Electrokinetic study of surfactant adsorption

Abstract

It has previously been shown that the limiting slope, measured at constant ionic strength, of the electrokinetic potential ζ of a charged interface through its isoelectric point (i.p.) with respect to pX, where X is a potential determining (p.d.) ion for the surface, can yield information about the inner part of the double layer. We have here used a similar treatment involving the rate of change of ζ with the concentration of an adsorbed (surfactant) ion, in the limit of ζ→ 0, at constant level of p.d. ion and of total ionic strength. Limiting forms of the derived relations are obtained for the cases where, respectively, the surface potential, or the density of primary surface charges, is not changed by the adsorption of ions other than p.d. ions. The model has been applied to electrokinetic measurements on AgI (constant potential approximation) and on latex particles (constant surface charge approximation) in the presence of organic surface active ions.