that govern gas absorption in liquids is critical to the development of high-capacity liquid absorbents for catalysis, electrochemistry, and separations. Here, we discuss experimental probes of liquid structure that demonstrate the importance of free volume in the O2 absorption capacities of ionic liquids. Specifically, we establish that isothermal compressibility—measured rapidly and accurately by small-angle X-ray scattering and supported by molecular dynamics simulations—reports on the size distribution of transient voids within ionic liquids and is correlated with their O2 absorption capacity. Additionally, the O2 absorption capacities of ionic liquids are correlated with their thermal expansion coefficients, reflecting the beneficial effect of weak intermolecular interactions on gas absorption capacity. Together, isothermal compressibility and thermal expansion explain trends in O2 absorption behavior observed for a broad range of ionic liquids, implying that the void size distribution—in particular the number of large voids—is more important than total free volume. These results emphasize that accurate measurements of gas absorption in ionic liquids coupled with reliable methods of investigating liquid structure can provide a powerful understanding of trends in absorption capacity and may assist in the development of higher-absorbing ionic liquids.

Understanding relationships between free volume and oxygen absorption in ionic liquids

ACS Spring 2022

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