Abstract: The structure of fluids confined in nanopores can change significantly as the number of adsorbed molecules increases. These adsorption-induced transformations may differ fundamentally from conventional capillary condensation, raising important questions about their microscopic mechanisms and thermodynamic behavior [1,2].

In this presentation, I will discuss how the competition between fluid–framework and fluid–fluid interactions, combined with the geometry and symmetry of adsorption sites, determines the structure of adsorbed molecular phases. Molecular simulations reveal that these structures are often highly dynamic and are better described by probability distributions of molecular positions than by static configurations. As pore filling increases, the redistribution of adsorbed molecules can produce distinct structural states, including metastable configurations and discontinuous transitions.

We investigate these phenomena using molecular simulations of CO₂ and CH₄ adsorption in a series of crystalline metal–organic frameworks (IRMOF-1, IRMOF-8, IRMOF-10, and IRMOF-14). The results demonstrate that the nature of adsorption-induced transformations depends strongly on temperature, pore geometry, molecular size, and the relative strength of intermolecular interactions. Discontinuous transitions observed at low temperatures may become continuous as temperature increases. Under certain conditions, additional intermediate states can emerge when attractive interactions are sufficiently strong. The occurrence and stability of metastable states are particularly sensitive to thermodynamic conditions near the adsorbate's triple point [3,4].

These findings provide a molecular-level understanding of structural transformations and metastability in confined fluids. They also demonstrate why identifying multiple adsorption states and their statistical distributions is essential for a reliable thermodynamic description of adsorption in ordered nanoporous materials.

References

1. Mazur, B. et al. J. Phys. Chem. Lett. 2022, 13, 6961–6965.
2. Kuchta, B. et al. Angew. Chem. 2017, 129, 16461.
3. Dorhauer, A. et al. ACS Omega 2025, 10, 63044–63050.
4. Stankiewicz, M. et al. ACS Nano 2026, 20, 4204–4216.