W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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2.3 INTERACTION FORCES 55<br />
separations, deviations did occur. This was attributed by the authors to<br />
an effect of the roughness of the surfaces involved, leading to a deviation<br />
from the assumed idealised geometry.<br />
There are a number of examples of the AFM being used along with the<br />
colloidal probe technique to probe van der Waals interactions between<br />
surfaces in a variety of media. Whilst most such studies examine a combination<br />
of van der Waals along with other surface forces, there are a few<br />
that concentrate on van der Waals measurements. Milling <strong>and</strong> colleagues<br />
[76] observed the interactions between colloidal gold spheres <strong>and</strong> poly(tetrafluoroethylene)<br />
(PTFE) surfaces in a variety of different liquids. Hamaker<br />
constants were found by fitting the appropriate force laws to the experimental<br />
data. As noted by the authors, the experimentally determined<br />
values for the Hamaker constant were only in agreement qualitatively<br />
with those calculated from theory. In addition, theoretical values varied<br />
significantly, depending upon whether an amorphous or crystalline form<br />
of PTFE was considered. For the majority of liquids used, which were of<br />
a polar nature (water, ethanol <strong>and</strong> dimethyl sulfoxide), only attractive<br />
forces were measured. In the case of dimethyl formamide, which was also<br />
a polar molecule, only repulsive interactions were observed. For these<br />
interactions, only water had been expected to show an attractive interaction.<br />
The authors concluded that in the case of the other polar liquids<br />
studied that showed attraction, the surfaces must have become contaminated<br />
by charged groups, leading to interactions that were not purely of<br />
a van der Waals nature. For interactions taking place in the perfluoroalkanes<br />
perfluorohexane <strong>and</strong> perfluorocyclohexane, as well as in air, shortrange<br />
interactions were also attractive. This was as predicted from theory,<br />
but the authors were unable to obtain experimentally determined values<br />
for A H. In the other low-polarity solvents examined (cyclohexane, dodecane,<br />
p-xylene <strong>and</strong> bromobenzene), all interactions were found to be repulsive,<br />
which was qualitatively in accordance with theoretical A H values calculated<br />
assuming amorphous PTFE surfaces.<br />
Karamen et al. [77] studied the interactions between model aluminium<br />
oxide surfaces in 1 mM solutions of potassium chloride as a function of<br />
pH. At separation distances greater than 5 nm, interaction forces were<br />
described very well by the traditional DLVO theory. From calculations<br />
of the surface potential from forces measured using DLVO theory, it was<br />
possible to calculate an isoelectric point at approximately pH 7, which<br />
was in agreement with values obtained from the literature.<br />
Many colloidal systems consist of particles with self-assembled monolayers<br />
covalently bound to the surface. Such a system is inherently more<br />
complex than those having interactions between single materials, as if the<br />
outer layer is thin, then interactions between the underlying surfaces will<br />
occur as well as between the monolayers. Kane <strong>and</strong> Mulvaney [78] studied<br />
the interactions between gold spheres <strong>and</strong> surfaces before <strong>and</strong> after