Cellulose nanofibrils (CNFs) have a high surface area and high mechanical properties, which make them attractive for a wide range of applications. However, in many cases, the surface modification of hydroxyl groups on CNFs is necessary to achieve good compatibility with the polymer matrix or other functional materials. Herein, we compare and contrast the reaction of CNFs with hexamethyldisilazane (HMDZ) and dimethoxydimethylsilane (DMDMS) carried out in the gas phase and in supercritical CO2 (sc-CO2). For CNF films dried from aqueous suspensions reacted with HMDZ, IR spectroscopic studies show a 450 times higher degree of substitution (DS) value of 1.4 using sc-CO2 compared to the gas-phase reaction and a DS value that is similar to those obtained for the same reaction conducted in liquid ammonia or ionic liquids. The gas-phase reaction occurs primarily with the outer surface of the CNF film, whereas in sc-CO2, X-ray diffraction (XRD) studies show that the HMDZ penetrates both the crystalline and amorphous regions of the fiber network. In contrast, the aggregated state of the CNFs is important in determining the DS for reactions with HMDZ in the gas phase, as the DS in ethanol- and acetone-exchanged dried films increased by at least 16 and 32 times, respectively, compared to CNF films dried from aqueous suspension. This increment can be attributed to the lower aggregation of fibers in acetone compared with ethanol-exchanged CNFs. In contrast, DMDMS reacts with the adsorbed water on CNFs to form a two-dimensional (2D) polymerized layer on the surface, and controlling the level of adsorbed water on the surface can, in turn, be used to tune the level of 2D polymerization of the alkoxysilanes with the CNFs. These findings suggest that the potential of sc-CO2 as a green reaction medium for high functionalization of CNFs reduces conventional solvent-based processes.
Senevirathne et al. (2026) studied this question.