  The effective electrostatic interaction between a pair of colloids, both of
them located close to each other at an electrolyte interface, is studied by
employing the full, nonlinear Poisson-Boltzmann (PB) theory within classical
density functional theory. Using a simplified yet appropriate model, all
contributions to the effective interaction are obtained exactly, albeit
numerically. The comparison between our results and those obtained within
linearized PB theory reveals that the latter overestimates these contributions
significantly at short inter-particle separations. Whereas the surface
contributions to the linear and the nonlinear PB results differ only
quantitatively, the line contributions show qualitative differences at short
separations. Moreover, a dependence of the line contribution on the solvation
properties of the two adjacent fluids is found, which is absent within the
linear theory. Our results are expected to enrich the understanding of
effective interfacial interactions between colloids.
