Molecular Simulation-Driven Design of Functionalized Cellulose Nanocrystals toward Improved Dispersibility and Stabilization in Ethanol.

Cellulose nanocrystals (CNCs) are commonly produced with carboxylic acid surface functionalities, as it aids the dispersion in aqueous solvents. However, the carboxylic acid-functionalized CNCs (CNC-COOHs) exhibit poor dispersibility in nonaqueous solvents due to their self-aggregation, limiting their integration into many biosourced solvent-based technologies, such as inkjet printing, spin-coating, etc. Aimed at improving the dispersibility of CNCs in nonaqueous solvents, molecular dynamics simulations were performed to study the underlying reasons for the low dispersibility of CNC-COOHs in biosourced ethanol and investigate how surface functionalization impacts their self-aggregation tendency in this solvent. The potential of mean force (PMF) calculations revealed that the aggregation of CNC-COOHs through their hydrophilic surfaces drives their low dispersibility in ethanol and that the functionalization of CNC-COOHs with alkyl groups (CNC-COOH-alkyl, with alkyl being ethyl, butyl, hexyl, and octyl) reduces their aggregation tendency. The lower binding tendency of the alkylated CNCs than CNC-COOHs stems from a higher binding entropy loss and a lower attraction between their surfaces, as alkylation increases the distance between bound CNCs. The higher binding entropy loss of CNC-COOH-alkyls is attributed to alkyl groups' reduced degrees of freedom at the contact point between CNC-COOH-alkyls. As a result, the highest dispersibility can be achieved when the CNC-COOHs are functionalized with the longer alkyl groups, i.e., hexyl and octyl. PMF-derived binding free energy values predict the dispersibility trend as CNC-COOH-octyl ≈ CNC-COOH-hexyl > CNC-COOH-butyl > CNC-COOH-ethyl > CNC-COOH. Experimental tests for synthesized CNC-COOH-alkyls with ethyl, butyl, and hexyl groups confirmed simulation predictions, wherein the increasing size of the alkyl chain increased CNC dispersibility. CNC-COOH-hexyl dispersed well in ethanol and remained stable for 1 day. The findings of this research enhanced our understanding of how functionalization of the CNCs improves their stability in biosourced nonaqueous solvents such as ethanol and opens the avenue for their integration in biosourced solvent-based technologies.