(174c) Adsorption Stress in Nanopores

Alexander V. Neimark

Department of Chemical and Biochemical Engineering,
Rutgers University, 98 Brett Road, Piscataway, New Jersey 08854-8058, USA

Email: aneimark@rutgers.edu; Web: http://sol.rutgers.edu/~aneimark/

Experimental demonstration of adsorption-induced deformation of microporous adsorbent has been known for a long time, starting from the pioneering work of Meehan in 1927 on charcoal swelling upon CO₂ adsorption. Later, it was shown with multiple examples that guest molecules adsorbed in pores exert a significant stress in the host solid matrix causing, depending on the conditions, not only swelling but also contraction, and sometimes matrix structural transformation and even collapse. With the advent of metal-organic frameworks, the adsorption deformation has attracted renewed attention due to the enigmatic phenomena of gate opening and breathing; the former discovered by Katsumi Kaneko in 2001. Adsorption deformation has important implications and potential for the development of high-performance adsorbents and drug delivery vehicles. It takes place in geological formations, like micro-mesoporous domains in coal and shale reservoirs, where the deformation induced by CO₂-methane displacement may cause significant reduction of permeability and even affect the mechanical integrity of reservoirs.

I will present a general thermodynamic approach to predicting adsorption stress and respective deformation in nanoporous materials based on the notion of the adsorption stress. The adsorption stress is defined as the derivative of the grand thermodynamic potential of the adsorbed phase with respect to the variation of the sample volume. It can be calculated based on adopted adsorbate-adsorbent models from the classical density functional theory or Monte Carlo atomistic simulations. The adsorption stress introduced into the linear elasticity theory provides a unified framework for extension of classical poromechanics to nanoporous materials. Special attention will be paid to the specifics of adsorption deformation in micropores of molecular dimensions and to the interpretation of the strain measurements in situ XRD and dilatometry experiments. I will show how the adsorption strain isotherms can be employed for evaluation of the micropore size distribution. Examples include micro- and mesoporous carbons and MOFs.