The structure and dynamics of a network formed by rod-shaped particles can indirectly be investigated by measuring the diffusion of spherical tracer particles. In this work, this approach has been used to characterize cellulose nanofibrils (CNFs) in both dispersed and arrested states, using tracer latex particles 20-500 nm in size. There is a transition at a CNF concentration of around 0.2 wt%, below which the CNFs are highly dynamic and have little effect on the tracer particle diffusion. At higher CNF concentrations, the diffusion of small particles (20-40 nm) follows the diffusion behavior predicted in static networks. This strongly indicates that a more rigid network has been formed with an effective mesh size in the same size range as the particles. Large tracer particles (100-500 nm) become trapped in this network due to the large number of contact points with the CNFs. Coarse-grained Brownian dynamics simulations were also performed where the influence of the effective particle size on the particle movements in the CNF networks was determined, taking the change in the Debye length with the CNF concentration into consideration. These simulations show a good agreement with the experimental results indicating that the smaller particles have sparse contacts with the fibrils whereas the larger particles are more or less constantly in contact with fibrils.<br/>Another set of experiments show that the sedimentation of silica microparticles is significantly affected by the presence of CNF networks. At CNF concentrations of 0.2 wt%, the sedimentation velocity of 5 μm particles was reduced by 99 % compared to that in pure water. The sedimentation was completely hindered when HCl was added to the CNF dispersion, which caused a transition to a colloidal gel. A CNF colloidal gel at a concentration of 0.05 wt% can prevent the sedimentation of 2 μm particles for several weeks. The results hence show a new mechanism for stabilization of colloidal dispersions using CNF of low particle concentrations, even below the concentration at which the CNF dynamics are arrested and a colloidal glass is formed.

Establishing the dynamic structure of cellulose nanofibril networks in dispersion and its unique stabilizing action on colloidal dispersions

ACS Spring 2022

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