  In this paper, at first, we focus on a FRW universe in which the dark energy
candidate satisfies the Polytropic equation of state and study thermodynamics
of dark energy. Bearing the thermal fluctuation theorem in mind, we establish a
relation between the thermal fluctuation of system and mutual interaction
between the dark energy and dark matter. Generalization to a viscous Polytropic
gas is also investigated. We point to a condition for decaying dark energy
candidate into the dark matter needed for alleviating coincidence problem. The
effects of dark energy candidates and their interactions with other parts of
cosmos on the horizon entropy as well as the second law of thermodynamics are
also addressed. Our study signals us to a correction term besides the
Bekenstein entropy which carries the information of the dark energy candidate,
its interaction with other parts of cosmos and its viscosity.
