Hydrophobization of Nanocellulose Aerogels.

Cellulose aerogels are biocompatible and biodegradable, but their extreme sensitivity to water limits their real-world applications. Effective hydrophobization typically requires high hydrophobe loadings, particularly for high surface area, mesoporous aerogels. Here, we react acid chlorides with surface hydroxy groups of cellulose nanofibril (CNF) gels to graft long-chain esters (C6/C12/C18). Apolar and/or aprotic solvents like heptane and toluene are preferred. FTIR and solid-state NMR confirm the grafting of ∼0.42/0.29/0.22 esters per d-glucopyranose unit for C6/C12/C18, respectively, corresponding to ∼19/21/23 wt % hydrophobe. The water contact angle increases from <20° to 125°, with a 350% reduction in humidity uptake and a 90% reduction in humidity-induced shrinkage following esterification. The hydrophobization has no direct effect on thermal conductivity but enables the aerogels to maintain their insulating properties after prolonged humidity exposure. A hydrophobic and superinsulating (18 mW·m-1·K-1) cellulose aerogel was produced by combining esterification and uniaxial compression. The protocol was extended to chitosan and alginate aerogels.