W. Richard Bowen and Nidal Hilal 4

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