Thermodynamic Interactions toward Nanocellulose Colloidal Properties

Abstract

In recent years, nanocellulose has emerged as a sustainable and environmentally friendly alternative to traditional petroleum-derived structural polymers. However, the widespread implementation of nanocellulose is hindered by the need for energy-intensive extraction and processing methods, limited quality reliability, and cost-effectiveness. In addition, the required fundamental understanding of process parameters that govern the morphology and structure-property relationships of nanocellulose systems, from colloidal suspensions to bulk materials, has not been developed and generalized for all forms of cellulose. This further hinders the more widespread adoption of this biopolymer in applications.This thesis aims to investigate how cellulose nanofibers (CNFs) disperse in solvents with different thermodynamic parameters. The objective is to highlight how thermodynamic interactions can be changed to tune the colloidal behavior of CNF dispersions. By adjusting the Hansen solubility parameters, the thermodynamic interaction between CNFs and solvents has been controlled. Here, we obtained CNFs from bacterial cultures to examine the hydrodynamic, electrokinetic, and thermodynamic effects that control the colloidal behavior of CNFs in different solvents. We varied the concentration of CNFs to explore dilute to semi-dilute regimes for each solvent. Specific viscosity was used to identify concentration-based transitions and compare the dispersions with different interaction parameters. This provided insights into how CNFs respond to different solvents. The excluded volume and viscosity were found to increase when there was a higher interaction between CNFs and solvents in dilute concentrations. This showcases the impact of the balance between CNF-CNF and CNF-solvent interactions on the colloidal properties. Deviations were noted in semi-dilute regimes due to weaker electrostatic stabilization and van der Waals attractions. These facilitate hydrocluster formation, which was confirmed via zeta potential measurements. Rheological characterization showed shear rate-based transitions, concentration effects on zero shear viscosity, stress overshoots, and viscoelasticity. This provides a foundation for phase behavior analysis and future implementation in structure-property relationships. Our findings provide a thermodynamic understanding of CNF colloidal properties with minimal surface charge density, that enable aggregation due to insufficient electrostatic repulsion, This work elucidates the intricate interplay of thermodynamic and electrokinetic interactions toward understanding and controlling the colloidal behavior of nanocellulose.