Effect of Ionic Surfactants on the Viscoelastic Properties of Chiral Nematic Cellulose Nanocrystal Suspensions

The gelation of cellulose nanocrystal (CNC) suspensions is explored in the presence of two ionic surfactants, namely, sodium dodecyl sulfate (SDS) with a negatively charged head and cetyltrimethylammonium bromide (CTAB) with a positively charged head. The viscosity profile of pure CNC suspensions indicated that at concentrations greater than 5 wt %, they exhibit gel-like behavior, where the viscosity shows a single shear-thinning profile. However, at concentrations lower than 1 wt %, the suspension shows isotropic behavior and experiences a transition to chiral nematic biphasic domains at increasing CNC concentration. In addition, the effect of CTAB, SDS, and NaCl on the surface charge of CNCs is analyzed and coupled with rheological measurements in order to study the relative importance of surfactants and ionic strength on the viscoelastic properties of the CNC suspensions. The mechanism of CNC/surfactants and CNC/CNC interactions leading to the gelation of CNC suspensions is also investigated. It was found that the addition of both ionic surfactants results in the gelation of biphasic chiral nematic CNC suspensions by fostering cross-linking between individual CNCs at smaller concentrations. In the case of CTAB, a gradual and ongoing increase in the viscoelastic moduli was observed with the increase of surfactant concentration, implying the induction of dominant attractive forces between CNCs by surfactant molecules. However, the presence of SDS stimulates both attractive and repulsive forces whose relative dominance controls the viscoelastic properties and gelation.