in a given chemical series. In the course of an investigation The behavior of two pure alkylethoxylated nonionic surfactants, on the adsorption of one of these surfactants at solid/water both as single components and as binary mixtures of various cominterfaces, it was find useful to study the behavior of mixposition, has been studied in solution and at silica/water interfaces. tures of two pure nonionic surfactants with a large difference In solution, the variation of cmc with surfactant composition between their hydrocarbon chain length. It is generally asshowed large negative and positive deviations from thermodysumed that mixtures of nonionic compounds in a given series namic ideality. These results, as obtained from surface tension such as polyoxyethylene derivatives in aqueous solutions measurements, may be interpreted by a partial demixing of the present small departures from thermodynamic ideality as micelles into segregated aggregates in the C 10 E 6 -rich composition defined at the critical micelle concentration (cmc) (1-4).
range and by an increased stability of the aggregates in the C 14 E 6rich domain. Careful determination of individual isotherms of the This situation is very different from that encountered when surfactants either as single components or as components of mixthe surfactants are mixtures of ionic and nonionic or of ionic tures was performed at a nonporous silica/water interface using compounds of oppositely charged types for which, undera reversed phase HPLC technique. The pure nonionic surfactants standably, strong interactions induce large deviation from displayed non superimposable isotherms when sufficiently dilute thermodynamic ideality. Therefore the free energy of mixing solution can be attained: a single adsorption plateau is observed of surfactants in a nonionic series should, in principle, be with the more hydrophobic component, and a two-plateau behavdeduced from the sole knowledge of the individual cmc's ior is obtained with the more hydrophilic compound. Some experiusing one of the thermodynamic models for surfactant mixments with the intermediate surfactant C 12 E 6 were also performed ing available. Good agreement is usually observed between showing a single-plateau profile. Experimental confirmation of the experiment and theory (1-4). However, most surfactant bidemixing process at the silica/water interface was found in the C 10 E 6 -rich end of the diagram. However, the larger than usual naries which were tested had cmc values which were relafavorable interaction between the surfactants in the C 14 E 6 -end of tively close to each other. In the case of detergency, surfacthe diagram could not be deduced directly from the shape of the tants of widely different cmc values may be present in the individual isotherms. The various isotherms were discussed maksolution. It is well-known that in the polyoxyethylene series, ing use of the known properties of these surfactants in solution as the variation of polar chain at constant hydrocarbon chain micelle-type aggregates and as lamellar phases. α§ 1996 Academic is not very large in comparison to the effect of changing the Press, Inc.