Adsorption Behavior of Two Binary Nonionic Surfactant Systems at the Silica−Water Interface

The adsorption of the mixtures C 14 E 6 −C 10 E 6 and C 12 E 5 −C 12 E 8 onto a silica surface has been studied by in situ null ellipsometry. Both equilibrium and kinetic features have been investigated. The adsorption isotherms of the two mixed systems exhibit different characteristics which can be attributed to the magnitude of the difference in critical micelle concentrations (cmc's) of the individual components in each pair. The isotherm of the C 14 E 6 −C 10 E 6 system exhibits a maximum at low total concentrations, while the C 12 E 5 −C 12 E 8 system acts as a single surfactant system with similar cmc but intermediate adsorbed amount compared with those of the individual components. The study of the kinetic behavior of the mixtures reveals that the adsorption characteristics are similar for the two systems, while during desorption they behave very differently. The amount of the C 14 E 6 −C 10 E 6 system adsorbed exhibits a maximum upon rinsing. The difference in desorption behavior between the two systems can be understood from the individual surfactant characteristics and the knowledge we have gained from studying the equilibrium adsorption behavior.