  We show theoretically that near a fluid-fluid interface a single active
colloidal particle generating, e.g., chemicals or a temperature gradient
experiences an effective force of hydrodynamic origin. This force is due to the
fluid flow driven by Marangoni stresses induced by the activity of the
particle; it decays very slowly with the distance from the interface, and can
be attractive or repulsive depending on how the activity modifies the surface
tension. We show that, for typical systems, this interaction can dominate the
dynamics of the particle as compared to Brownian motion, dispersion forces, or
self-phoretic effects. In the attractive case, the interaction promotes the
self-assembly of particles into a crystal-like monolayer at the interface.
