Interfacial behavior of core-shell composite nanoparticles under compression and shear: Influence of polymer shell thickness.

Hypothesis: The mobility of core-shell nanoparticles partitioned at an air-water interface is strongly governed by the compliance of the polymer shell.

Experiments: The compressional, relaxation and shear responses of two polymer-coated silica nanoparticles (CPs) were studied using a Langmuir trough and needle interfacial shear rheometer, and the corresponding structures of the particle-laden interfaces were visualized using Brewster angle and scanning electron microscopy.

Findings: The mobility of CPs partitioned at an air-water interface correlates to the polymer MW. In compression, the CPs40-laden interface (silica nanoparticles coated with 40 kDa PVP) showed distinct gas-liquid-solid phase transitions and when the surface pressure was reduced, the compressed particle-laden interface relaxed to its original state. The compressed-state of the CPs8-laden interface did not relax, and wrinkles in the particle-laden film that had formed in compression remained due to greater adhesion between the compressed particles. The increased mobility of the CPs40-laden interface translated to lower surface shear moduli, with the viscoelastic moduli an order of magnitude or more lower in the CPs40-laden interface than the CPs8-laden interface. Ultimately this contributed to changing the stability of particle-stabilized foams, with less mobile interfaces providing improved foam stability.