THERMODYNAMIC STUDY OF NONIONIC ANIONIC SURFACTANT MIXTURES - MICELLIZATION AND ADSORPTION ON SILICA

The aim of this work is to compare bulk micellization and surface aggregation of non-ionic-anionic surfactant mixtures. First, the solution properties of a mixture of n-octyltetraethylene and n-octylbenzene sulphonate (OBS) were studied by microcalorimetry in order to follow the behaviour of a typical non-ionic-anionic system and to choose a suitable method for the determination of micellization enthalpies. Then the TX100-OBS system was studied by determining adsorption isotherms on silica, micellization enthalpies and displacement enthalpies. It was shown that the adsorption of the anionic surfactant on silica is made possible by the formation of mixed surface aggregates. The change in micellization enthalpies and surface aggregation enthalpies, as the anionic surfactant content increases, can be explained by a rehydration of ethoxy groups of aggregates. This rehydration could be due to water molecules accompanying ions in the polar shell of the micelles. Both enthalpic and electrophoretic data are consistent with a higher counterion binding in surface micelles than in bulk micelles.