The adsorption of anionic and nonionic polydisperse surfactants vity experiments were performed either with neutron or light was studied by ellipsometry on titania and silica, respectively. The (12, 13). Among these methods, ellipsometry has drawn effect of polydispersity is mainly observable in the low concentrabenefit of the progress of electronic industry which allows tion range. In the case of polyethoxylated nonionic surfactant adsamples with very low roughness and good contrast to be sorption on silica, the long chain components are preferentially prepared.
adsorbed at low concentration. Ethoxy-sulfated surfactants do not
In a set of recent papers on surfactant adsorption (15adsorb on silica in the same conditions (pH รบ 3). Their ethoxy 18), the interest of this approach to determine adsorption chains seem to have no interaction with the titania surface at pH isotherms, kinetic of adsorption, and even thickness of the 2.5 where adsorption is mainly due to an electrostatic interaction between sulfate head and positive sites of the surface. At satura-adsorbed layer was stressed. Surfactants are fairly well suited tion, surface concentrations and thicknesses of the adsorbed layer to determinations of adsorbed layer thickness since they genlead to the same interpretations as those deduced from results erally form at the interface self-assembly structures (at least obtained on powders with the same surfactants or on flat surfaces at the plateau of the adsorption isotherm) which are probably with monodisperse surfactants: the surface seems to be covered very homogeneous in the direction perpendicular to the surwith aggregates of limited extension whose thickness is comparable face allowing simple multilayer models to be applied. to micelle diameter. It is also worth noticing that the linear evolu-Nevertheless, going from divided systems to surface scition of apparent areas per molecule with ethoxy number is very ence, a number of difficulties are introduced at the level of similar for nonionic and anionic sets. แญง 1997 Academic Press both experimental conditions and extrapolation of results to Key Words: adsorption on silica; adsorption on titania; ellippractical systems. For example, working with a few square sometry; nonionic surfactant; anionic surfactant.
centimeters of surface area, the pollution of the surface is quite easy. This is why most works using this approach have been carried out with pure monodisperse surfactants whereas EXPERIMENTAL ESR (12) are example of experiments which can bring informations on adsorbed amount, stability, thickness, or local Materials Four nonionic surfactants of the Triton set (provided by