(166g) Exploiting Unique Properties of Liquid-Gas Interfaces for Efficient Gas Absorption and Separation: Insights from Molecular Dynamics

Interface states in liquid- and solid-gas systems control important physics and chemistry of adsorption and absorption separation processes. The properties of these surfaces (and films) are very different from the properties of bulk solvents. Therefore, the objective of this work is to gain an understanding of the microscale behaviour and properties of gas-liquid interfaces which could pave the way to designing novel solvents and adsorbents.

In this presentation, we consider ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) as a solvent, interacting with gases such as carbon dioxide and nitrogen. One of the key properties of ionic liquids is essentially zero vapour pressure and supported ionic liquids systems have been proposed for catalysis, gas separations and other applications. To explore adsorption of carbon dioxide and nitrogen on the surface of a thin layer of [BMIM]⁺[PF₆]^- we employed molecular dynamics simulations, coupled with detailed characterisation of the structure of the interface, density profiles of the adsorbed gases, dynamics and other aspects of the system.

Analysis of the global and intrinsic density profiles of gas-liquid systems across the simulated interfaces reveals the preferred active sites on ionic liquid. We demonstrate that CO₂ approaches the terminal carbon atom in the butyl chain of [BMIM]⁺[PF₆]^- 2 Å more closely than N₂. We estimate that the concentration of carbon dioxide molecules on the surface is 13 and 7 higher compared to the gas phase and the bulk liquid at 298.15 K and 0.1 MPa respectively. The increase in temperature from 298.15 to 393.15 K leads to desorption of gas molecules from the interface. In comparison, in the liquid phase, carbon dioxide prefers to stay near the anion [PF₆]^- while nitrogen has a preference for the imidazole ring. Both gases, nitrogen and carbon dioxide, express similar transport properties, as quantified by the studies of self-diffusion coefficients as a function of concentration and temperature. We also explore the extent of enhancement of CO₂/N₂ selectivity of interfacial and bulk systems.

In the systems, where interfaces are extensive, they will dominate the behaviour and performance of the whole system. Properties such as orientation of molecules, density behaviour with respect to the surface, distribution of molecular species at the surface influence the thermodynamics of gas adsorption and control the gas absorption into the bulk. This work shows the importance of the gas-liquid interface and intrinsic surface properties in designing materials with high gas uptake and selectivity.