Surfactant Driven Liquid to Soft Solid Transition of Cellulose Nanocrystal Suspensions

Cellulose nanocrystals (CNCs) have recently attracted wide interest due to their abundance, biocompatibility, and extraordinary physical properties. In particular, easy manipulation of their surface properties, hydrophilicity, and high aspect ratio make them ideal rheology modifiers; yet, the gelation mechanisms and microscopic origin of the complex rheological behavior in the presence of secondary components, such as polymers and surfactants, are far from well understood. In this work, we used light scattering, small-angle neutron scattering, and bulk rheology to study the phase behavior and mechanical behavior of aqueous CNC solutions in the presence of cationic 1-decyl trimethyl imidazolium chloride and 1-decyl trimethyl imidazolium ferric tetrachloride. The micelles of these surfactants form at similar cmc's (about 50 mM) and adopt identical hydrodynamic sizes (on the order of a few nanometers) and prolate-shaped ellipsoids but vary in their intermicelle interactions (charged vs neutral), thus allowing us to clarify the unprecedented effect of the surfactant micelle charge on the gel behavior of the aqueous CNC-surfactant complexes. Our results show that the positively charged micelles greatly strengthen the gel network while excessive free micelles weaken the gels due to repulsive micelle-micelle interaction. In the meantime, analysis of the transition from linear to nonlinear deformation regimes suggests that the gels gradually become more fragile with surfactant concentrations due to electrostatic repulsion of the charged micelles. Such a surfactant concentration-dependent gel fragility was not observed in the presence of the neutral micelles. These results provide a great step further in our understanding of the phase behavior and rheology of complex CNC-surfactant mixtures and obtaining biocompatible hydrogels with tunable mechanical properties.