Surface Force Measurement between Atomic Layer Deposition Prepared Hafnia Surfaces

Abstract

Surface forces play a fundamental role in colloidal systems as they control the stability, adhesion, friction and rheology of colloids. Information on all of these can be obtained from an analysis of the normal forces measured between particles. Therefore, processing of colloidal products can be informed by knowledge of the forces between the constituent particles. For wet particles systems, the interaction forces between two particles can rarely be predicted from theory; rather, it requires experimentation or direct measurement. This requires that the surfaces used have the same surface properties as the particles. In practice this is rarely possible, as surface force measurements require surfaces with extremely low roughness and precise geometry and the majority of materials do not conform to these requirements. To address these challenges, this thesis investigates the forces measured between surfaces of low roughness and controlled chemistry produced by the use of atomic layer deposition (ALD). This thesis reports the forces between hafnia surfaces produced by ALD and shows that like ALD produced titania surfaces and silica surfaces, the expected van der Waals forces at high pH are not manifest, suggesting that most real surfaces have unexpectedly repulsive surface forces at high pH and small separations. This will fundamentally alter how these particulate systems behave when being processed, reducing the adhesion and the friction and enhancing the stability compared to the expected interaction from DLVO theory. Here, the interaction forces between very smooth Hafnia surfaces have been measured using the colloid probe technique and the forces evaluated within the DLVO framework, extended to include both hydration forces and the influence of roughness. The measured forces across a wide range of pH at different salt concentrations are well described with a single parameter for the surface roughness. These findings show that even small degrees of surface roughness significantly alter the form of the interaction force and therefore indicate that surface roughness needs to be included in the evaluation of surface forces between all surfaces that are not ideally smooth. The knowledge gained in the first part of this work as to how to account for the roughness effect, was then applied in investigating the influence of adsorbed citric acid and palmitic acid coatings on the surface forces between hafnia surfaces. The knowledge of the surface forces and citric acid adsorption that we obtained will be useful in understanding the stability and flocculation of colloids and nanoparticles which will influence the rheology of the colloidal dispersions and the distribution of colloids and nanoparticles in the environment. The measured surface force between hafnia surfaces that are hydrophobised by palmitic acid coating promises a very easy way to hydrophobise hafnia surfaces. The investigation into the forces measured between these smooth hydrophobic surfaces provides insight into the origin of the long-ranged hydrophobic force measured between surfaces covered with a monolayer of amphiphiles. A previously unrecognized interaction mechanism of interactions between single patches formed by the mobile amphiphile has been proposed based on the measured forces. Show more