Debye screening of colloid repulsion (${\kappa }^2=2n{e}^2/(ϵk_{B}T)$)

For a symmetric monovalent reservoir with number density $n$ of each species, linearizing Poisson-Boltzmann theory gives the displayed inverse screening length. A planar potential decays exponentially; a spherical far field also has a geometrical inverse-distance factor. Increasing salt reduces the range of repulsion. Fixed surface potential, fixed charge and charge regulation need not give identical interaction amplitudes.