Schematic representation of the Poisson–Boltzmann model for the investigation of the potential distribution across the spore–solution or spore–spore interface of noninteracting and interacting
spores. The electric potential is considered to arise from three different charge contributions: (i) a surface charge density σ at the outer boundary of the outer spore wall layer (OL), (ii) the nonuniform space charge density of ionized fixed carboxyl groups in the surface coating (SC), ρ
fix, and (iii) the space charge densities ρ
+ and ρ
−, covering the charge contributions from mobile positive and negative ions in the electrolyte solution (EL) and in the ion-penetrable surface coating.