Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 130

force- distance curve fi ts well the DLVO theory for distances 2 nm. However,

below 2 nm, a repulsive force is probed and was attributed to hydration force
(Meagher, 1992). Further, hydration forces were observed between an alumina
(Al 2 O 3 ) tip and a mica surface at different pH values and between a silicon nitride
tip and a mica surface at high concentrations of divalent cations (

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3 M) (Butt, 1991 ),
between silica surfaces in 1,2-ethanediol and water (Atkins and Ninham, 1997),
between gold surfaces in sodium chloride (Biggs et al. , 1994), between silica surfaces
and silicon on titanium dioxide at high pH (Larson et al. , 1993) and between an
alumina surface and an aluminum or silicon nitride tip (Karaman et al. , 1997 ).
The hydration force was also measured between two mica surfaces in electrolyte
solution (Israelachvili and Pashley, 1983; Pashley, 1981; Pashley and Israelachvili,
1984a). They measured a short range repulsive force in addition to van der Waals
forces at high salt concentrations, which varied with the type of cation in solution.
The more hydrated cations, such as Mg 2+ and Ca 2+ , gave stronger repulsive forces
than the less hydrated monovalent ions, such as K + and Cs + .
Other studies have suggested that hydration forces are oscillatory and can be
either attractive or repulsive (Israelachvili and Wennerstrom, 1996; Pashley and
Israelachvili, 1984b). These hydration forces should be present in aquatic colloidal
particles and can be dominant in those with high negative charge densities. Clearly
further theoretical and experimental work is needed to explore hydration forces in
environmental colloids.

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4.5.4.2 Hydrophobic Interactions

A hydrophobic surface is one that has low affi nity for water and has no polar or
ionic groups or hydrogen bonding sites. The nature of water in contact with such a
surface is different from the bulk water. Bulk water is signifi cantly structured via
the formation of hydrogen bonds between the water molecules, resulting in the
formation of large clusters of hydrogen bonded water molecules. The presence of
a hydrophobic surface will most likely restrict such phenomena and water confi ned
between two hydrophobic surfaces will not be able to form clusters larger than a
certain size, causing water molecules to tend to migrate to the bulk water where
there is unrestricted hydrogen bonding opportunities and a lower free energy
(Elimelech et al. , 1995a ).
Hydrophobic forces can be important, giving an extra attraction between sur-
faces or particles. Attraction between hydrophobized mica sheets, via surface
adsorption of hydrocarbon and fl uorocarbon surfactants, has been directly mea-
sured (Israelachvili and Pashley, 1984) and was found to operate over a long range
of about 80 nm and to be much stronger than the van der Waals force (Claesson
and Christenson, 1988). The magnitude of hydrophobic forces was found to decrease
with the increase in electrolyte concentration (0.01- 0.1 M magnesium sulfate)
(Christenson et al. , 1990). Figure 4.9 shows the long range (about 80 nm) attractive
hydrophobic forces measured between a silicon nitride (Si 3 N 4 ) tip and a hydropho-
bic mica surface prepared by depositing a monolayer of cetyltrimethylammonium
bromide (CTAB) on the surface of freshly cleaved mica surfaces (Teschke and de
Souza, 2003). More details can be found elesewhere (Christenson and Claesson,