Confined water cluster formation in water harvesting by metal organic frameworks: CAU-10-H versus CAU-10-CH 3 .

Several metal-organic frameworks excel in harvesting water from the air or as heat pumps as they show a steep increase in water uptake at 10-30 RH%. A precise understanding of which structural characteristics govern such behaviour is lacking. Herein we studied CAU-10-H and CAU-10-CH 3 with -H, -CH 3 corresponding to the functions grafted to the organic linker. CAU-10-H shows a steep water uptake around 18 RH% making it interesting for water harvesting. Yet the subtle replacement of -H by -CH 3 in the organic linker drastically changes the water adsorption behaviour to water uptake at much higher humidity values, that is also less steep. We unravelled the materials' structural deformation and water ordering during adsorption with in situ sum-frequency generation, in situ X-ray diffraction, and molecular simulations. In CAU-10-H, an energetically favourable water cluster is formed in the hydrophobic pore, tethered via H-bonds to the framework μ-OH groups, while for CAU-10-CH 3 , such a favourable cluster cannot form. By relating our findings to the features of water adsorption isotherms of a series of MOFs we conclude that favourable water adsorption occurs when sites of intermediate hydrophilicity are present in an hydrophobic structure, and the formation of energetically favourable water clusters is possible. This article is protected by copyright. All rights reserved.