  The electrostatic interaction between colloidal particles trapped at the
interface between two immiscible electrolyte solutions is studied in the limit
of small inter-particle distances. Within an appropriate model exact analytic
expressions for the electrostatic potential as well as for the surface and line
interaction energies are obtained. They demonstrate that the widely used
superposition approximation, which is commonly applied to large distances
between the colloidal particles, fails qualitatively at small distances and is
quantitatively unreliable even at large distances. Our results contribute to an
improved description of the interaction between colloidal particles trapped at
fluid interfaces.
