Two-Step Wetting of Nanoporous Carbons: Small-Angle\nScattering Analysis of Capillary Rise

Numerous\napplications of nanoporous materials require their pores\nto be filled with liquids. In spite of its huge technological importance,\nthe conditions for the wetting of nanometer-sized pores and its phenomenology\nare still poorly understood. We report on capillary rise experiments\nwith water in carbon xerogels, with synchrotron small-angle scattering\nused to follow the process in situ at the nanometer scale. The data\nreveal a two-step wetting process whereby water permeates first into\nmolecular-sized micropores, which is followed by the imbibition of\nlarger mesopores. A Cassie–Baxter analysis shows that the presence\nof water in the micropores is central, as it turns the mesopores from\nbeing hydrophobic to hydrophilic. Based on the so-calculated contact\nangles, the mesopore wetting kinetics are found to be quantitatively\ndescribed by a classical Washburn model. Modeling of the experimental\nwater profile ahead of the Washburn front reveals strong surface barriers\nopposing water transfer from the mesopores to the micropores.