  We consider the thermal Casimir effect in systems of parallel plates coupled
to a mass-less free field theory via quadratic interaction terms which suppress
(i) the field on the plates (ii) the gradient of the field in the plane of the
plates. These boundary interactions correspond to (i) the presence of an
electrolyte in the plates and (ii) a uniform field of dipoles, in the plates,
which are polarizable in the plane of the plates. These boundary interactions
lead to Robin type boundary conditions in the case where there is no field
outside the two plates. In the appropriate limit, in both cases Dirichlet
boundary conditions are obtained but we show that in case (i) the Dirichlet
limit breaks down at short inter-plate distances and in (ii) it breaks down at
large distances. The behavior of the two plate system is also seen to be highly
dependent on whether the system is open or closed. In addition we analyze the
Casimir force on a third plate placed between two outer plates. The force
acting on the central plate is shown to be highly sensitive to whether or not
the fluctuating scalar field is present in the region exterior to the two
confining plates.
