  The thermodynamics of nucleic acid processes is heavily affected by the
electric double-layer of micro-ions around the polyions. We focus here on the
Coulombic contribution to the salt-polyelectrolyte preferential interaction
(Donnan) coefficient and we report extremely accurate analytical expressions
valid in the range of low salt concentration (when polyion radius is smaller
than the Debye length). The analysis is performed at Poisson-Boltzmann level,
in cylindrical geometry, with emphasis on highly charged poly-ions (beyond
``counter-ion condensation''). The results hold for any electrolyte of the form
$z_-$:$z_+$. We also obtain a remarkably accurate expression for the electric
potential in the vicinity of the poly-ion.
