Structure and Dynamics of pH-Responsive Nanoparticle Assembly at Oil-Water Interfaces

Responsive core-shell particles can vary their adsorption behavior and assembly structure at fluid interfaces in response to changes in environmental conditions, which benefits many applications including petroleum processes and drug delivery. Using a dissipative particle dynamics model with long-range electrostatic forces included, we discover the microstructure and dynamics of polyelectrolyte-grafted nanoparticle monolayers formed at a planar oil-water interface. The in-plane structure of the monolayer is analyzed by the structure factor, Voronoi diagram, and bond orientational order parameter. It shows a disorder-to-order phase change induced by the electrostatic forces as the degree of ionization of grafted polyelectrolytes increase. The dynamics of the monolayer is quantified by velocity autocorrelation function and the collective behavior is examined by mean orientation of particle velocity vectors. Due to randomly grafted chains, the particles do not experience isotropic electrostatic forces and defects are observed accordingly. We thus vary the polymer grafting density to probe the effect of particle anisotropy on the monolayer structure. Our findings can provide insight into controlling the structure and stability of PGNP monolayer by tuning the pH of the solution.