Polyelectrolyte
(PE) brushes are widely used in biomaterials and
nanotechnology to regulate the surface properties and interactions.
Here, we apply the electrostatic correlation augmented self-consistent
field theory to investigate the interactions between opposing PE brushes
in a mixture of 1:1 and 3:1 salt solutions. Our theory predicts a
hysteretic feature of the normal stress induced by strong ion correlations.
In the presence of trivalent ions, the force profile is discontinuous:
repulsive in the compression branch and adhesive in the separation
branch. The molecular origin of the hysteretic force is the coexistence
of two collapsed modes: two separated condensed layers on each surface
in the compression and a single bundled condensed layer in the separation.
With the systematic inclusion of ion correlations, our theory captures
well the hysteretic force, adhesive separation, “jump-in”
and “jump-out” features, and the “specific ion
effect”, all in good agreement with the reported experimental
results.