To describe the electrostatic properties of a lipid membrane,
mean-field continuum electrostatics is widely used. Its implementation
-- the classical Poisson-Boltzmann model -- predicts the structure
and corresponding free energy of the diffuse layer of counterions and
coions in the vicinity of a uniformly charged surface. The mean-field
character of the Poisson-Boltzmann approach and its ability
to predict free energies make the model a convenient starting point for
extensions and applications to complex soft matter systems. In this talk
we focus on one specific extension that is related to the dipole character
of zwitterionic lipids. We demonstrate how mobile surface dipoles can
be included into the Poisson-Boltzmann framework. To this end, we
suggest an extended mean-field model that can be used to describe
mixed charged-zwitterionic membranes, their stability with respect
to lateral phase separation, and their interactions with macroions
such as DNA and proteins