  Based on the random phase approximation, we develop a molecular theory of
self-coacervation in zwitterionic polymer solutions. We show that the interplay
between the volume interactions of the monomeric units and electrostatic
correlations of charged groups on a polymer backbone can result in
liquid-liquid phase separation (self-coacervation). We analyse the behavior of
the coacervate phase polymer concentration depending on the electrostatic
interaction strength -- the ratio of the Bjerrum length to the bond length of
the chain. We establish that in a wide range of polymer concentration values --
from a semi-dilute to a rather concentrated solution -- the chain connectivity
and excluded volume interaction of the monomeric units have an extremely weak
effect on the contribution of the electrostatic interactions of the dipolar
monomeric units to the total free energy. We show that for rather weak
electrostatic interactions, the electrostatic correlations manifest themselves
as Keesom interactions of point-like freely rotating dipoles (Keesom regime),
while in the region of strong electrostatic interactions the electrostatic free
energy is described by the Debye-H{\"u}ckel limiting law (Debye regime). We
show that for real zwitterionic coacervates the Keesom regime is realized only
for sufficiently small polymer concentrations of the coacervate phase, while
the Debye regime is approximately realized for rather dense coacervates. Using
the mean-field variant of the density functional theory, we calculate the
surface tension (surface free energy) of the $"$coacervate-solvent$"$ interface
as a function of the bulk polymer concentration. Obtained results can be used
to estimate the parameters of the polymer chains needed for practical
applications such as drug encapsulation and delivery, as well as the design of
adhesive materials.
