Very long-ranged forces in highly charged colloidal dispersions at finite concentration

As early as in 1983 a coexistence of an ordered and disordered distribution of charged
colloidal dispersions [1,2], as well as the formation of void structures [3,4] were reported.
These phenomena imply that there are long-ranged repulsive and attractive potentials, of
the order of thousands or even tens of thounsands of angstroms. However, molecular
interaction potentials are relativelly short ranged [5]. Even for charged fluids, at low
colloidal concentration, the particles’ correlation is at most a few hundreds of angstroms.
The origin and attributes of this correlation have been discussed in the literature [6], and
semi-phenomenological potentials have been proposed to explain the experimental
results. In the past we applied an integral equations formalism to a Colloidal Primitive
Model, to study the structure charged colloidal particles, at finite volume fraction [7]. In
this presentation we extend our calculations to investigate now the force between
macroions. We find a very long-ranged oscillatory force. We specifically explore the
nature of this like-charged particles’ repulsive-attractive interaction, i.e., we calculate its
electrical and entropic components, and discuss their contribution to the total force, in
terms of an energy-entropy balance.