(383c) Capillary Condensation and Hysteresis in Nanoporous Materials: New Simulations and New Insights

Capillary condensation and hysteresis are important phenomena in the field of adsorption. Pioneering work by Keith Gubbins and his colleagues demonstrated the power of grand canonical Monte Carlo (GCMC) simulations and classical density functional theory to study hysteresis and capillary condensation and provided important insights into these phenomena. Here, we build upon this work by applying molecular simulation to study capillary condensation and hysteresis of C1 to C6 n-alkanes in the complex pore structures of metal-organic frameworks (MOFs). Two complementary simulation approaches were employed. First, we used GCMC simulations and obtained the hysteresis loops (if any) by performing the simulations sequentially “up and down” the isotherm, where the final configuration from a given fugacity is used as the initial configuration for the simulation at the next fugacity. In the second approach, we used canonical ensemble simulations at fixed loadings combined with Widom insertions to determine the fugacities. Note the complementary nature of the approaches: GCMC sets the fugacity and calculates the loading, while canonical Widom insertions set the loading and calculate the fugacity. We show that the canonical Widom approach allows us to access the full van der Waals (vdW) loop for the isotherm. GCMC simulations can generate hysteresis loops, but they may suffer from sampling inefficiencies: at high loading, GCMC insertion and deletion moves have low acceptance rates due to the high density in the pores. A canonical simulation avoids these inefficiencies by fixing the number of molecules in the simulation box. The Widom method allows us to access meta- and unstable thermodynamic states (loadings) and observe bubble formations that are impossible to capture using GCMC. Canonical isotherms also allow us to interpret and explain hysteresis loops and step changes given by GCMC simulations. Using these methods, we were able to characterize and analyze complicated phase behaviors of alkanes in complex MOF structures.