THEORY OF PLANAR POLYELECTROLYTE BRUSH IMMERSED IN SOLUTION OF ASYMMETRIC SALT

The analytical self-consistent field theory of polymer brushes and the Scheutjens-Fleer numerical theory are applied in a study of the equil ibrium behaviour of a planar polyelectrolyte brush immersed in a solut ion of an asymmetric salt. A scaling-type diagram comprising various r egimes of brush behaviour is constructed and verified by numerical cal culations. It is shown that for a highly asymmetric salt with multival ent coions there are two different salt-controlled regimes of brush be haviour. At large salt contents, highly charged coions penetrate into the brush and dominate the screening of the electrostatic repulsion be tween grafted polyions (salted brush; SB). In this regime the brush th ickness H scales as H approximately I-1/3, where I is the ionic streng th of the outer solution (made up of coion and counterion contribution s). At relatively small salt contents, highly charged coions are exclu ded from the brush whereas counterions equal out their concentrations in the brush and outer solution (counterion brush; CB). In this regime the screening of electrostatic repulsion in the brush is performed by mobile counterions and the brush thickness scales as H approximately phi(s)-1, where phi(s) is the concentration of salt counterions only i n the outer solution. Possible experiments to verify these theoretical predictions are discussed.