  Bulk properties and free interfaces of mixtures of charged platelike colloids
and salt are studied within density-functional theory. The particles are
modeled by hard cuboids with their edges constrained to be parallel to the
artesian axes corresponding to the Zwanzig model. The charges of the particles
are concentrated in their center. The density functional is derived by
functional integration of an extension of the Debye-H\"uckel pair distribution
function with respect to the interaction potential. For sufficiently small
macroion charges, the bulk phase diagrams exhibit one isotropic and one nematic
phase separated by a first-order phase transition. With increasing platelet
charge, the isotropic and nematic binodals are shifted to higher densities. The
Donnan potential between the coexisting isotropic and nematic phases is
inferred from bulk structure calculations. Non-monotonic density and nematic
order parameter profiles are found at a free interface interpolating between
the coexisting isotropic and nematic bulk phases. Moreover, electrically
charged layers form at the free interface leading to monotonically varying
electrostatic potential profiles. Both the widths of the free interfaces and
the bulk correlation lengths are approximately given by the Debye length. For
fixed salt density, the interfacial tension decreases upon increasing the
macroion charge.
