  The implications of soft `patchy' interactions on the orientational
disorder-order transition of strongly elongated colloidal rods and flat disks
is studied within a simple Onsager-van der Waals density functional theory. The
theory provides a generic framework for studying the liquid crystal phase
behaviour of highly anisometric cylindrical colloids, which carry a distinct
geometrical pattern of repulsive or attractive soft interactions localised on
the particle surface. In this paper, we apply our theory to the case of charged
rods and disks for which the local electrostatic interactions can be described
by a screened-Coulomb potential. We consider infinitely thin rod-like cylinders
with a uniform line charge and infinitely thin discotic cylinders with several
distinctly different surface charge patterns. Irrespective of the backbone
shape, the isotropic-nematic phase diagrams of charged colloids feature a
generic destabilization of nematic order, a dramatic narrowing of the biphasic
density region and a reentrant phenomenon upon reducing the electrostatic
screening. At higher particle density the electrostatic repulsion leads to a
complete suppression of nematic order in favour of spatially inhomogeneous
liquid crystals.
