Abstract:
Understanding the interaction between surfaces at the intermolecular level in ambient
conditions is not only a fundamental science, but is of increasing value to water treatment
systems. Here the uses of the atomic force microscopy (AFM) modified with particles of
interest are assessed, and compared to bench-scale experimental techniques. In the first
part of this study, the results from force measurements performed with calcite-modified
probes in synthetic hard water (SHW) on selected substrates showed there was no
correlation with macroscale scaling rate experiments. However, unmodified tips showed
some correlation with non-metal substrates, where carbon coatings (Dymon-iC and
Graphit-iC) were least adhesive. Although unmodified tips were unlikely to represent one
of the surfaces of interest in water treatment systems, the findings suggest they can be
used to screen materials with Ra < 50 nm. Contact angle measurements complemented
force data, indicating the origin of repulsive forces on carbon coatings was due to
hydrophilic repulsion because carbon and calcite were highly basic. Enhanced adhesion
was caused by hydrophobic attraction and the presence of acidic surface groups. In the
2nd part of this study, force measurements were performed on natural organic matter
(NOM) polyanions such as humic acid fraction (HAF), fulvic acid fraction (FAF) and
hydrophilic acid (HPIA) using modified and unmodified tips. The results showed in
symmetric NOM-NOM interactions with modified tips, HPIA-HPIA dominated both
adhesion and detachment lengths, while FAF-FAF and HAF-HAF gave similar adhesion
profiles. It is thought these intermolecular interactions can be transferred to floc size data,
where HPIA flocs were bigger than FAF flocs. In non-symmetric systems adhesion
between FAF-NOM was indiscriminate, compared to HAF and HPIA polyanions,
indicating FAF polyanions were most likely to control coagulation performance during
NOM removal.
