Structures of adsorption layers of surfactant mixtures on nonpolar solid surfaces

The structure of the adsorbed layer of mixtures of a low molecular mass nonionic surfactant (alkyl heptaglycolether, C12EO7) with low molecular mass anionic (sodium dodecylsulfate, SDS, and sodium dodecylbenzenesulfonate, SDBS) and high molecular mass nonionic surfactants (EO37PO56EO37) was studied by atomic force microscopy in the combination with adsorption isotherms and adsorption enthalpies by isothermal titration calorimetry. The single surfactants C12EO7 and SDS show domains of parallel stripes on the graphite lattice in the atomic force microscopy (AFM) images. These parallel stripes can be related to hemicylindrical aggregates of a width of about 5 nm repeated in a periodic order which was proposed by different authors before. These structures are backed up by the results of the adsorption isotherms and the calorimetric measurements of the adsorption enthalpies. For SDBS and the high molecular mass nonionic surfactant, no such structures are observed in the AFM images despite an adsorption of surfactants on the surface. Mixtures of C12EO7 and the anionic surfactants give similar parallel stripes as the nonionic surfactant alone even in combination with SDBS, which indicates ideal mixing behavior on the surface. The concentration ratio of the surfactants on the surface differs from that in solution. For a mixed system of C12EO7 and the high molecular mass nonionic surfactant, a demixing of the surfactants on the surface can be assumed.