IMAGE CHARGE EFFECTS ON COLLOIDAL CRYSTAL ORDERING

We theoretically investigated the effect of the container wall on the structural ordering of aqueous dispersions of negatively charged elect rostatically stabilized colloidal spherical particles. The colloidal c rystal contained between two quartz plates is modeled as a set of crys tal planes oriented parallel to the quartz walls. We consider the elec trostatic interactions between the particles and the container wall, a nd the particles and their induced image charges. The position-depende nt interaction energies of a plane with its neighbors and the induced image charges are calculated under the Debye-Huckel approximation. We also theoretically investigate the effect of charged container walls o n the ordering of the colloidal particle dispersion. For zero wall sur face charge, the colloidal sphere plane nearest to the container wall is held in a deeper potential well than are interior colloidal planes. A negative wall surface charge creates a shallower well for the neare st colloidal sphere plane, which is still deeper than that of the inte rior planes. A positive wall surface charge creates the deepest potent ial well. These results rationalize our recent observations of the ini tial nucleation of crystalline colloidal array by formation of 2D hexa gonal colloidal layers near the container wall. We calculate the root- mean-square displacement of the planes at various distances from the w all and use the Lindermann-type melting criteria to examine ordering. (C) 1998 American Institute of Physics. [S0021-9606(98)51139-7].