Electrokinetic and surface potentials at liquid interfaces

Abstract

A homologous series of four cationic surfactants of the n-dodecyltrialkylammonium family has been prepared. Measurements of the adsorption of these surfactants at the n-decane/aqueous electrolyte interface have been made as a function of both surfactant and electrolyte concentration, using the Gibbs equation. Measurements have also been made of the electrophoretic mobility of hydrocarbon droplets stabilised by these surfactants and of the interfacial (ΔV) potentials, as a function of adsorption and of bulk phase electrolyte concentration. The nature of the ΔV potentials is considered and surface potentials ϕ_p are estimated from the ΔV data. Plots of ϕ_p as a function of adsorption show maxima in all cases. These maxima are discussed with reference to the discrete-ion theory. Comparison of the Gouy potential ϕ_G and of ϕ_p with the electrokinetic potentials is made in terms of the position of the apparent plane of shear in the interface. The distance of this plane of shear from the plane of primary charge depends on both surfactant structure and on adsorption and can be considerable when calculated from ϕ_G; however, if ϕ_p is used instead of ϕ_G, it is found that the plane of shear is always at a distance of less than the “exclusion diameter” of the head group of the surfactant and a counter-ion.