  Hydrophobic nanoporous materials can be intruded by water only by exerting an
external action, typically increasing pressure. For some materials, water
extrudes when the pressure is lowered again. Controlling intrusion/extrusion
hysteresis is central in a number of technological applications, including
materials for energy applications and for high performance liquid
chromatography, and experimental techniques, as liquid porosimetry, but is
still far from being understood. In this work, we consider water intrusion and
extrusion in common mesoporous materials grafted with hydrophobic chains,
showing that the macroscopic properties of the system are significantly
affected by subnanometric heterogeneities in the grafting. For example,
intrusion and extrusion pressures can vary more than 20 MPa depending on the
chain length and density of the grafting. Coarse-grained molecular dynamics
simulations reveal that local changes of radius and contact angle produced by
grafting heterogeneities can pin the interface during intrusion or facilitate
bubble nucleation in extrusion. These unprecedented microscopic insights can
directly impact the design of energy materials and chromatography columns, as
well as the interpretation of porosimetry results.
