Gas-phase esterification of cellulose nanocrystal aerogels for colloidal dispersion in apolar solvents†
With the goal of an efficient and selective surface hydrophobization for subsequent colloidal dispersion into non-polar media, cellulose nanocrystals (CNCs) were transformed into aerogels of high surface area, prepared by freeze-drying dilute CNC suspensions in mixtures of water and t-BuOH of various compositions. In the best case, namely with dispersions in a medium consisting of 20 parts of water and 80 parts of t-BuOH (w/w), freeze-dried aerogels having specific areas as high as 160 m2 g−1 and a widely open texture were obtained. These aerogels were then subjected to a gas-phase esterification process resulting from the action of palmitoyl chloride vapors. By varying the process parameters, palmitoylated CNCs presenting degrees of substitution (DS) ranging from 0.1 to nearly 2 – deduced from solid state 13C NMR analysis – could be obtained. Of particular interest, the palmitoylated CNCs having a DS in the range of 0.3/0.8 still kept their native crystalline core, but their surface was hydrophobized. These characteristics allowed them to be readily dispersible into hydrophobic non-polar liquids, such as toluene, to yield non-flocculating suspensions showing transient birefringence under stirring. For higher DS values, the palmitoylation progress had the effect of diminishing the lateral size of the CNC crystalline cores, while adding at their remaining surface shells of highly modified cellulose chains easily soluble or swollen in apolar solvents. The resulting products thus yielded viscous isotropic media no longer birefringent under stirring when dispersed into toluene. Comparative classical derivatization experiments with palmitoyl chloride, performed with CNCs suspended in DMF, gave similar results. Thus, the CNCs in the optimized aerogels showed accessibility and reactivity comparable to those of CNCs dispersed in appropriate liquids. The gas-phase derivatization of nanocellulose aerogels appears therefore as an attractive and efficient process for preparing surface treated nanocelluloses.