  The electrostatic interaction between two non-identical, moderately charged
colloids situated in close proximity of each other at a fluid interface is
studied. By resorting to a well-justified model system, this problem is
analytically solved within the framework of linearized Poisson-Boltzmann (PB)
density functional theory (DFT). The resulting interaction comprises a surface
and a line part, both of which, as functions of the inter-particle separation,
show a rich behavior including monotonic as well as non-monotonic variations.
In almost all cases, these variations cannot be captured correctly by using the
superposition approximation. Moreover, expressions for the surface tensions,
the line tensions and the fluid-fluid interfacial tension, which are all
independent of the inter-particle separation, are obtained. Our results are
expected to be particularly useful for emulsions stabilized by oppositely
charged particles.
