Adsorption of nonionic surfactants with ethylene oxide headgroup chemistry at the titania-water interface

The adsorption of nonionic surfactants such as poly-(ethylene oxide) alkyl ether surfactant (C_nE_m) and polysorbate (commonly referred to as Tween) was studied at the titania?water interface using optical reflectometry and atomic force microscopy (AFM). Previous reports have indicated little to no adsorption of these surfactants to titania, however under certain conditions, the surface excess was high. Typically significant adsorption was only observed when the titania surface was not strongly hydrated, that is, at point of zero charge and under low ionic strength conditions. For these amorphous titania surfaces prepared using atomic layer deposition, the pzc was at pH 5.1. Furthermore, the adsorbed amount of nonionic surfactant decreased with increasing ionic strength. This was attributed to the increased hydration of the titania interface from the specific adsorption of ions inhibiting the adsorption of the strongly hydrated ethylene oxide headgroup of the surfactant. AFM force measurements at the pzc in the presence of the surfactant as a function of concentration confirmed adsorption of the surfactant. Furthermore, soft contact imaging suggests that there was a spherical aggregate adsorbed layer structure above the critical surface aggregation concentration at pH 5.1 and no additional background electrolyte. No structure was observed at lower concentrations or under other solution conditions.