  We construct a phenomenological Landau-de Gennes theory for hard colloidal
rods by performing an order parameter expansion of the chemical-potential
dependent grand potential. By fitting the coefficients to known results of
Onsager theory, we are not only able to describe the isotropic-nematic phase
transition as function of density, including the well-known density jump, but
also the isotropic-nematic planar interface. The resulting theory is applied in
calculations of the isotropic core size in a radial hedgehog defect, the
density dependence of linear defects of hard rods in square confinement, and
the formation of a nematic droplet in an isotropic background.
