W. Richard Bowen and Nidal Hilal 4

60 2. MEASUREMENT OF PARTICLE ANd SURFACE INTERACTIONS
A repulsive force dominant at short ranges between silica surfaces in
aqueous solutions of NaCl has been reported by Grabbe and Horn [101],
which was also found to be independent of electrolyte concentration over
the range investigated. They attributed this force to a hydration repulsion
resulting from hydrogen bonding of water to the silica surface, and fitted
the additional component to a sum of two exponentials to work out the
formula for the hydration forces in the system.
The physical mechanisms underlying the hydration force are still a
matter for debate. One possible mechanism is the anomalous polarisation
of water near the interfaces, which completely alters its dielectric
response [102–104]. These theories imply an electrostatic origin of the
hydration force. However, other authors report [105] that there is no
evidence for a significant structuring of water layers near interfaces, or
a perturbation of its dielectric response, as envisaged by previous theories.
Instead, they suggest that the repulsive forces are due to entropic
(osmotic) repulsion of thermally excited molecular groups that protrude
from the surfaces [106]. This theory explains many experimental observations
in neutral systems [107], but its validity in charged systems is not
certain. Given the available evidence from experiments and simulations,
it is not possible to reach a definitive conclusion on the precise role of
these mechanisms in determining the hydration forces. Until recently,
computer simulations of water films coated with ionic surfactants
showed that protrusions are not significant in these systems [108]. On
the other hand, computer simulations show that water has an anomalous
dielectric behaviour near charged interfaces [109], but the observed electrostatic
fields obviously differ from the predictions of electrostatic theories
on hydration forces [103, 110]. The effect of this anomalous dielectric
behaviour of water on the electrostatic force between surfaces or interfaces
is still unknown.
The use of AFM to measure solvation forces between surfaces has been
of some interest amongst researchers in recent years for the purposes of
both studying the phenomena between closely interacting probes and
surfaces that may cause artefacts when imaging with an AFM and investigating
the effect of these solvation forces on the interactions between
colloidal particles. O’Shea and colleagues [111] observed an oscillatory
force on close approach of a sharp AFM probe to a graphite surface in
water, octamethylcyclotetra siloxane (OMCTS), and dodecanol. A series
of repulsive barriers were observed as approach was made for OMCTS
and dodecanol, with each repulsive barrier becoming successively larger,
owing to the closer-bound solvation layers becoming harder to displace.
The authors calculated that the energy required to remove the solvation
molecules was 5–25 times kT for OMCTS and 5–1000 times kT for
dodecanol, suggesting that only a small number of molecules were being
displaced. In addition, it was noted that the distance between successive