ADSORPTION-ISOTHERMS AT A SILICA WATER INTERFACE OF THE OLIGOMERS OF POLYDISPERSED NONIONIC SURFACTANTS OF THE ALKYLPOLYOXYETHYLATED SERIES/

Adsorption isotherms of the oligomers of three commercial alkylethoxyl ated nonionic surfactants at a silica/water interface have been determ ined using a HPLC method. The surfactants investigated are of the type used in detergency. The compounds, here noted A, B, and C, were mixtu res of surfactants. A and B were of the general formulas C13E7 + C15E7 and C13E10 + C15E10, respectively, Compound C was a mixture of oligom ers ranging from C12E7 to C16E7 in various proportions, About 10 isoth erms are presented for the most important oligomers for each of these detergents, Compound C could be more thoroughly discussed than compoun ds A and B because adsorption data for the pure surfactant of the same surfactant series with even numbers of carbon atoms in the alkyl chai n were made available, Thus, the behavior of representative oligomers of C could be compared with that of the same pure surfactants at the s ilica/water interface. It is shown that at low surface coverage, polyd ispersity effects of either alkyl chains or ethoxy groups on global su rfactant adsorption are negligible and the oligomers behave essentiall y as pure, single surfactants. At higher surface coverage, the simulta neous presence of oligomers with long chain lengths or with small numb ers of ethoxy groups does not favor the formation of large structures, disks, or patched bilayers at the silica surface as do single pure su rfactants, However, it is demonstrated that the opposing effects of ol igomers with small and large alkyl chain lengths cancel out in the glo bal isotherms, So it may be concluded that polydispersity in the case of commercial nonionic surfactants of the type used in the present inv estigation does not affect to any great extent the behavior of such co mpounds at a silica/water interface. The consequences of these finding s can be extended to other nonionic surfactant series. (C) 1997 Academ ic Press.