Integration of Mesopores into Paper Materials Using Sol-Gel Chemistry

Cellulose based materials are beneficial economically and ecologically due to their renewability, recyclability and biodegradability. The hierarchical porosity of cellulosic fibers in paper is a prerequisite and key factor to optimize application potential e.g. in the field of lab-on-chip-technology, sensors, point of care diagnostics, etc. The reason for this impact of fiber structure is its influence on capillary fluid imbibition, wettability, or mechanical stability of paper. Upon paper production, porosity can be drastically reduced, especially on the mesoscale. Attempts to reintroduce porosity into paper often come with the use of environmentally problematic chemicals or great technical effort. Here, we present a method to modify cellulose fiber and thus paper porosity by coating with (mesoporous) silica using sol-gel chemistry and evaporation induced self-assembly (EISA) combined with dip-coating. Applying different sol-gel solution compositions using two different types of cellulose fibers, namely cotton linter and eucalyptus fibers, not only mesoporosity was designed into paper but systematic insights into the mechanism of pore formation and fluid flow control were obtained. The modified paper sheets were characterized with respect to their porosity, silica distribution and fluid imbibition using small-angle x-ray scattering (SAXS), argon and krypton gas sorption, and confocal laser scanning microscopy (CLSM) to localize the silica coating relatively to the fiber structure within the paper sheet. The combined characterization techniques reveal that successful mesopore formation within paper has been achieved, that specific surface area can be adjusted by sol-gel chemistry and dip-coating which allows to adjust fluid transport velocity in paper. Interestingly, mesopore formation thereby relies on a synchronization of solvent evaporation rate, critical micellar concentration and paper-intrinsic capillary fluid imbibition velocity. The simple process allowing tuning of fluid imbibition and designing nanoscale pore space for application engineering in paper together with the fundamental mechanistic understanding is expected to impact the design of future hybrid materials for new sensing or separation devices.