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 61<br />
oscillatory peaks compared well with the sizes of the molecules of interest,<br />
suggesting that each solvation shell consisted of a single layer of<br />
molecules. A later set of measurements in OMCTS <strong>and</strong> dodecanol on<br />
HOPG reached a similar conclusion [112, 113]. No displacement of solvation<br />
layers could be observed in water owing to the relatively large range<br />
<strong>and</strong> magnitude of the attractive jump-in event observed. In a later set of<br />
experiments [114], an oscillating lever was used to probe the solvation<br />
forces in OMCTS <strong>and</strong> dodecanol, allowing the mechanical compliance of<br />
the solvation shells to be measured. Here, periodic changes in the amplitude<br />
of lever oscillations were observed, which was explained as being<br />
because of an increase in the effective viscosity of the solutions when the<br />
surfaces were in close approach. It was noted that the presence of oscillatory<br />
structural forces even at the highly curved geometries present, even<br />
when using a sharp AFM probe, could have a detrimental influence on<br />
the very high resolution imaging of surfaces when using AFM.<br />
Solvation forces were measured in primary alcohols between silicon<br />
nitride probes <strong>and</strong> mica <strong>and</strong> HOPG surfaces by Franz <strong>and</strong> Butt [115]. For<br />
the measurements made against the hydrophilic mica, no solvation oscillation<br />
forces were observed at separation distances of less than 4 nm. At<br />
distances less than this, repulsive maxima followed by sudden jump-in<br />
events were observed. These repeated maxima were ascribed to the presence<br />
of solvation shells, with at least two of these layers present. These<br />
solvation forces were determined to be greater in magnitude than the<br />
attractive van der Waals forces present. It was noted that the period of<br />
the force oscillations increased linearly with chain length, with the period<br />
greater than the chain length. It was concluded that for the measurements<br />
made on mica, the molecules did not take on a flat conformation,<br />
but were at least partially upright. On the hydrophobic HOPG surface,<br />
measurements in 1-propanol <strong>and</strong> 1-pentanol oscillations had a period<br />
of 0.45 nm, suggesting that on the hydrophobic surface they did lie flat<br />
against the surface.<br />
Valle-Delgado <strong>and</strong> colleagues [116] investigated the solvation forces<br />
in water between hydrophilic silica spheres versus plane silica surfaces.<br />
A repulsive force was measured (�2 nm) at ranges shorter than the<br />
observed double layer repulsion <strong>and</strong> attractive van der Waals forces.<br />
A number of theoretical models were applied to the experimental data<br />
<strong>and</strong> it was concluded that for the situation under investigation, the observations<br />
were best explained by the formation <strong>and</strong> rupture of hydrogen<br />
bonds between SiOH groups on the silica surfaces <strong>and</strong> a single layer of<br />
water molecules.<br />
Jarvis et al. [117] used a carbon nanotube to probe solvation forces in<br />
water against a self-assembled monolayer bound to a gold surface, which<br />
had been characterised by imaging using the nanotube probe. When the<br />
forces were normalised, they were found to be in reasonable agreement