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.