  At nuclear matter density, electrically neutral strongly interacting matter
in weak equilibrium is made of neutrons, protons and electrons. At sufficiently
high density, such matter is made of up, down and strange quarks in the
color-flavor locked phase, with no electrons. As a function of increasing
density (or, perhaps, increasing depth in a compact star) other phases may
intervene between these two phases which are guaranteed to be present. The
simplest possibility, however, is a single first order phase transition between
CFL and nuclear matter. Such a transition, in space, could take place either
through a mixed phase region or at a single sharp interface with electron-free
CFL and electron-rich nuclear matter in stable contact. Here we construct a
model for such an interface. It is characterized by a region of separated
charge, similar to an inversion layer at a metal-insulator boundary. On the CFL
side, the charged boundary layer is dominated by a condensate of negative
kaons. We then consider the energetics of the mixed phase alternative. We find
that the mixed phase will occur only if the nuclear-CFL surface tension is
significantly smaller than dimensional analysis would indicate.
