Force measurements between titania surfaces produced by atomic layer deposition : the Van der Waals Enigma

Abstract

The surface forces between colloidal particles are important in flotation, lubrication, adhesion, rheology, materials science and cell interactions. In order to perform a fundamental investigation of surface forces, model surfaces are usually employed instead of natural surfaces. This is because natural surfaces are unsuitable due to the degree of surface roughness, inappropriate geometry or chemical heterogeneity. Given that the number of materials for which model surfaces are available is small, it is advantageous to expand the range of model surfaces that are available. This will enable a wider range of properties to be investigated and provide analogs for materials that are of interest in industrial processes, as well as providing for more stringent tests of our theoretical understanding of surface forces. With this as the goal, Atomic Layer Deposition (ALD) has been applied to the production of model surfaces of mineral oxides for surface force measurements in this thesis. Smooth surfaces of titania were produced using ALD and the surface forces measured in a range of conditions. Direct force measurements performed at the isoelectric point (IEP) of the titania surface revealed the van der Waals interaction. This measurement agreed with the calculated interaction predicted using Lifshitz theory. At pH values slightly above or below the IEP, a diffuse double layer repulsion was observed which is attributed to charging of the surface. At high pH, the forces were found to be repulsive up until contact, with no van der Waals attraction or adhesion being observed. It appears that the van der Waals attraction is either reduced at high pH or an additional repulsive force arises in these conditions. Reasons that could explain the absence of the van der Waals interaction at high pH include surface roughness, hydration of the surface or formation of a gel layer due to surface swelling. Surface roughness alone does not account for the absence of the van der Waals forces as the ALD prepared titania surfaces are very smooth. Neither do the hydration forces, as the expected dispersion forces should be exhibited at a range in which the hydration forces are absent. Formation of a gel layer through surface swelling is a possible explanation, however there is no evidence for this occurring as no change in the mechanical properties of the surfaces with pH were detected. Surfactants are employed as a facile means to alter surface properties; they have applications in mineral processing, detergency and lubrication. Adsorption isotherms for the cationic surfactant cetyltrimethylammonium bromide (CTAB) on ALD titania surfaces were measured using optical reflectometry. Measurements were performed at a range of concentrations below and above the common intersection point where adsorption is dominated by electrostatic and hydrophobic interactions respectively, as well as above and below the IEP. Interestingly, significant levels of adsorption were observed below the IEP where the electrostatic interactions are unfavorable. The adsorption results were used to interpret the force measurements between titania surfaces in aqueous CTAB solutions, which were measured using the colloid probe technique at different pH and electrolyte concentrations. The surface force data was compared to DLVO theory. Poor fits are obtained when Lifshitz theory is used to determine the effective Hamaker constant for the dispersion forces. However, all of the data are fit well with a dispersion force of reduced magnitude. The observation that the dispersion forces exhibited away from the IEP or in the presence of surfactant are much reduced has important implications for flocculation, adhesion and rheology of colloidal systems. Whilst several explanations for this anomaly have been proposed and tested in this thesis, the observation currently remains unexplained.