Nanoparticle-Induced Charge Redistribution of the Air-Water Interface

The airwater interface is believed to carry a negative electrostatic potential that is nontrivial to invert through pH, electrolyte, or electrolyte strength. Here, through a combined experimental and theoretical study, we show that the close approach of a negatively charged nanoparticle induces a charge redistribution of the airwater interface. Using different electrolytes to control the interfacial potential of the nanoparticles, X-ray photoelectron spectroscopy (XPS) results establish that nanoparticles with a more negative zeta potential adsorb closer to the airwater interface than do the same particles with a less negative zeta potential. The short-ranged attractive force between two (nominally) negative surfaces is caused by charge redistribution under the strong electric field of the nanoparticle that locally inverts the charge density of the airwater interface from negative to positive. The nature of the nanoparticles counterions modulates the attractive interaction, which thus could be used to control reactivity, stability, and nanoparticle self-assembly at airwater interfaces.