Surface modification of cellulose nanocrystals with cetyltrimethylammonium bromide

Cellulose nanocrystals (CNCs) prepared by sulfuric acid hydrolysis of cotton were surface modified with cetyltrimethylammonium bromide (CTAB). Essentially, the counterions of the CNC surface sulfate ester groups are exchanged for cetyltrimethyl-ammonium (CTA+), which acts as a bulky, amphiphilic cation. The CTAB-modified CNCs were thoroughly purified to remove surfactant that was non-electrostatically bound. The surface modification could be tailored from 50 to 100% charge coupling efficiency by varying the reaction conditions. The main factor that influenced coupling efficiency was ionic strength; increasing the ionic strength screened electrostatic interactions, which led to decreased surfactant adsorption. Adsorption isotherms of CTAB on model CNC films, measured by surface plasmon resonance spectroscopy, indicated an increase in adsorbed surfactant amount with increasing bulk CTAB concentration without achieving saturation in the concentration range studied. CTAB-modified CNCs were unstable in water but formed stable colloidal suspensions in ethanol, which transitioned into a continuous gel-like chiral nematic liquid crystal at relatively low concentrations (~4 wt. %) but did not phase separate into isotropic and anisotropic phases. The particle size and morphology of the CTAB-modified CNCs were unchanged compared to the native CNCs but were more thermally stable and less hydrophilic after the surface modification reaction.