  We calculate the entropy and the temperature dependent part of the free
energy for a free standing plane plasma sheet and for the a free standing plane
slab of finite thickness with dispersion described by the plasma model. In case
the plasma sheet describes a charged fluid, the entropy is positive. In case it
describes a polarizable dipole sheet, the entropy takes negative values in a
certain parameter region. Negative entropy is also observed for the slab,
however with the reservation that the contribution from the surface plasmons
was not accounted for.
