The electrostatic interactions between two colloidal particles of zero netcharge

The electrostatic interactions between two charged planes with zero net cha rge density are investigated on the basis of the Poisson-Boltzmann equation under the Debye-Huckel approximation. The electrostatic free energy and in teraction energy are obtained by the charging process. Two cases have been investigated: (1) particles with periodic surface charge density, and (2) p articles coated with ion-penetrable layers. For case (1), the interaction e nergy contains both repulsive and attractive contributions, the strength of which depends on the phase lag between the two particles. They experience repulsive forces in the proximity of the in-phase mode. However, in the pro ximity of the out-of-phase mode, the two particles experience attractive fo rces at sufficiently long distances and repulsive forces at closer distance s. For case (2), the two particles repel each other and the repulsive force s decay monotonicaly with increasing interparticle distances. However, the compromise between the net charge effect and the screening effect leads to the existence of an ionic strength (or amount of salt addition) at which th e at-contact repulsion is maximum for a specified thickness of the layer.