Adsorption of Charged anisotropic nanoparticles at A/W and O/W Interface

Abstract

Cellulose Nanocrystals (CNC) are renewable, anisotropic nanoparticles gained from wood pulp. CNC allows for Pickering stabilization of fluid interfaces, as well as resulting foams and emulsions. Despite successful application, the underlying mechanisms of CNC adsorption and stabilization are matter of current debate. We present our recent advances on the adsorption and rheology of CNC at air/water and oil/water interfaces. CNC adsorption occurs at the time-scale of hours and induces a decrease in surface tension. Herein, we present parameters for the adsorption of CNC at the A/W interface. Initial CNC adsorption is limited by diffusion, followed by monolayer saturation and decrease in surface tension, indicating a Pickering stabilization. The adsorption of CNC may be accelerated by salt-induced charge screening. Neutron reflectometry revealed that CNC form a discontinuous monolayer with crystallites oriented in the interfacial plane. Besides numerous reports on CNC Pickering emulsions, the adsorption and stabilization of O/W interfaces by CNC remains mostly unknown. We investigated the adsorption of CNC at oils with varying polarity and chemical structure. CNC adsorption was found to be independent of the chain length of aliphatic n-alkanes, but strongly dependent on oil polarity. Surface pressures decreased for more polar oils due to lower particle adsorption energies. No surface pressure was measurable and the O/W emulsification capacity ceased for the most polar octanol, suggesting limited CNC adsorption. Further, salt-induced charge screening enhanced CNC adsorption and surface coverage due to lower interparticle and particle-interface electrostatic repulsion. An empiric power law is presented which predicts the induced surface pressure of charged nanoparticles based on the specific oil-water interface tension. These findings underline the prediction of nanoparticle-oil interactions and choice of the right oil-particle combination for application targeted properties.