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

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