  We study the dependence of the surface tension of a fluid interface on the
density profile of a third suspended phase. By means of an approximated model
for the binary mixture and of a perturbative approach we derive close formulas
for the free energy of the system and for the surface tension of the interface.
Our results show a remarkable non-monotonous dependence of the surface tension
on the peak of the density of the suspended phase. Our results also predict the
local value of the surface tension in the case in which the density of the
suspended phase is not homogeneous along the interface.
