Modulating the zeta potential of cellulose nanocrystals using salts and surfactants

The zeta potential of cellulose nanocrystal (CNC) aqueous dispersions was studied as a function of solution conditions, including changing pH and different electrolyte identities and concentrations. A range of electrolytes that spans typical Hofmeister/hydrophobic effects was explored, along with both cationic and anionic surfactants. A subtle interplay of electrostatic and hydrophobic effects in ion adsorption was uncovered, including evidence of charge reversal and supercharging when hydrophobic surfactants are added to aqueous CNC dispersions. The apparent effects of zeta potential on dispersion stability were explored by using atomic force microscopy (AFM) to determine the roughness of resulting CNC films. The root mean square roughness (RMS) of these cellulose films was unaffected by the presence of surfactants (achieving a constant value of ∼9 nm), but scaled inversely and non-linearly with the zeta potential of the CNC suspension while using the ionic salts from ∼2 nm to 10 nm, indicating a facile method for the control of cellulose film roughness.