Adsorption-Induced Deformation of Flexible Nanoporous Materials: from Human Hair and Coal to Metal-Organic Frameworks and Zeolites - Alexander V. Neimark (Rutgers University, USA)

Guest molecules adsorbed in pores of a solid body exert substantial stress on the host matrix that causes its swelling or contraction depending on the nature of host-guest interactions. This phenomenon of adsorption-induced deformation has been known for a long time since Leonardo da Vinci’s studies of water sorption on human hair. Various manifestations of adsorption-induced deformation are currently actively explored with respect to the design of novel nanoporous adsorbents and membranes for hydrocarbon separation, actuators, nanobumpers, energy storage devices, as well as the enhancement of gas recovery and carbon dioxide sequestration in shales and coal mines. Despite of the evident importance of the deformation effects during adsorption-desorption cycles, a rigorous theoretical description of this phenomenon is lacking. A similar problem arises in case of intrusion-extrusion of nonwetting fluids, the process involving high applied pressures. 

I will present a general thermodynamic approach to predicting adsorption stress and respective deformation in nanoporous materials of different origin based on molecular models of adsorption. The theoretical modeling is complemented by direct molecular simulation of adsorption-induced deformation using hybrid Monte Carlo and molecular dynamics methods. The focus is on elaborating modeling techniques for quantitative predictions of adsorption deformation effects and comparison of the theoretical and molecular simulation results with experimental data. Examples of comparison of the theoretical predictions with experiments include metal-organic frameworks (MOFs), microporous carbons and zeolites, mesoporous crystals, coal, and hierarchical micro-mesoporous monoliths.

A special attention will be paid to the peculiar counterintuitive effects of non-monotonic deformation of a porous body upon fluid saturation with consequent stages of contraction and expansion. Contrary to the expected gradual swelling during adsorption, adsorbent deformation is non-monotonic, it is characterized by contraction at low loadings, followed by partial expansion upon the pore filling. Moreover, guest molecules may engender softening of the host structure to, in some cases, a nearly 100% increase in compressibility. Most prominent is a so-called "breathing" behavior of MOFs during gas adsorption-desorption cycles.

Contact: Benoit Coasne