We have employed Atomic Layer Deposition (ALD) to produce mineral like surfaces that are extremely smooth for use in surface force studies. Our investigations to date have focused on alumina and titania surfaces.
Alumina Surfaces Alumina surfaces were found to be unstable in aqueous solution – they slowly dissolved. This prevented surface force investigations in simple aqueous solutions. However the surface could be passivated against dissolution through the adsorption of short chain carboxylic acids. These acids are of industrial interest as they have significant effects on the rheology of alumina dispersions. Examination of the surface forces between alumina surfaces in solutions of muconic acids revealed DLVO type forces under some conditions Non‐DLVO forces where a strong attraction was evident between the surfaces and is significantly stronger than van der Waals attraction. This attraction was attributed to the formation of a capillary consisting of an oil‐like muconic acid phase forming between the surfaces. This phase change is induced by the close proximity of the surfaces and is possible because the muconic acid is present at concentrations that approach the solubility limit in these solutions. The presence of a capillary between the surfaces results in a strong attraction. Attempts were made to form stable alumina surfaces that would enable surface force measurements to be conducted in water and electrolyte solutions. This included looking at much thicker layers and using different binding layers (such as titania). To date none have been successful. We are still pursuing this though it may be possible that all alumina surfaces – not just ALD surfaces – have this property. A slow rate of dissolution would not be revealed in many studies and therefore may have previously gone unnoticed. Evidence from Optical Reflectometer (OR) shows that the surface dissolves at a rate of ~8 nm per hour. So indeed the rate of dissolution is slow, but sufficient to prevent surface force or optical reflectometry measurements.
Titania Surfaces In contrast, titania surfaces are stable and this has allowed us to perform a range of surface force studies at both low and high salt concentrations. At low salt concentrations a long range, pH dependent electrostatic force was observed. This data could be fit using the DLVO theory, which enabled the surface potential to be determined. This showed that the isoelectric point was between pH 5 and pH 6. At short range a repulsive interaction dominated the attractive van der Waals force. This is attributed to hydration forces. At high salt concentrations adhesion was seen that was dependent on both the pH and specific salt present. We find that this trend does not follow the Hofmeister series.