On thermodynamical properties of dark energy

We have investigated the thermodynamical properties of the dark energy. Assuming that the dark energy temperature $T\sim a^{-n}$ and considering that the universe volume enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For the dark energy with constant equation of state $w>-1$ and the generalized Chaplygin gas dark energy models, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of the dark energy, the thermal radiation and the geometric entropy on the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom dark energy with constant equation of state, the positivity of entropy, the entropy bound and the generalized second law cannot be satisfied simultaneously