Kirkwood-Buff integrals: From fluctuations in finite volumes to the thermodynamic limit

The Kirkwood-Buff theory is a cornerstone of the statistical mechanics of liquids and solutions. It relates volume integrals over the radial distribution function, so-called Kirkwood-Buff integrals (KBIs), to particle number fluctuations and thereby to various macroscopic thermodynamic quantities such as the isothermal compressibility and partial molar volumes. Recently, the field has seen a strong revival with breakthroughs in the numerical computation of KBIs and applications to complex systems such as bio-molecules. One of the main emergent results is the possibility to use the finite volume KBIs as a tool to access finite volume thermodynamic quantities. The purpose of this Perspective is to shed new light on the latest developments and discuss future avenues.

Sizable dynamics in small pores: CO2 location and motion in the α-Mg formate metal-organic framework

Metal-organic frameworks (MOFs) are promising materials for carbon dioxide (CO₂) adsorption and storage; however, many details regarding CO₂ dynamics and specific adsorption site locations within MOFs remain unknown, restricting the practical uses of MOFs for CO₂ capture. The intriguing α-magnesium formate (α-Mg₃(HCOO)₆) MOF can adsorb CO₂ and features a small pore size. Using an intertwined approach of ¹³C solid-state NMR (SSNMR) spectroscopy, ¹H-¹³C cross-polarization SSNMR, and computational molecular dynamics (MD) simulations, new physical insights and a rich variety of information have been uncovered regarding CO₂ adsorption in this MOF, including the surprising suggestion that CO₂ motion is restricted at elevated temperatures. Guest CO₂ molecules undergo a combined localized rotational wobbling and non-localized twofold jumping between adsorption sites. MD simulations and SSNMR experiments accurately locate the CO₂ adsorption sites; the mechanism behind CO₂ adsorption is the distant interaction between the hydrogen atom of the MOF formate linker and a guest CO₂ oxygen atom, which are ca. 3.2 Å apart.

Experimental evidence for the influence of charge on the adsorption capacity of carbon dioxide on charged fullerenes

The adsorption of CO₂ is sensitive to charge on a capturing model carbonaceous surface, such as C₆₀ fullerenes.

A procedure to find thermodynamic equilibrium constants for CO2 and CH4 adsorption on activated carbon

Thermodynamic equilibrium for adsorption means that the chemical potential of gas and adsorbed phase are equal. A precise knowledge of the chemical potential is, however, often lacking, because the activity coefficient of the adsorbate is not known. Adsorption isotherms are therefore commonly fitted to ideal models such as the Langmuir, Sips or Henry models. We propose here a new procedure to find the activity coefficient and the equilibrium constant for adsorption which uses the thermodynamic factor. Instead of fitting the data to a model, we calculate the thermodynamic factor and use this to find first the activity coefficient. We show, using published molecular simulation data, how this procedure gives the thermodynamic equilibrium constant and enthalpies of adsorption for CO2(g) on graphite. We also use published experimental data to find similar thermodynamic properties of CO2(g) and of CH4(g) adsorbed on activated carbon. The procedure gives a higher accuracy in the determination of enthalpies of adsorption than ideal models do.