Department of Chemical Engineering and Chemical Technology, Imperial College London
Doi
Abstract
This Ph.D. study investigates the adsorption of monomeric (isostearic acid, ISA) and oligomeric fatty
acids (poly(hydroxystearic acid), PHS 1400 and PHS 3500) to nanoparticles of industrial interest in
carriers with different solvent properties. It was found that it was necessary to apply a number of
methods to study the adsorption and subsequent stabilisation of the particles by the dispersing
molecules, namely adsorption isotherm methods, rheological techniques, atomic force microscopy and
UV/vis spectroscopy. The particles (surface) studied consist of uncoated titania and a commercially
available titania coated with a combination of alumina and silica. The use of gel permeation
chromatography (GPC) to study the adsorption behaviour of polydisperse dispersants provided
information not only about the amount of the dispersant adsorbed to the surface, but also the
preferential adsorption of low molecular weight components. The displacement of large molecules by
smaller ones could be monitored in order to gain insight into the types of adsorption mechanisms at
work. It was found that small molecules were unable to fully displace their larger counterparts,
suggesting that there was more than one adsorption mechanism in effect. This was supported by the
adsorption study of one of the fatty acid dispersants which was esterified with methanol to remove the
acid functionality. Adsorption at the particle surface still occurred, but at a much reduced rate. The
preferential adsorption of the smaller molecules was also found to be largely eliminated.
Adsorption isotherms showed Langmuir-like adsorption behaviour of the molecules to the surface,
probably through a combination of acid-base interaction and other specific interactions between
surfactant molecule and surface. Adsorption of the dispersant molecules at the particle surface was
found to vary with solvent properties reflecting the equilibrium which is established between solubility
of the dispersant in solution (χ) and that adsorbed on the surface (г) as a result of adsorption affinity
(χs). Steric layer thickness δ could be varied by altering dispersion medium (and hence solvency of
the stabilising chain) and by altering molecular weight of the surfactant. This had a significant effect
upon the bulk properties of the suspended particles measured by rheological methods. For optimised
dispersion performance with high solids load at low viscosity whilst maintaining descrete dispersed
particles, as determined by assessing the optical properties of the suspension, the stabilising layer
required is dependent upon the particle type and size: small particles require small steric layers. It was
found that some degree of polydispersity was important for the oligomeric fatty acid dispersants.
Although good adsorption characteristics were observed for the monodisperse monomeric dispersant,
adsorption appeared to be optimum for the 1400 Mw oligomeric which had a high proportion of low
molecular weight components as well as larger molecules present. Removal of the low molecular
weight components from PHS 3500 resulted in interactions measured by AFM that appeared to
indicate bridging behaviour due to reduced packing efficiency
