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

The aim of this work is to compare bulk micellization and surface aggr egation of non-ionic-anionic surfactant mixtures. First, the solution properties of a mixture of n-octyltetraethylene and n-octylbenzene sul phonate (OBS) were studied by microcalorimetry in order to follow the behaviour of a typical non-ionic-anionic system and to choose a suitab le method for the determination of micellization enthalpies. Then the TX100-OBS system was studied by determining adsorption isotherms on si lica, micellization enthalpies and displacement enthalpies. It was sho wn that the adsorption of the anionic surfactant on silica is made pos sible by the formation of mixed surface aggregates. The change in mice llization enthalpies and surface aggregation enthalpies, as the anioni c surfactant content increases, can be explained by a rehydration of e thoxy groups of aggregates. This rehydration could be due to water mol ecules accompanying ions in the polar shell of the micelles. Both enth alpic and electrophoretic data are consistent with a higher counterion binding in surface micelles than in bulk micelles.