Ultrafine and Fairly Long Carboxylated Cellulose Nanofibers Prepared by a Hydrated Multi-carboxylic Acid Deep Eutectic Solvent

Abstract Sustainable carboxylated cellulose nanofibers (C-CNFs) with access to variable surface chemical modifications and exquisite mechanical properties have great and promising applicability across various fields. However, the current fabrication processes either require harsh reaction conditions or severely impair the high aspect ratio of products, resulting in low yields, environmental issues and/or limited uses. Herein, we address these limitations by using a hydrated multi-carboxylic acid deep eutectic solvent comprised of only choline chloride, citric acid, and water to produce ultrafine and fairly long C-CNFs. The resultant C-CNFs have fine diameters of ~ 3 nm, high aspect ratios up to 4000, high carboxyl content of 1.5 mmol/g, and high yield of 90%. Notably, we can easily achieve large-scale production of C-CNFs for preparing large area, high-performance structural materials. Superior stability of the C-CNFs suspensions even at high concentrations allows the fabrication of 4 m-long flexible C-CNF films showing excellent mechanical strength, great optical properties (92% transparency and 90% haze) and high thermal stability ( T max =348 ℃) as well as of large area C-CNF aerogels with ultralow bulk density of 0.002 g/cm 3 , ultrahigh porosity of 99.86% and super low thermal conductivity of 0.0289 W/mK. These unique advantages as high yield, great cost-efficiency, scalable productivity and recyclability of eutectic solvents open up a new route to producing ultrafine C-CNFs on an industrial scale.