  We study charged nanoparticles adsorbed onto surfactant bilayers using
small-angle scattering of synchrotron radiation. The in-plane interaction of
the particles is well described by a DLVO component (measured independently in
solution) and a repulsive dipolar interaction due to the presence of the
interface, with an amplitude close to the theoretical prediction. We prove that
charged nanoparticles at soft interfaces are well described by the classical
model of Poisson-Boltzmann and van der Waals terms; as a corollary, they do not
experience the like-charge attraction reported in the literature for some
systems of micron-sized spheres at interfaces.
