Many-bodied effects and the structure of colloidal crystals

Experimental evidence indicates that under low ionic strength conditions suspensions of highly charged colloidal particles exhibit a two-state structure: a crystalline phase coexisting with a gas-like phase. We applied herein the juxtaposition of potential fields (JPF) method to two crystalline structures: simple-cubic (sc) and body-centered-cubic (bcc). It is concluded from the JPF approach that an inhomogeneous distribution of counterions exists within each lattice structure, with the higher ion concentration in its interior. It is also suggested that this inhomogeneity results in a long range attraction between the constituents of the crystalline structure due to the multi-bodied sharing of counterions in a manner similar to that of chemical bonding in molecules. Thus the "‘vibration’' motions of the colloidal particles must therefore vary within the JPF crystalline structure giving rise to paracrystalline behavior. The JPF crystal appears to share the properties of a phase transition with added salt as reported for polystyrene latex spheres by Matsuoka and co-workers. We also conclude that the sc structure is less resistant to shear than the bcc structure because of the interplane sharing of counterions for the latter configuration.