A guest article from the University of Calgary in Alberta (Canada)
Interfacial tension measurements are conventionally conducted to quantify the adsorption of surfactants and particles at liquid-fluid interfaces. Although these tests provide valuable information regarding the formation of emulsions, they do not reveal the structure of the generated particle-surfactant interfacial layer representing the stability of emulsions. We study the influence of cellulose nanocrystals (CNC) as a green and biocompatible source of nanoparticle dispersion on the oil-water interfacial viscoelasticity and consequently on the emulsion formation and stability. By comparing the viscoelasticity data with confocal and cryogenic scanning electron microscopic images of the oil-water interface, we found the high viscoelasticity modulus corresponds to interconnected structures of surface-activated CNC particles at the interface. Interfacial rheology measurements are conducted in oil-water systems where both fluids have low viscosity to minimize the effect of bulk viscous forces. Due to the wide applications of high viscosity oils in food, pharmaceutical, and oil industries, we develop an emulsification map and extend our findings to high viscosity oils, where the knowledge of interfacial viscoelasticity is required.
Cellulose nanocrystals (CNCs) are natural-based, rod-shaped, and highly crystalline colloidal particles extracted from wood, cotton, or other plant sources. CNCs have received lots of attention as emulsion stabilization agents. In CNC-stabilized emulsions, the adsorption of CNCs to the interface, their surface charge, and their considerably large size prevent the coalescence of droplets, resulting in high stability of the thus formed Pickering emulsions. The effect of electrostatic forces and steric repulsion on the CNC-stabilized emulsions have been extensively studied in the literature [1, 2]. However, the mechanical properties of CNCs at liquid-liquid interfaces have received less attention.
Interfacial viscoelasticity, representing the mechanical properties of the interface, may significantly influence the properties of liquid-liquid interfaces. The response of mobile interfaces under specific deformations is employed to characterize the strength of the interfacial film [3, 4].
Dilatational interfacial rheology sheds light on two interfacial properties, i.e., elasticity and viscosity, resulting from the change in the surface area. These properties have a different influence on emulsion formation and stability. The dependency of the interfacial tension (IFT) on the degree and speed of change in surface area are called surface elasticity and viscosity, respectively. Elastic (storage) and viscous (loss) modulus are obtained by changing the interface area sinusoidally and measuring the resulting IFT over time. The response of the interface is expressed in terms of summation of viscous and elastic contributions as