Adsorption of non-ionic surfactants to the sapphire/solution interface--effects of temperature and pH

The adsorption of the non-ionic surfactants tetraoxyethylene glycol monododecyl ether (C(12)EO(4)), pentaoxyethylene glycol monododecyl ether (C(12)EO(5)), and hexaoxyethylene glycol monododecyl ether (C(12)EO(6)) to single crystal sapphire substrates has been studied using specular neutron reflection for solutions at the critical micelle concentration. The effects of temperature and pH of the solutions were studied as well as the differences between two different crystal faces, the C and the R planes. At neutral pH, significant adsorption was only observed when the temperature was raised above the cloud temperature. This adsorption was reversible and surfactant was displaced on cooling. Reducing the pH to 3 results in significantly increased adsorption of C(12)EO(5) at 25°C with a central layer consisting mainly of surfactant (about 90%) on the C-plane substrate. A slightly smaller surface excess was observed for the R-plane. This contrasts with the significantly lower density observed even at high temperatures at neutral pH on both substrates. The results suggest that for neutral solutions surfactant association above the cloud point is the primary driving force for adsorption. At low pH, specific interactions with protonated surfaces are important. The structures of the highly hydrated layers are similar to those found for the surfactants at hydrophilic silica surfaces.