  Monodisperse suspensions of rodlike chiral $fd$ viruses are condensed into a
rod-length thick colloidal monolayers of aligned rods by depletion forces.
Twist deformations of the molecules are expelled to the monolayer edge as in a
chiral smectic $A$ liquid crystal, and a cholesteric band forms at the edge.
Coalescence of two such isolated membranes results in a twist wall sandwiched
between two regions of aligned rods, dubbed $\pi$-walls. By modeling the
membrane as a binary fluid of coexisting cholesteric and chiral smectic $A$
liquid-crystalline regions, we develop a unified theory of the $\pi$-walls and
the monolayer edge. The mean-field analysis of our model yields the molecular
tilt profiles, the local thickness change, and the crossover from smectic to
cholesteric behavior at the monolayer edge and across the $\pi$-wall.
Furthermore, we calculate the line tension associated with the formation of
these interfaces. Our model offers insights regarding the stability and the
detailed structure of the $\pi$-wall and the monolayer edge.
