Local grafting heterogeneities control water intrusion and extrusion in nanopores

Hydrophobic nanoporous materials can only be intruded by water forcibly, typically increasing pressure. For some materials, water extrudes when the pressure is lowered again. Controlling intrusion/extrusion hysteresis is central in technological applications, including energy materials, high performance liquid chromatography, and liquid porosimetry, but its molecular determinants are still elusive. Here, we consider water intrusion/extrusion in mesoporous materials grafted with hydrophobic chains, showing that intrusion/extrusion is ruled by microscopic heterogeneities in the grafting. For example, intrusion/extrusion pressures can vary more than 60 MPa depending on the chain length and grafting density. Coarse-grained molecular dynamics simulations reveal that local changes in radius and contact angle produced by grafting heterogeneities can pin the water interface during intrusion or facilitate vapor bubble nucleation in extrusion. These microscopic insights can directly impact the design of energy materials and chromatography columns, as well as the interpretation of porosimetry results.