INTERFACIAL AGGREGATION OF NONIONIC SURFACTANTS ONTO SILICA-GEL - CALORIMETRIC EVIDENCE

The search for the origins of the macroscopic interfacial phenomena, the adsorption layer structure, and the nature of interactions between solid and surfactant molecules and between molecules at adsorbed states has led us to a calorimetric investigation of adsorption. The adsorption of nonionic surfactants onto the hydrophilic surfaces of silica gel has been studied by batch microcalorimetry for a wide range of coverages. Differential molar enthalpies of adsorption corresponding to the adsorption processes have been obtained. The microcalorimetric method for the investigation of adsorption from solution is shown to be a very sensitive tool for the elucidation of the adsorption mechanism. The calorimetric results indicate that there are at least two kinds of interactions between nonionic surfactant molecules and the silica surface, the first one being due to a direct interaction between the polar parts of molecules and the surface (exothermic effect) and the second resulting from lateral interactions between the hydrophobic chains, leading to the formation of interfacial aggregates (endothermic effect). The principal conclusion resulting from the present investigation is that after the adsorption of individual "gaseous" surfactant molecules which probably constitute nucleation sites, there is the formation of interfacial micelles. The main driving forces for the formation of aggregates are of the same nature as those for the formation of micelles in the bulk solution. © 1990.