Adsorption Isotherms at a Silica/Water Interface of the Oligomers of Polydispersed Nonionic Surfactants of the Alkylpolyoxyethylated Series

Abstract

Adsorption isotherms of the oligomers of three commercial alkylethoxylated nonionic surfactants at a silica/water interface have been determined using a HPLC method. The surfactants investigated are of the type used in detergency. The compounds, here noted A, B, and C, were mixtures of surfactants. A and B were of the general formulas C13E7+ C15E7and C13E10+ C15E10, respectively. Compound C was a mixture of oligomers ranging from C12E7to C16E7in various proportions. About 10 isotherms are presented for the most important oligomers for each of these detergents. Compound C could be more thoroughly discussed than compounds A and B because adsorption data for the pure surfactant of the same surfactant series with even numbers of carbon atoms in the alkyl chain were made available. Thus, the behavior of representative oligomers of C could be compared with that of the same pure surfactants at the silica/water interface. It is shown that at low surface coverage, polydispersity effects of either alkyl chains or ethoxy groups on global surfactant adsorption are negligible and the oligomers behave essentially as pure, single surfactants. At higher surface coverage, the simultaneous presence of oligomers with long chain lengths or with small numbers of ethoxy groups does not favor the formation of large structures, disks, or patched bilayers at the silica surface as do single pure surfactants. However, it is demonstrated that the opposing effects of oligomers with small and large alkyl chain lengths cancel out in the global isotherms. So it may be concluded that polydispersity in the case of commercial nonionic surfactants of the type used in the present investigation does not affect to any great extent the behavior of such compounds at a silica/water interface. The consequences of these findings can be extended to other nonionic surfactant series.