Local Grafting Heterogeneities Control Water Intrusion and Extrusion in Nanopores

Hydrophobic nanoporous materials can be intruded by water only by exerting anexternal action, typically increasing pressure. For some materials, waterextrudes when the pressure is lowered again. Controlling intrusion/extrusionhysteresis is central in a number of technological applications, includingmaterials for energy applications and for high performance liquidchromatography, and experimental techniques, as liquid porosimetry, but isstill far from being understood. In this work, we consider water intrusion andextrusion in common mesoporous materials grafted with hydrophobic chains,showing that the macroscopic properties of the system are significantlyaffected by subnanometric heterogeneities in the grafting. For example,intrusion and extrusion pressures can vary more than 20 MPa depending on thechain length and density of the grafting. Coarse-grained molecular dynamicssimulations reveal that local changes of radius and contact angle produced bygrafting heterogeneities can pin the interface during intrusion or facilitatebubble nucleation in extrusion. These unprecedented microscopic insights candirectly impact the design of energy materials and chromatography columns, aswell as the interpretation of porosimetry results.