Here, ϵ is the solvent permittivity which was set to 80, ψ i and ψ j are the surface potentials for the two particles which unless otherwise indicated were set to +50 mV and -50 mV for positive and negative particles, respectively, and λ D is the Debye length. h ij is the distance between the surfaces of the particles. a ij is the ionic radius coming from the Derjaguin approximation which is expressed as