  Conformational characteristics of polyzwitterionic molecules in aqueous
solutions are investigated using the variational method. Analytical relations
are derived for the radius of gyration of a single polyzwitterionic chain as a
function of the chain length, electrostatic interaction strength, added salt
concentration, dipole moment and degree of ionization of the zwitterionic
monomers. In the absence of the small ions (counterions and coions) near the
polyzwitterionic chain, attractive dipole-dipole interactions are shown to
induce a collapse of the polyzwitterionic chain. However, in the presence of
the small ions, the radius of gyration is shown to be an interplay of the
screening of the electrostatic interactions and the counterion adsorption on
the zwitterionic sites. In addition to the well-known Debye-Huckel screening of
the charge-charge interactions, screening of the charge-dipole and
dipole-dipole interactions are found to play important roles in determining the
size of the chain. Functional forms for the screened charge-dipole and
dipole-dipole interaction potentials are presented. Furthermore, counterion
adsorption on the zwitterionic monomers is predicted to be asymmetric depending
on the nature of the added salt and the zwitterionic groups. Qualitative
remarks regarding the solubility of these molecules in aqueous solutions along
with the classical "anti-polyelectrolyte" effect (increase in the solubility in
water with the addition of salt) are presented.
