Adsorption of sodium dodecyl sulfate and sodium dodecyl phosphate on aluminum, studied by QCM-D, XPS, and AAS

The adsorption of two anionic surfactants, sodium dodecyl sulfate (SDS) and sodium dodecyl phosphate (SDP), at surfaces of aluminum and aluminum oxide has been studied by means of atomic absorption spectrometry (AAS), X-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance with dissipation monitoring (QCM-D). It was shown that more SDP than SDS binds to the surface and that SDP prevents dissolution of aluminum in water whereas SDS does not. This was not obvious, since the adsorption isotherms of the two surfactants to aluminum pigment powder are quite similar, as shown in an earlier work. The decreased aluminum dissolution with SDP compared to SDS was explained by the formation of a more compact protective layer with less permeability on the aluminum surface with SDP than with SDS. This is explained by differences in complexing ability between the surfactants and the aluminum pigment surface. While SDP is expected to form an inner-sphere complex with aluminum, leading to a lower accessibility of aluminum sites to water, SDS is likely to form a weaker outer-sphere complex.