  We develop a model of interacting zwitterionic membranes with rotating
surface dipoles immersed in a monovalent salt, and implement it in a field
theoretic formalism. In the mean-field regime of monovalent salt, the
electrostatic forces between the membranes are characterized by a non-uniform
trend: at large membrane separations, the interfacial dipoles on the opposing
sides behave as like-charge cations and give rise to repulsive membrane
interactions; at short membrane separations, the anionic field induced by the
dipolar phosphate groups sets the behavior in the intermembrane region. The
attraction of the cationic nitrogens in the dipolar lipid headgroups leads to
the adhesion of the membrane surfaces via dipolar bridging. The underlying
competition between the opposing field components of the individual dipolar
charges leads to the non-uniform salt ion affinity of the zwitterionic membrane
with respect to the separation distance; large inter-membrane separations imply
anionic excess while small, nanometer size separations, favor cationic excess.
This complex ionic selectivity of zwitterionic membranes may have relevant
repercussions on nanofiltration and nanofluidic transport techniques.
