  In mixtures of colloids and nonadsorbing polyelectrolytes, a Donnan potential
arises across the region between surfaces that are depleted of polyelectrolyte
and the rest of the system. This Donnan potential tends to shift the
polyelectrolyte density profile towards the colloidal surface and leads to
local accumulation of polyelectrolytes. We derive a zero-field theory for the
disjoining pressure between two parallel flat plates. Polyelectrolyte is
allowed to enter the confined interplate region at the cost of a conformational
free energy penalty. The resulting disjoining pressure shows a crossover to a
repulsive regime when the interplate separation gets smaller than the size of
the polyelectrolyte chain, followed by an attractive part. We find a
quantitative match between the model and self-consistent field computations
that take into account the full Poisson-Boltzmann electrostatics.
