Diffusion of Nanoparticles at an Air/Water Interface Is Not Invariant under a Reversal of the Particle Charge

The diffusion of charged nanoparticles at an aqueous/air interface is not invariant under a charge reversal of the particles. Negatively charged particles slow down with the ionic strength of the aqueous phase, while positively charged particles speed up. The diffusion constant of the particles reflects their immersion into the aqueous/air interface. We argue that the opposing behavior of oppositely charged particles is proof that the immersion depth of the particles scales with the contrast in electric surface potential of the particle to the electric surface potential of the air/water interface, not with the particle’s charge density. We therefore propose to incorporate the potential drop across the air/water interface into theories of electrodipping.