What phenomena can arise from dynamic properties in porous solids? Adsorption describes the adhesion of atoms, ions or molecules from a gas, liquid or solution state (fluid phase) to a surface of a solid (solid phase). Adsorption is dominated by the bulk properties of both the fluid and solid state and the interfacial interactions. The stronger the interactions the higher the ability for a solid to adsorb a particular species. Because most gases (in particular low molecular weight gases) exhibit a high level of entropy at ambient temperature the adsorption capacity at ambient pressure is ver low. Adsorption interactions can either be enhanced by increasing the solid-fluid interactions e.g via chemical functionalisation of the surface, or by introducing the surface are with which the fuid can interact. The most established strategy is the construction of porous solids that exhibit pore diameters in the range of the kinetic diameter of the fluid, which results in very strong adsorption interactions. The design of porous materials and their consideration for storage, separation and sensing primarily of gases is a large field with long tradition. However, recently novel adsoprtion phenomena were discovered that originate from the change of the porous solid during the adsorption process. We recently summarized the adsorption properties of such soft porous materials in the review article: Chemistry of Soft Porous Crystals – Structural Dynamics and Gas Adsorption Properties

Illsutration of adsorption phenomena in soft porous crystals see original work here

More or less by accident I discovered that under certain conditions porous solids can contract their pores during the adsorption process and release adsorbed gas from their pore as a consequence of reduced pore diameter. The phenomenon was concequently called „negative gas adsorption“. S. Krause, V. Bon, I. Senkovska, U. Stoeck, D. Wallacher, D. M. Többens, S. Zander, R. S. Pillai, G. Maurin, F. o.-X. Coudert, S. Kaskel, A pressure-amplifying framework material with negative gas adsorption transitions. Nature 2016,532 (7599), 348-352. To date most adsorption phenomena in soft porous materials are discovered by accident and fundamental understanding of the properties and structural motives that govern these non-classical properties are still lacking. With the help of computational methods we try to explore criteria that efine such „soft adsorption processes“.

Toy-model to represent pore expansion and contraction in soft porous materials. see original work

J. D. Evans, S. Krause, S. Kaskel, M. B. Sweatman, L. Sarkisov, Exploring the thermodynamic criteria for responsive adsorption processes. Chemical Science 2019,10 (19), 5011-5017. A key feature in these systems is the complex energy landscape that depends on many different variables of the solid but also liquid phase. Brining both together is a key step in designing novel adsorption properties and utilize them for applications and technologies that currently do not exist.

Illustration of the free energy landscape of soft porous crystals. copyright Simon Krause