Poly(ethoxy) anionic surfactants: micellization and adsorption at the solid/liquid interface

The properties of three surfactants of the nonylphenolpolyethoxy sulphate type with 4, 10 and 25 ethoxy units are studied both in solution and at the solid/aqueous solution interface. Critical micelle concentrations, adsorption isotherms (on silica, alumina and kaolinite), enthalpies of micellization or adsorption, are determined in various conditions. As for many surfactant systems, the adsorption can be described on a large range of concentration by the formation of surface aggregates, which are probably close in nature to bulk micelles. The possibility of adsorption is mainly driven by the interaction of the anionic group with the surface (when the latter owns positively charged sites), whereas the final structure at saturation is governed by the ethoxy chain length like for non-ionic surfactants.