  Polyelectrolyte brushes consist of a set of charged linear macromolecules
each tethered at one end to a surface. An example is the glycocalyx which
refers to hair-like negatively charged sugar molecules that coat the outside
membrane of all cells. We consider the transport and equilibrium distribution
of ions, and the resulting electrical potential, when such a brush is immersed
in a salt buffer containing monovalent cations (sodium and/or potassium). The
Gouy-Chapman model for ion screening at a charged surface captures the effects
of the Coulombic force that drives ion electrophoresis and diffusion, but
neglects non-Coulombic forces and ion pairing. By including the distinct
binding affinities of these counter-ions with the brush, and their so-called
Born radii, which account for Born forces acting on them when the permittivity
is non-uniform, we propose modified Poisson-Nernst-Planck continuum models that
show the distinct profiles that may result depending on those ion-specific
properties.
