The Role of Liquid-like Surface-Adsorbed Water and Alkylsilanes in the Plasticity of Cellulose

Alkylsilane surface-treated paper is widely used in microfluidic devices and paper-based analytical devices (μ-PAD) for point-of-care diagnostics or paper-based electronic device (EPED) implants. These devices generally utilize surface modification through silane reactions to maintain structural stability in the presence of water. However, the fundamental role of surface-adsorbed water and the effects of silane treatment on the mechanical properties of paper are not well understood. In this work, we explore the primary role of surface-adsorbed water and the role of surface energy mismatch between capillary water and cellulose fiber in the mechanical properties of the paper. Our findings demonstrate that the reduction in surface energy and consumption of interfiber capillary water lead to a loss of capillary forces and subsequent decay in the plasticity of paper. Based on these observations, we concluded that surface-adsorbed liquid-like capillary water plays a central role in the mechanical properties of paper. The effective capillary size in silane-treated cellulose fibers was estimated to be ∼31 nm, which is an order of magnitude larger than ice-like water (∼2 nm), validating the role of liquid-like water. Moreover, we established that the silanization treatments exclusively alter the surface properties without impacting the fiber bulk despite lowering the tensile strength by more than 50%. This work demonstrates a major caveat in using silane-cellulose reactions and serves as a guide in designing reactions for surface modification while maintaining the plasticity in cellulose-based materials.