The system is however of wide practical and theoretical The electrostatic interaction energy for a charged sphere interinterest where important examples are the interaction beacting with a low dielectric charged planar surface in an electrolyte tween a surface and micelles, charged polymers, or charged solution is calculated. The calculations are based on a solution of colloid particles, respectively. Our interest for this system the linearized Poisson-Boltzmann equation under the condition stems from the view that electrostatic interaction is of great of constant charge density on both surfaces. The influence of importance in the description and understanding of protein sphere size, its dielectric constant, and net charge as well as planar adsorption. The reason is that many proteins are globular surface charge density and electrolyte concentration on the interand charged and it is reasonable to assume that they can be action energy is demonstrated. A comparison is made between the interaction energies for a point charge interacting with a charged approximated as charged spheres. In an earlier paper (1) we planar surface, calculated from the Gouy-Chapman theory, and showed that retention changes of proteins in ion exchange our solution for a charged 2.5 A ˚sphere. The calculations show an chromatography, due to changes in electrolyte concentration asymmetric interaction energy for spheres of opposite sign of in the mobile phase, could be described by using a model charge, i.e., when the surface and sphere are of the same (opposite) based on the electrostatic interaction between two oppositely sign the interaction is stronger (weaker) than what is obtained charged planar surfaces. The charged sphere-planar surface from the Gouy-Chapman theory. On the other hand, the asymmesystem may therefore be a complement to this model in the try is overestimated when the Deryaguin procedure is applied to description of electrostatic interactions in protein adsorption calculate the interaction energy between a charged sphere and a studies. charged planar surface. In the latter case the asymmetry is mainly Even though advances have been made in the molecule due to overestimation of the repulsive energy when the sphere and the planar surface have the same sign of charge. It is shown that, dynamic and Monte Carlo calculations of electrostatic as a first approximation, the interaction energy can be obtained interactions in surface and colloid chemistry, solutions of by adding the contribution from three separate components: a the Poisson -Boltzmann ( P -B ) equation are also useful point charge, located in the center of the sphere, interacting with tools for the calculation of these interactions. Although the charged surface plus the interaction energy between an unthe limitations of Poisson -Boltzmann approach are well charged sphere and the charged surface plus the interaction energy known, i.e., the mean field approximation and the asbetween a charged sphere and the uncharged surface. ᭧ 1995 sumption of point charges, many studies have shown that Academic Press, Inc.