Heterogeneous structure in colloidal systems: the role of the microion disposition

Under certain conditions colloidal systems exhibit a heterogeneous structure sometimes referred to as a "two state" structure, "spinodal instability," or a "phase separation." The present study focuses on the "orbital model" for the description of two geometries of colloidal clusters: a 7-particle diamond shape array and an 8-particle simple cubic array. The orbital model envisions the distribution of the microions as being dictated by the specific configuration of all the macroions in the system, in much the same way that electron distributions in molecules are determined by the array of atoms. Brownian dynamics simulations were performed as two similar clusters approached each other in rectangular cells at the volume fraction of phi(p)=0.01. The number distributions of both the counterion and coion species were determined three ways: the one-dimensional projection along the long axis of the computations cell; the three-dimensional number distributions in "real" space; and a "constant concentration" contour profile. It was found that as the two clusters approached each other the diamond cluster system became less stable whereas the simple cubic cluster system became more stable. This difference in behavior is attributed to the relative abilities of these structures to "share" counterions and the exclusion of the coions.