Ion Specific Effects on the Rheology of Cellulose Nanofibrils in the Presence of Salt

Cellulose nanofibrils (CNFs) are high-aspect-ratio semiflexible filaments that can modify the rheology of fluids in which they are suspended. This work addresses the role of ionic strength on the rheology of CNF suspensions and the ion-specific nature of such rheology. Salt-free CNF suspensions exhibit viscoelastic, shear-thinning behavior. The concentration dependences of the storage modulus and specific viscosity exhibit similar power-law relationships in two regimes, G'~η_sp~c^a, with exponents of a≈1 and a≈5 below and above, respectively, a critical concentration of roughly 0.5 wt.% that delineates “dilute” and “semi-dilute” characteristics. In the semi-dilute regime, salt addition increases the elastic modulus due to increased filament-filament association enabled by electrostatic screening of the repulsive interactions between weakly charged filaments. Notably, the modulus increases exponentially with salt concentration, or nominal ionic strength, I. In the dilute regime, the intrinsic viscosity decreases with ionic strength, reflecting the adoption of more compact conformations at the single-filament level due to screened electrostatics. At a fixed ionic strength, both storage modulus and intrinsic viscosity show a marked dependence on ion identity, for which ion hydration enthalpy, 〖∆H〗_h, is used as a proxy. The intrinsic viscosity increases with ∆H, while the storage modulus decreases with an exponential dependence. Notably, the orderings of both parameters mimic the ion sequence of the Hofmeister series. This highlights a strong correlation between the ability of different ions to modify the hydrogen-bonding-network structure of water and their ability to screen inter- and intra-filament electrostatic interactions. The exponential dependences of modulus on salt concentration and hydration enthalpy suggest that the nominal ionic strength I can be corrected using 〖∆H〗_h yield an effective ionic strength, I_eff, that captures the role of salt addition agnostic of ion identity. This work provides new insight regarding ion-specific effects in CNF suspension rheology that can be used to rationally modify the properties of CNF-based complex fluids.