Self-Assembly and Aggregation of Nanocelluloses into Functional 3D Soft Materials and at 2D Fluid Interfaces

Nanocelluloses (NCs) have emerged throughout soft materials sciences as a green building block for the bottom-up design of functional materials. This thesis addresses the self-assembly and aggregation of NCs into (I) 3D soft materials and (II) at 2D fluid interfaces to foster the understanding of NC colloidal behavior and facilitate this bottom-up approach. In part (I) optical, mechanical, and structural analyses are combined to map the phase behavior of cellulose nanocrystal (CNC) aqueous dispersions. CNCs undergo a transition from isotropic dispersion to cholesteric liquid crystals at increasing volume fraction. Ion-induced charge screening initially decreases the cholesteric volume fraction, but induces the arrest of CNCs into attractive glasses beyond a critical ionic strength. CNC attractive glasses have received little attention, but may be of great interest as they exhibit gel-like mechanical properties while remaining optically active owed to entrapped nematic domains. At higher ionic strength CNCs aggregate into hydrogels. Using four salts with different size and valency, it was confirmed that gelation derives entirely from CNC attractive forces, refuting previous speculations of ionic bridging. Ultimately, the hydrogels were demonstrated to qualify as injectable biocompatible drug carriers for sustained and pH-responsive drug delivery. NCs have been widely used for the stabilization of biocompatible Pickering emulsions in the past decade, however, their behavior at fluid interfaces remained poorly understood. In part (II), the first ever protocol for the formation of NC interfacial films is presented. This provided insights into NC adsorption kinetics, isotherms, and energy by dynamic surface tension measurements, as well as enabling the analysis of NC interfacial structure and coverage by reflectivity and film displacement techniques. NC adsorption is limited by its surface charge, and is promoted at lower charge density or by salt-induced charge screening. NCs orient laterally in the interfacial plane and form discontinuous monolayers. The inability of NCs to stabilize foams, an ongoing enigma in the field, could be attributed to an energetically more favorable contact angle at oil-water compared to air-water interfaces. Adsorption experiments at different oils revealed that NC adsorption is less stable at oils with increasing polarity, which in turn impedes emulsion stability. These insights provide the long-missed fundamental understanding of NC adsorption at fluid interfaces, and will facilitate the formulation of NC Pickering emulsions in the future