Surface-restricted modification of nanocellulose aerogels in gas-phase esterification by di-functional fatty acid reagents

The gas-phase esterification of nanocellulose aerogels was implemented, using several fatty acid derivatives varying (1) the size of their aliphatic tails, (2) their reactive groups, namely acyl chloride or anhydride, (3) their mono or bi-functionality. The process parameters were adapted to the vaporization properties of each reagent in order to obtain efficient reagent transfer to the nanocellulose surface, as measured from the amount of deposited reagents. For each of these, the esterification extent was deduced from gravimetric measurements and spectroscopic analysis (FTIR, ¹³C CP/MAS NMR). The esterification with the mono-functional fatty acid derivatives progressed, as demonstrated earlier, from the surface of the substrates to their core. Strikingly, when bi-functional esterifying reagents such as cyclic anhydride and diacyl chloride were used, the derivatization was strictly limited to the nanocellulose surface, as evidenced from the correlation between the degree of substitution and the specific surface of the aerogel. The green gas-phase derivatization scheme reported here is thus a versatile process that can be easily adapted to many esterifying reagents, leading to interesting possibilities for selective surface topochemistry of the substrates.