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Breathing Solids: from Human Hair to Designer Nanoporous Materials

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Phenomenon of adsorption-induced deformation attracted recently a considerable attention owing to its relevance to practical problems of mechanical stability and integrity of novel nanoporous materials and their adsorption properties. Guest molecules adsorbed in nanopores cause a substantial stress in the host matrix leading to its swelling or contraction depending on the specifics of host-guest interactions. Although various experimental manifestations of adsorption-induced deformation have been known for a long time since Leonardo da Vinci’s studies of water sorption on human hair, a rigorous theoretical description of this phenomenon is lacking. I will present a general thermodynamic approach to predicting adsorption stress and respective deformation in various microporous and mesoporous materials based on molecular models of adsorption within elastic nanoscale confinements. Examples include carbons, zeolites, mesoporous crystals, metal-organic frameworks, and bio-inspired hierarchical nanoporous solids. A special attention will be paid to the enigmatic phenomenon of breathing of metal-organic frameworks during gas adsorption-desorption cycles illustrated in the figure. Upon gas consumption, the crystal first contacts exhibiting a counter-intuitive abnormal “inhaling” transition from large pore (lp) phase to narrow pore (np) phase, and then expands back to lp phase that is associated with normal “inhaling”. On the desorption pass, the reverse normal lp-np and abnormal np-lp “exhaling” transitions take place with a prominent hysteresis. Breathing solids are currently widely explored as actuators, sensors, selective adsorbents, and drug delivery vehicles. Literature: 1. P.I. Ravikovitch and A. V. Neimark, – Density Functional Theory Model of Adsorption Deformation, Langmuir, 2006, 22, 11171. 2. A.V. Neimark, F.-X. Coudert, A. Boutin, A.H. Fuchs, Stress-based Model for the Breathing of Metal–Organic Frameworks, JPC Letters, 2010, 1, 445. 3. C. Triguero, F.-X. Coudert, A. Boutin, A.H. Fuchs, A.V. Neimark, Mechanism of Breathing Transitions in Metal-Organic Frameworks, JPC Letters, 2011, 2, 2033. 4. G.Yu. Gor and A.V. Neimark, – Adsorption-Induced Deformation of Mesoporous Solids: Macroscopic Approach and Density Functional Theory, Langmuir, 2011, 27, 6926. 5. A.V. Neimark, F.X. Coudert, C. Triguero, A. Boutin, A.H. Fuchs, I. Beurroies, and R. Denoyel, – Structural Transitions in MIL -53 (Cr): View from Outside and Inside, Langmuir, 2011, 27, 4734. 6. F.-X. Coudert, A H . Fuchs, A. Boutin, and A.V. Neimark, Adsorption Deformation and Structural Transitions in Metal-Organic Frameworks: From the Unit Cell to the Crystal, JPC Letters, 2013, 4, 3198. 7. F.-X. Coudert, A.H. Fuchs, and A.V. Neimark, Adsorption Deformation of Microporous Composites, Dalton Transactions, 2016, 45, 4136. 8. C. Balzer, R. Cimino, G Y . Gor, A.V. Neimark, and G. Reichenauer, Deformation of microporous carbons during N2, Ar, and CO2 adsorption: Insight from the density functional theory, Langmuir, 2016, 2, 8265. 9. C. Balzer, A.M. Waag, S. Gehret, G. Reichenauer, R. Morak, L. Ludescher, F. Putz, N. Hüsing, O. Paris, N. Bernstein, G.Y. Gor, and A.V. Neimark, Adsorption-Induced Deformation of Hierarchically Structured Mesoporous Silica – Effect of Pore-Level Anisotropy, Langmuir, 2017, V.33, p.5592-5602.

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