Free energy and entropy for thin sheets

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.