The surface potentials of long chain sulphates and their relation to dispersion stability

The monolayer characteristics of sodium octadecyl sulphate have been studied at the airlwater interface. The effect of sodium chloride on the phase boundary potentials provides evidence of the penetration of sodium ions into or above the plane of negative sulphate groups. This effect is found at high charge densities and ionic strengths. The changes of surface pressure and potential produced on replacing sodium by the hydrogen ion suggest the formation of a hydrogen-bonded monolayer. The surface potentials of adsorbed films of sodium dodecyl sulphate (SDS) agree closely with those for sodium octadecyl sulphate, and confirm the previous calculations using a modified Gibbs' adsorption isotherm for the monolayer densities of sodium dodecyl sulphate. These data were used to predict ionic strength regions of minimum stability for sulphate-stabilized colloid systems. These predictions were borne out by experiments on foams, solid suspensions and emulsions stabilized by sodium dodecyl sulphate.