Electrokinetic and surface potentials at liquid interfaces

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 ϕp are estimated from the ΔV data. Plots of ϕp 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 ϕG and of ϕp 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 ϕG; however, if ϕp is used instead of ϕG, 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.