Electrostatic interactions to modulate the reflective assembly of nanoparticles at the oil–water interface

The driving force for the adsorption of nanoparticles (5–10 nm) at the oil–water interface can be small and is particularly sensitive to the surface chemistry of the particles. Here we show that the interfacial assembly of 5 and 10 nm diameter gold nanoparticles functionalized with stoichiometric ion-pairs is reversible and can be tuned with the solution pH. We also show that at the interface the nanoparticles form a reflective layer with a mirror-like reflectance. Using titration we demonstrate that the mechanism for particle desorption from the interface is the electrostatic repulsion between the nanoparticles, likely due to pH-dependent adsorption of hydroxide ions. By controlling electrostatic repulsion we can control both the extent of adsorption at the interface and the separation between particles within the interfacial film. As such, we demonstrate two avenues to reversibly control the optical properties of the fluid interface: (1) increase the pH of the aqueous solution to desorb particles from the interface, and (2) decrease the ionic strength in the aqueous phase to increase the spacing of the nanoparticles at the oil–water interface.