Rice straw cellulose nanofibrils from the optimal 2,2,6,6-tetramethylpiperidine-1-oxyl
oxidation/blending process carrying 1.17 mmol/g surface carboxyls
were protonated to varying charged (COO–Na+) and uncharged (COOH) surfaces. Reducing the electrostatic repulsion
of surface charges by protonation with hydrochloric acid from 11 to
45 and 100% surface carboxylic acid most prominently reduced the aerogel
densities from 8.0 to 6.6 and 5.2 mg/cm3while increasing
the mostly open cell pore volumes from 125 to 152 and 196 mL/g. Irrespective
of charge levels, all aerogels were amphiphilic, super-absorptive,
stable at pH 2 for up to 30 days, and resilient for up to 10 repetitive
squeezing-absorption cycles. While these aerogels exhibited density-dependent
dry [11.3 to 1.5 kPa/(mg/cm3)] and reduced wet [3.3 to
1.4 kPa/(mg/cm3)] moduli, the absorption of organic liquids
stiffened the saturated aerogels. These data support protonation as
a critical yet simple approach toward precise control of aerogels’
dry and wet properties.