We obtained mean-field expressions for the compression normal forces betwee
n two identical opposing quenched polyelectrolyte brushes in the presence o
f monovalent salt. The brush elasticity is modeled using the entropy of ide
al Gaussian chains, while the entropy of the microions and the electrostati
c contribution to the grand potential is obtained by solving the nonlinear
Poisson-Boltzmann equation for the system in contact with a salt reservoir.
The interplay between these distinct contributions upon variations of the
ionic strength determines the brush thickness. For the polyelectrolyte brus
h we considered both an isotropic charged slab as well as a longitudinally
heterogeneous charge profile obtained using a self-consistent field theory.
Using the Derjaguin approximation, we related the planar-geometry results
to the realistic two-crossed cylinders experimental setup. Theoretical pred
ictions are compared to experimental measurements of the salt dependence of
the compression normal forces between two quenched polyelectrolyte brushes
formed by the adsorption of diblock copolymers poly(tert-butyl styrene)-so
dium poly(styrene sulfonate) onto an octadecyltriethoxysilane hydrophobical
ly modified mica surface. (C) 2001 American Institute of Physics.