  We consider the phase separation in an asymmetrically charged lipid bilayer
membrane consisting of neutral and negatively charged lipids that are in
contact with in/out ionic solutions having different ionic strengths. The two
asymmetric leaflets are coupled through electrostatic interactions. Based on a
free energy approach, the critical point and phase diagrams are calculated for
different ionic strength of the two solutions and coupling parameter. An
increase of the coupling constant or asymmetry in the salt concentration
between the in/out solutions yields a higher phase separation temperature
because of electrostatic interactions. As a consequence, the phase-coexistence
region increases for strong screening (small Debye length). Finally, possible
three-phase coexistence regions in the phase diagram are predicted.
