A simple method for calculating the interaction force and energy per unit a
rea between two dissimilar plates with high potentials at constant surface
potential presented. Using Derjaguin's method and the improved Derjaguin' m
ethod, the expressions of the interaction free energy between two dissimila
r spheres with high surface potentials are derived. These formulae may be d
ivided into two groups: those for the strong interaction and those for the
weak interaction. The juncture of strong and weak interactions is at kappah
= 4 for dissimilar plates and at kappah = 2.8 for spheres The relative err
or is largest at this point, about 10%. (C) 2001 Academic Press.