Effect of nanoparticle surface properties on adsorption, structure, and rheology at fluid interfaces

Abstract

Nanoparticles such as cellulose nanocrystals are increasingly employed for the Pickering stabilization of foams and emulsions. The particles' surface properties have a substantial effect on their adsorption, structure, and rheology at fluid-fluid interfaces. Here, we review the interfacial behaviour of cellulose nanocrystals (CNCs) with various surface properties. Native CNCs are primarily hydrophilic, negatively charged, and anisotropic nanoparticles, while chemically methylated or esterified CNC with different degrees of C16 fatty acids are yielding in total three different CNC surface chemistries. Adsorption kinetics and interfacial structure, i.e. coverage and contact angle, were determined and complemented with interfacial shear and dilatational rheology to establish a structure-rheology relationship for interfacial nanoparticle layers. Native unmodified CNCs showed negligible viscoelasticity owed to their low surface coverage, however, their viscoelasticity could be increased by charge screening due to a shift from repulsive to attractive CNC interactions. Methylated CNCs formed dense monolayers with higher dynamic moduli compared to native CNCs and could be thermo-gelled into multilayers with even higher moduli at 37°C. The esterified CNCs adsorbed at a contact angle higher than 90° and formed aggregated clusters at the interface, resulting in a Maxwellian frequency behavior with distinctive relaxation times, a rarely observed phenomenon for interfacial layers.