CO-ION EXCLUSION IN ORDERED COLLOIDAL DISPERSIONS

The theoretical treatment of the exclusion of co-ions in terms of explicit boundaries, as defined in the model of dissociative electrical double layers (DEDL), suggests the formation of net co-ion volume charge densities, when diffuse double layers overlap extensively in charged colloidal systems. Such co-ion regions may appear farthest away from the charged particles, giving rise to periodic charge distributions that resemble the periodic structure of solid ionic crystals. This analogy suggests the existence of attractive electrostatic forces in ordered colloidal phases. Preliminary theoretical calculations in spherical geometry suggest a plausible mechanism for the long-range ordering of colloidal particles at low ionic strengths. These results are discussed in relation to experimental observations. It is suggested that new experiments be designed to demonstrate directly the existence of the periodic co-ion exclusion boundaries and charge distributions in colloidal dispersions.