  The "polymer reference interaction site model" (PRISM) integral equation
formalism is used to determine the pair structure of binary colloidal
dispersions involving large and small polyions of opposite charge. Two examples
of such bidisperse suspensions are considered in detail, namely mixtures of
charged spherical colloids and oppositely charged polyelectrolyte chains, and
binary mixtures of oppositely charged large and small clay platelets. In both
cases clear evidence is found for aggregation of the larger particles induced
by the polyionic counterions, signalled by a strong enhancement of long
wavelength concentration fluctuations.
