Characterization of surface forces in the presence of low molecular weight carboxylic acids through yield stress and atomic force microscopy

Low molecular weight (LMW) carboxylic acids are known to cause significant changes to the rheological behaviour of colloidal dispersions. Hence, they attract interest as potential additives that can be used in various industrial applications to improve processing and performance. However, the key factors that control how LMW carboxylic acids alter the surface forces operating between colloid particles have not been fully explored. Data from direct force measurements via atomic force microscopy (AFM), when correlated to that from yield stress measurements, can provide valuable insight into the interactions between surfaces. Therefore, it is surprising to find a paucity of studies that combine the AFM and yield stress techniques to probe the interaction between surfaces. In this body of work, both the AFM and yield stress techniques are used to investigate how adsorbed LMW carboxylic acids of different molecular architecture dictate the surface forces between colloid particles and ultimately contribute to the bulk behaviour of the concentrated dispersions. The solubility of the LMW carboxylic acids is revealed to be an important controlling factor. Poorly soluble LMW carboxylic acids, when present in high concentrations at low pH, invoke an attractive force that may be interpreted within the current paradigm as either van der Waals attraction or highly directed bridging. However, our studies reveal that this attractive force is most likely due to capillary bridging and/or precipitate bridging which in turn give rise to a heightened yield stress in the corresponding metal oxide dispersions. On the contrary, when the adsorbed LMW carboxylic acids are highly soluble or remain soluble within the experimental conditions, the molecular structure is important.