Very long-range attractive and repulsive forces in model colloidal dispersions

Experiments with polymer latex solutions show the coexistence of order-disorder structures of macroions. Because of the large macroions’ sizes, this order–disorder phase coexistence imply the existence of very long-range attractive and repulsive forces, which cannot be explained in terms of conventional direct interaction potentials, which are short-range. Here, we apply an integral equations theory to a simple model for colloidal dispersions, at finite concentrations, calculate the particles distribution functions and the involved effective forces. We find very long-range attractive and repulsive forces among the like-charged macroions. The distribution functions are in qualitative agreement with experimental results. The origin of these forces are discussed in terms of an energy–entropy balance.