ABSTRACT We study the effect of lipid
demixing on the electrostatic interaction of two oppositely-charged membranes in
solution, modeled here as an incompressible two-dimensional fluid mixture of
neutral and charged mobile lipids. We calculate, within linear and nonlinear
Poisson-Boltzmann theory, the membrane separation at which the net
electrostatic force between the membranes vanishes, for a variety of different
system parameters. According to Parsegian and Gingell, contact between oppositely-charged surfaces in an
electrolyte is possible only if the two surfaces have exactly the same charge
density (). If this condition is not fulfilled, the surfaces can repel
each other, even though they are oppositely charged. In our model of a
membrane, the lipidic charge distribution on the membrane surface is not
homogeneous and frozen, but the lipids are allowed to freely move within the
plane of the membrane. We show that lipid demixing allows contact between
membranes even if there is a certain charge mismatch,
, and that in certain limiting cases, contact is always
possible, regardless of the value of
(if
). We furthermore found that of the two interacting
membranes, only one membrane shows a major rearrangement of lipids, whereas the
other remains in exactly the same state it has in isolation and that, at
zero-disjoining pressure, the electrostatic mean-field potential between the
membranes follows a Gouy-Chapman potential from the more strongly charged
membrane up to the point of the other, more weakly charged membrane.