Imbibition and dewetting of silica colloidal crystals: An NMR relaxometry study

The wettability of the inner surfaces in a porous network is challenging to be accessed but essential to understand the complex performance of e.g. particulate systems.

Here we investigate the water behavior in the macroporous (50-100 nm) voids of dense particle packings by performing low-field ¹H NMR relaxometry in solid silica colloid crystals. The systems chosen guarantee a regular, known void size distribution with controllable water affinity (through thermal annealing), where the NMR experiment clearly discriminates the void water from micropore or bound water contributions, allowing separate monitoring.

Analysis of the saturated state indicates that the interparticle voids are completely filled after imbibition, even for less hydrophilic spheres. Due to the interaction with the silica surface, proton relaxation in void water is up to 100 times faster than that in bulk water, serving for assessment of the hydrophilicity within the sample. The relaxation time evolution upon dewetting provides an empirical measurement of the wettability inside the ensemble, revealing a progressively inhomogeneous wetting of increasingly hydrophobic surfaces. Our results provide insight into the imbibition state and dewetting performance in meso- and macroporous systems, with emphasis in the marked influence of the surface nature on the pore wetting distribution.