Zeta Potential Time Dependence Reveals the Swelling Dynamics of Wood Cellulose Nanofibrils

In this paper, we present the swelling dynamics of individual wood cellulose nanofibrils (CNFs) following solvent substitution into various organic solvents and drying, by employing the time dependence of the zeta potential (ζ). We succeeded in smoothly redispersing the coaggregating CNFs dried in solvents, including acetone, acetonitrile, DMSO, ethanol, and t-butanol into water. ζ-t plots of the redispersed CNFs measured in a 1 mM KCl solution indicated different values of Δζ (volume fraction of hydration capacity), corresponding to the dielectric constant of the substituted solvents. Differential scanning calorimetry confirmed that the redispersed CNFs swell to different degrees, corresponding to Δζ. This swelling behavior is characterized by expansion of hemicelluloses, the amorphous polysaccharides located on the CNF surface, with a different degree of aggregation during drying. The specific swelling ratio, radius, and diameter of the CNFs in water were calculated using the values of ζ(0) and ζ(∞) by introducing surface chemical analysis. The calculated diameters of the CNFs at t = 0 coincided well with the median diameters measured directly by transmission electron microscope. Swellability of hemicelluloses exponentially increased with the decrease in dielectric constant of solvent during drying. The analysis method combining zeta potential time dependence and a surface chemical approach proved useful for specifically evaluating the swelling dynamics of polymers on a bulk surface.