Electric Potential (ψδ and ψd) at Outer Helmholtz Plane and Midplane on the Clay Colloid Surface with Overlapping Flat Double Layers

An anion negative adsorption equation in the condensed colloidal suspension with overlapping flat double layers was derived according to Gouy–Chapman theory. The electric potential at the outer Helmholtz plane (OHP), ψδ, and the electric potential at the midplane, ψd, were numerically solved by computer using the anion negative adsorption equation on the basis of experiments. The results showed that ψδ and ψd increase with the decrease of the distance between two clay plates, λ, at first in the given electrolyte concentration. When λ is smaller than 50–70 Å, ψd remains almost unchanged while ψδ declines remarkably with the further decrease of λ. The change of ψd/ψδ with λ can explain and manifest overlapping degree of flat double layers more appropriately than ψd in previous works. Due to compression of the flat double layer on the clay colloid surface at increasing electrolyte concentration, the magnitude of the electrical potentials at OHP and midplane is considerably reduced at a given λ.