The structural amphiphilicity of cellulose nanocrystals characterized from their cohesion parameters

Cellulose nanocrystals (CNCs), usually considered as isotropically polar nanoparticles, are sheet-like crystalline assemblies of cellulose chains. Here, we link the anisotropy of the CNC structure to an amphiphilic behavior in suspension. The Hansen solubility parameters (HSP: δ_D;δ_P;δ_H) of wood-based H₂SO₄-hydrolyzed CNCs were measured from sedimentation tests in a wide set of 59 solvents and binary mixtures. Two sets of cohesion parameters corresponding to a polar surface (18.1; 20.4; 15.3) ± (0.5; 0.5; 0.4) MPa^1/2 and to a mildly non-polar one (17.4; 4.8; 6.5) ± (0.3; 0.5; 0.6) MPa^1/2 were determined, with respective solubility radii of 7.8 and 2.1 MPa^1/2. The polar sphere is thought to correspond to the (110) & (11¯0) surfaces of cellulose I_β nanocrystals, while the smaller non-polar sphere is coherent with the exposure of (200) surfaces. The HSP graph provides new insights on the amphiphilic nature of CNCs and a mapping of their chemical affinity for solvents and polymer matrices.