Concentration of Cations at Negatively Charged Surfaces

Abstract

WHEN ions of one sign are fixed at a surface, the concentration of ions of the opposite sign in the surface phase may be calculated from the Gibbs-Donnan equilibrium between the surface and bulk phases¹, or from the electrokinetic potential. By both methods it is predicted that the pH at a surface containing fixed anions will be less than in the bulk phase, and Hartley and Roe have shown² that the pH at the surface of cetyl sulphate micelles, as calculated from the ζ potential (circles of accompanying figure) is in reasonable agreement with the values observed using a surface colorimetric indicator (crosses of accompanying figure). We have now calculated the pH at the same surface from the Gibbs-Donnan equilibrium (continuous line of accompanying figure) and find values in reasonable agreement with those obtained by the other two methods. The thickness of the surface phase was taken as that given by the theory of Debye and Hückel.