Structural transformations of methane adsorbed in MOF-5 model framework
Résumé
Phase changes are ubiquitous in nature, and transformations between solid structures represent a large group of them. They are usually reported on phase diagrams, as coexistence lines between different structures. The phase diagrams, well established for macroscopic 3-dimensional (3D) materials, change drastically when the material’s dimensions are reduced to few nanometers, or if the material is confined in a nano-pore. At the nanoscale the positions of coexistence lines on the phase diagrams are shifted and their new locations depend mainly on the size and shape of the nano-confinement, the structure of the confining walls, and their interaction with the confined substance. Here we show that it is possible to induce structural transformations in a confined system by simply varying the number of molecules adsorbed in the pore. We found that the mechanism of these novel, adsorption-induced structural transformation in nano-pores differs from that of well-known capillary condensation. The confined, equilibrium structures are not characterized by mean positions of molecules but rather by a probability distribution of molecular positions around adsorption centres. The character of transformation depends on temperature: it is strongly discontinuous at low temperature but evolves into a continuous transition when the temperature increases.