  We study the effective interaction between differently charged
polyelectrolyte-colloid complexes in electrolyte solutions via Monte Carlo
simulations. These complexes are formed when short and flexible polyelectrolyte
chains adsorb onto oppositely charged colloidal spheres, dispersed in an
electrolyte solution. In our simulations the bending energy between adjacent
monomers is small compared to the electrostatic energy, and the chains, once
adsorbed, do not exchange with the solution, although they rearrange on the
particles surface to accomodate further adsorbing chains or due to the
electrostatic interaction with neighbor complexes. Rather unexpectedly, when
two interacting particles approach each others, the rearrangement of the
surface charge distribution invariably produces anti-parallel dipolar doublets,
that invert their orientation at the isoelectric point. These findings clearly
rule out a contribution of dipole-dipole interactions to the observed
attractive interaction between the complexes, pointing out that such
suspensions can not be considered dipolar fluids. On varying the ionic strength
of the electrolyte, we find that a screening length, short compared with the
size of the colloidal particles, is required in order to observe the attraction
between like charged complexes due to the non-uniform distribution of the
electric charge on their surface ('patch attraction'). On the other hand, by
changing the polyelectrolyte/particle charge ratio, the interaction between
like-charged polyelectrolyte-decorated (pd) particles, at short separations,
evolves from purely repulsive to strongly attractive. Hence, the effective
interaction between the complexes is characterized by a potential barrier,
whose height depends on the net charge and on the non-uniformity of their
surface charge distribution.
