Electrostatic interactions strongly effect the immersion depth of nanoparticles into an interface. We prove this statement with measurements of the diffusion constant of charged nanoparticles at the sodium chloride solution / air interface. Interfacial diffusion of nanoparticles slows down with increasing ionic strength of the sodium chloride solution. Recent hydrodynamic calculations are used to estimate the immersion depth from the diffusion constant suggesting that nanoparticles with a carboxylate surface slightly immerse into a bare air/water interface. With increasing molarity of sodium chloride the immersion depth increases to almost complete immersion for a 1molar solution. The experiments show that an intricated interplay between the electrostatic properties of the solution/air and solution/solid interfaces and the classical contact angle determine the location of nanoparticles at interfaces.