The swelling properties of smectite clay minerals are relevant to many engineering applications including environmental remediation, repository design for nuclear waste disposal, borehole stability in drilling operations, and additives for numerous industrial processes and commercial products. We used molecular dynamics and grand canonical Monte Carlo simulations to study the effects of layer charge location, interlayer cation, and temperature on intracrystalline swelling of montmorillonite and beidellite clay minerals. For a beidellite model with layer charge exclusively in the tetrahedral sheet, strong ion–surface interactions shift the onset of the two-layer hydrate to higher water contents. In contrast, for a montmorillonite model with layer charge exclusively in the octahedral sheet, weaker ion–surface interactions result in the formation of fully hydrated ions (two-layer hydrate) at much lower water contents. Clay hydration enthalpies and interlayer atomic density profiles are consistent with the swelling results. Water adsorption isotherms from grand canonical Monte Carlo simulations are used to relate interlayer hydration states to relative humidity, in good agreement with experimental findings.

The adsorption of noble gases (Ar, Kr, Xe, and Rn) and N 2 by a diverse range of Metal−Organic Frameworks (MOFs) containing open metal sites (OMS) was systematically investigated using volumetric gas porosimetry and grand canonical Monte Carlo simulation. The ten MOFs considered are grouped into two series. The first, MOF-74-x, is based upon MOF-74/CPO-27, which has divalent metal ions Mg, Co, Ni, and Zn, and was chosen to explore the effect of metal identity within a constant topology. The second series, nbo-MOFs, consists of NOTT-100, NOTT-101, NOTT-102, NOTT-103, and PCN-14 and probes the effect of pore size while maintaining constant OMS identity and similar coordination environment. Comparisons of the nbo-MOFs with HKUST-1 are also made, since this MOF has a similar chemical structure, although the topology is different from the nbo structures. Gas uptake and Henry's constant (k H ) for nbo-MOFs increase with decreasing pore size, and as pore size decreases, the effect of gas polarizability on k H becomes more profound. This implies that tailored pore size can strongly influence k H , especially for the more polarizable gases Kr and Xe. A detailed analysis of the void space in the nbo-series reveals large cages connected by narrow pores, resulting in tortuous, zigzag diffusion pathways that likely increase the strength of the MOF−gas interaction. Data for the MOF-74-x series show that both the density of OMS and their accessibility in this topology enhance the framework interaction with the noble gases. However, because the interaction is largely due to (point charge)−(induced-dipole) interactions and the charge on the metal ion varies little, the specific identity of the metal ion is relatively unimportant. Consequently, new MOFs tailored to maximize noble gas uptake from air should target structures with both a high density of OMS and pores that are approaching the size of the gas of interest.

A collection of >3000 MOFs with experimentally confirmed structures were screened for performance in three binary separations: Ar/Kr, Kr/Xe, and Xe/Rn. 70 materials were selected for further analysis, and calculations were performed to account for inaccessible regions. Single component GCMC calculations were performed to parametrize IAST calculations on these 70 materials. An approach that avoids possible imprecision in IAST due to curve-fitting of single component isotherms is introduced. The precision of IAST for these gas pairs was confirmed with extensive binary GCMC calculations. For each binary separation, materials were identified with predicted performance that surpasses the state of the art. A significant number of materials were found to be "reverse selective" in the sense that a smaller gas species is preferably adsorbed over a larger species. The physical origin of this phenomenon is explained. The effect of temperature on separation performance was also examined.

A systematic investigation of the effects of linker polarizability on the adsorption properties of weakly interacting gases (N 2 , Ar, Kr, and Xe) is reported. Experimental and simulated adsorption properties were measured for a complete isoreticular series of monohalogenated metal−organic frameworks (MOFs). Variations on IRMOF-2, in which one linker hydrogen is replaced with −F, −Cl, −Br, or −I, comprise the series. Both experimental and simulated results indicate that increasing linker polarizability correlates with increased gas uptake. Evidence of increased adsorbate interaction with increased linker polarizability is also observed in the Kr/N 2 , Xe/N 2 , and Xe/ Kr selectivity data and in isosteric heats of adsorption. Unexpectedly, comparison between simulated and experimental isotherms reveals that the agreement between the two improves with the size of the adsorbate, with essentially identical results for Xe. This is apparently due to the creation of regions inaccessible to any of the noble gases as a result of halogen functionalization. Simulated adsorption isotherms are also reported for radon, which is difficult to measure experimentally due to its radioactivity.

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